National Inventory (1)


General Limnology, Revised Edition (1997)


Copyright ©

  • 1995 Pasi Lehmusluoto and Badruddin Machbub
  • 1997 Pasi Lehmusluoto and Badruddin Machbub (Revised edition)

ISBN 951-45-7237-8

Cover photo:
Pura Ulun Danu, temple dedicated to the goddess Dewi Danu of Lake Bratan, Bali by
Pasi Lehmusluoto

Printed and bound by Edita Oy


The field work of the Indonesian national study program “Major Lakes and Reservoirs in  Indonesia, a Limnological Study”, generally called “Expedition Indodanau”, was executed in 1991-1994 as a joint Indonesian-Finnish research project.

In the project’s program, altogether 38 natural lakes and reservoirs were selected. They are  situated in Sumatra, Java, Bali, Lombok, Flores, Sulawesi and Irian Jaya. Some of the lakes could not be visited due to logistical problems.

This report deals with the results of the four field phases of the project, comprising of more than  8,000 measurements of 39 physical and chemical variables, in addition to the phytoplankton  identification and enumeration. However, more data are necessary to fully understand and sustain the  ecological health of these equatorial water bodies, and their interrelationship with geographic,  meteorological and weather patterns as well as the effects of land-use. It has to be emphasized that the  Indonesian lakes, even though they may have great volumes, are not bottomless sinks into which all manner of waste materials can be dumped, but ecological entities and they shall be treated as such.

The project has given indications that the northern and southern temperate limnology is not, as  such, entirely relevant to Indonesia, and cannot always be readily applied. This is especially the case with  the temperature, oxygen and nutrient regimes, and possibly with the biological diversity. Without  adequate information misconcepts may lead, in some occasions, to misunderstandings in decisionmaking. Clear study and research concepts shall be emphasized to rectify this situation.

The Indonesian lakes and reservoirs are reliable sources of protein-rich food. Lake quality control shall thus be also an integrated part of the sustainable fisheries development.

After a short summary of the report, in Part 1 a description of the background of the project and  the project itself is given. Part 2 gives an overview on the major findings of the natural lakes and  reservoirs, and discusses them, and Part 3 describes shortly the individual natural lakes and reservoirs.  Part 4 deals with the objectives of the management of the natural lakes and reservoirs. Part 5 is the list of references used and other useful literature. Finally, there is one Annex.

The entire team has accomplished all parts, except Part 4, which has been prepared by Pasi Lehmusluoto.

This report is, basically, a descriptive one, trying to make the information available also for a  wider public. The more analytical approaches are elaborated in the future publications.

In the second edition, no major changes have been made. Only some newly generated data have  been included in the text. In this revised and updated edition, some corrections and additions have been made.


Following persons, institutions and companies made the mission possible by assisting in various ways in the implementation of the program;

The entire project team in Indonesia; Dr. Badruddin Machbub, Ir. Nana Terangna, Drs.  Sudarmadji Rusmiputro (responsible counterpart to his sudden death on 19.5.1993), Drs. Firdaus  Achmad (responsible counterpart from 1.6.1993 on), Dra. Lusia Boer (deputy responsible counterpart  from 1.6.1993 on), Dr. Simon S. Brahmana, Drs. Bambang Priadi, Drs. Bambang Setiadji, Mr. Oman  Sayuman, Mr. Agus Margana, and the drivers Ade, Tatang and Tisna. In addition to the Indonesian  project team the following persons in Indonesia and Finland shall be mentioned, Dr. Michio Hashizume  of the UNESCO Regional Office for Science and Technology for Southeast Asia, Jakarta, Dr. Risto  Lemmelä of the Finnish IHP-Committee for the UNESCO/International Hydrological Programme (IHP)  for arranging the funds for printing of this report, LicSc. Toini Tikkanen and Prof. Pertti Eloranta, in  part, for phytoplankton identification, enumeration and drafting of the contents in Chapter 2.6. dealing  with phytoplankton, Mr. Ismo Malin, Mr. Jouko Saren and Mr. Pentti Orava for remarkable craftsmanship.

The Indonesian Embassy, Helsinki, Finland, the Embassy of Finland, Jakarta, Indonesia, RIWRD,  Bandung, Indonesia, LIPI in Jakarta and Bogor, Indonesia, Jasa Tirta Public Corporation, Malang,  Indonesia, Department of International Development Cooperation (former FINNIDA), Ministry for  Foreign Affairs, Helsinki, Finland, the Academy of Finland, Helsinki, Finland, the University of Helsinki, Finland and the Tallinn Technical University, Estonia.

Top Solutions Oy, Finland, Hyxo Oy, Finland, AS EL-KE Sensor, Estonia, Matkantekijät Oy,  Finland, OK-Matkat Oy, Finland, Malaysian Airlines System (MAS), Finland, Czechoslovak Airlines  (ÈSA), Finland, Sempati Air, Indonesia, Merpati Airlines, Indonesia, Karhumetalli Oy, Finland, Hanna  Instruments Asia Pacific PTE Ltd., Singapore, the following hotels in Indonesia for their hospitality  during the demanding field trips: Hotel Pangeran’s Beach, Padang, Hotel Pusako, Bukittinggi, Hotel  Marco Polo, Bandar Lampung, Hotel Kartika Plaza, Hotel Indonesia and Hotel Sari Pan Pacific,  Jakarta, Hotel Istana and Hotel Royal Dago Inn, Bandung, Makassar Gate Beach Hotel, Ujung  Pandang, Hotel Wisata and Palu Golden Hotel, Palu, Hotel Kawanua City, Manado, Hotel Sindhu  Beach and Hotel Bali Beach, Bali, and various guest houses, private and of the Ministry of Public Works.

Numerous persons in e.g. the Provincial Offices of the Governors, Social Politics and Public  Works, who helped in many ways in the formalities and logistics, especially Ir. Aisyah in Medan,  Sumatra, President Director Roedjito of Jasa Tirta Public Corporation in Malang, Java, Drs. Hamza  Yusuf Kali in Ujung Pandang, Sulawesi and Drs. Fauzi Bachtiar in Jayapura, Irian Jaya.


Dra. (Female) Indonesian lowest university degree in economy, chemistry, mathematics, etc.
Drs. (Male) Indonesian lowest university degree in economy, chemistry, mathematics, etc.
Ir. Indonesian lowest university degree in civil engineering
FINNIDA The former Finnish International Development Agency
IHP International Hydrological Programme
LIPI Lembaga Ilmu Pengetahuan Indonesia (The Indonesian Institute of Sciences)
ORP Oxydation-reduction potential
RIWRD Research Institute for Water Resources Development
RTR Relative thermal resistance Page 7


General Limnology

By Pasi Lehmusluoto1 in cooperation with Badruddin Machbub2, Nana Terangna2, Sudarmadji Rusmiputro(†)2, Firdaus Achmad2, Lusia Boer2, Simon S. Brahmana2, Bambang Priadi2, Bambang Setiadji2, OmanSayuman2 and Agus Margana2

1Expedition Indodanau, P.O.Box 717, FIN-00101 Helsinki, Finland; Contact by E-mail: (
2Research Institute for Water Resources Development, Jl. Ir. H.Juanda 193, Bandung 40135, Indonesia

The limnological information of the Indonesian lakes and reservoirs has been rather limited. There  are only some studies from Java, Sumatra and Bali from 1928-1929 (Ruttner 1931), and some sporadic  but by area and depth restricted studies from the 1970s, 1980s and 1990s. The present Expedition  Indodanau is covering 38 major lakes and reservoirs in Sumatra, Java, Bali, Lombok, Flores, Sulawesi and Irian Jaya.

The major objectives of the study are stipulated in the Joint Project Agreement.

The long-range objectives are;

  • To promote knowledge and environmental awareness of the problems of the major and economically important lakes and reservoirs.

The immediate objectives of the project are;

  • To develop and implement a pilot project for limnological study of lakes and reservoirs in Sumatra, Java, Bali, Lombok, Sumba (later substituted for Flores), Sulawesi and Irian Jaya.
  • To assist in developing a workable data collecting and reporting system for all water related data, which are produced in several Government Directorates.
  • To promote in-service and on-the-job training of researchers and managerial level staff, for improving operating and decision making capabilities.
  • To undertake organizational review and strengthen capabilities for implementing lake and reservoir management programs in Indonesia.

The project was executed according to the Joint Project Agreement and Plan of Operation which  the contracting partners had mutually agreed upon and which was outlined in the Project Proposal. The   fieldwork of the project was carried out in 1991-1994. For the first time the lakes were studied by the  same team using same sampling techniques and analytical methods, thus avoiding the uncalibrated situation in results comparison and evaluation.

The majority of lakes in the project program were visited during the field studies. However, due  to various reasons Tawar Laut, Segara Anak, Lindu and Tigawarna lakes were not visited. Some of the  lakes, such as Kerinci, Gajah Munkur and Sidenreng were visited but could not be vertically sampled  due to logistical hardships. From the lakes, 39 physical and chemical variables were measured either in  situ, or from the collected samples at the lakes or in the laboratory in Bandung. In addition,  phytoplankton was identified and enumerated from the surface samples. Altogether more than 8,000 measurements have been made.

The ecological health of the large natural lakes is still quite good, and the reservoirs are not yet  heavily polluted, eutrofied or contaminated. The circulation and mixing patterns of the lakes are generally irregular, and mixing tends to be incomplete. The reservoirs are oligomictic.

The major threats to the natural lakes are the control dams, which do not generally affect water quality, population and agriculture. However, the drainage areas are generally small and isolated from  major human activities, contrary to the reservoirs. Only Singkarak, Rawa Pening, Sidenreng, Tempe,  Matano and Sentani lakes may be under some notable influence from their drainage areas. Floating  vegetation heavily infests Rawa Pening. Most of the shallow reservoirs are in danger to silt up due to the  activities in their large watershed areas, especially Saguling, Cirata and Jatiluhur reservoirs in the  Citarum river basin and Lahor, Sutami and Wlingi in the Brantas river basin. In Saguling Reservoir cage  fish cultures are common. The Selorejo reservoir may be prone to extensive eutrophication. Based on the observations no really hazardous lakes could be found.

The management of the lakes and reservoirs shall be based on multiple-objective and integrated  planning in which non-economic objectives shall get much more weight. It is to be based on reductio  of  point and non-point loading, better understanding of the land-use and water interrelationship, and assimilative capacity and vulnerability of the receiving waters.

It is necessary to identify and prioritize;

  • Information and research needs, with pertinence also to the requirements of decision makers and other users of data,
  • Monitoring of freshwaters, and
  • Ways to assess the quality of lakes to provide timely and appropriate information.

To fulfill this, it is more than justified to outline a strategy plan agenda, Indonesian lake basin  action plan, based on the existing information and data on the lakes and reservoirs and on their  respective drainage areas, which are continuously updated by national and provincial activities. This kind  of activities would plausibly extend the activities of Expedition Indodanau, and assist in the sustainable development and utilization of the Indonesian inland water resources.

In order to achieve all that for the prosperity of the Indonesian people, it is necessary to compile  the existing information in one data base, make an inventory of the necessary background data of the  lakes and their drainage areas, carry out comprehensive diel, short-term and long-term ecological studies  of the lakes and reservoirs in a prioritized order, and establish a computer based lake basin atlas of  Indonesia, which is the backbone for the action plan agenda.



In the large island state of Indonesia there is a rather limited number of major ecologically and  economically important lakes, less than one hundred. One third of them are reservoirs, most of which  are situated in Java. The total number of all the lakes is estimated to be 521 (Nontji 1994, Giesen 1994),  but most of them are merely ponds. The limnology of the lakes and reservoirs is largely unknown. Their  research has until recently been scarce. Depth charts are almost non-existing. The lakes and reservoirs of this project and their approximate locations are shown on page 5.

The study program “Major Lakes and Reservoirs in Indonesia. A Limnological Study”, generally  called “Expedition Indodanau”, is a joint study of the Bandung based Research Institute for Water  Resources Development (RIWRD) at the Agency for Research and Development of the Ministry of  Public Works of Indonesia and the Department of Limnology and Environmental Protection (former  Department of Limnology) at the Faculty of Agriculture and Forestry of the University of Helsinki,  Finland. It has been included as a national project in the Blue Book Bappenas 1991/1992 (No.  BTA-244). The local sponsoring institution, in addition to RIWRD, is the Indonesian Institute of Sciences (LIPI) with its research permit 3913/11/1992 of 14.7.1992.

The project was funded by the Government of Indonesia through the Research Institute for Water  Resources Development of the Ministry of Public Works, the Academy of Finland from the funds  provided by the Department of International Development Cooperation (former FINNIDA) of the  Ministry for Foreign Affairs through the grants 1011935-8 and 1944, the Department of Limnology and  Environmental Protection of the University of Helsinki, and private funds, and the printing costs of this  report, after the request of the UNESCO Regional Office for Science and Technology for Southeast  Asia in Jakarta, Indonesia, by the Finnish IHP-Committee for the UNESCO/International Hydrological Programme (IHP).

The objectives of the study are stipulated in the Joint Project Agreement signed from the  Indonesian side by the Director General of the Agency for Research and Development, Ministry of  Public Works and the Director of the Research Institute for Water Resources Development, and from  the Finnish side by the Head of the Department of Limnology (at present the Department of Limnology  and Environmental Protection) at the Faculty of Agriculture and Forestry of the University of Helsinki, and the Project Coordinator of the Expedition Indodanau.

The long-range objectives are;

  • To promote knowledge and environmental awareness of the problems of the major and economically important lakes and reservoirs.

The immediate objectives of the project are;

  • To develop and implement a pilot project for limnological study of lakes and reservoirs in Sumatra, Java, Bali, Lombok, Sumba (later substituted for Flores), Sulawesi and Irian Jaya.
  • To assist in developing a workable data collecting and reporting system for all water related data, which are produced in several Government Directorates.
  • To promote in-service and on-the-job training of researchers and managerial level staff, for improving operating and decision making capabilities.
  • To undertake organizational review and strengthen capabilities for implementing lake and reservoir management programs in Indonesia.

The project was executed according to the Plan of Operation, which the contracting partners had  agreed upon and which was outlined in the Project Proposal. The fieldwork of the project was carried  out in 1991-1994. In this report, the project and its major findings during the four field phases in  1991-1994 are presented.


The natural resources of the lakes and reservoirs for the Indonesian national economy are  manifold; e.g. water abstraction for irrigation, water supply for domestic and industrial use, for  generation of hydroelectric energy, for fisheries, transport, tourism, recreation, and for conservation of biological diversity.

Chapter XIV, article 33, paragraphs 2 and 3 of the 1945 Indonesian Constitution state that  branches of production which are important for the State and which affect the life of most people shall  be controlled by the State, and that land and water and the natural riches contained therein shall be controlled by the State and shall be made use of for the people.

Later, on page 33, it is explained that only those enterprises, which do not affect the life of most  people may be in the hands of individuals. The earth and waters and the natural riches contained therein  are the fundamentals of the people’s prosperity. Therefore, they should be controlled by the State and be made use of for the greatest possible prosperity of the people (Republic of Indonesia 1988).

As already mentioned, the general limnology, long-term physical, chemical and biological trends  and limnological processes of the Indonesian inland waters are, largely, fairly little known. Especially the  physical processes are unique where Coriolis force is low and prevailing winds are unidirectional for extended periods of time. Chemistry is strongly affected by biological and, perhaps, geothermal

For the management and sustainable beneficial use of the lakes and reservoirs, more detailed  recent information is necessary. As a whole, there is much to be learned about the limnology in  Indonesia. The concept of sustainable development was spelled out in the 25-year development program  and a directive was prepared for the sixth five-year development plan, which started in April 1993. The  year 1993 was also the Year of the Environment in Indonesia. Thus, there was a good momentum for the social marketing of the environmental issues connected to the water resources.

All the acts of man will affect the nature, and they have often caused problems. Usually the natural  sciences are cautious of detailed studies, but the entities to which the issues belong, are often left unobserved. Conservative moderation in a humane way may be wise when dealing with nature.

For example, the present rice consumption in Indonesia is about 150-190 kg/person and year, and  its production may not be continuously increased. The dry season in 1992 was probably not as severe as predicted, and Indonesia did not have to import rice from abroad.

Nonetheless, rice is the main component in the Indonesian diet. When all the other sources of  carbohydrates are accounted for, the proportion of carbohydrates will be about 70-80 % of the diet.  However, the diet should include more proteins, plants, meat and fish. If the diet would correspond to  the recommendations of FAO and contain about 50 % of carbohydrates and 50 % of proteins, the rice  consumption could be decreased to the national target of 138 kg rice/person and year. This would also  be the prerequisite for the self-sufficiency of the rice reserves. It means, that the production, availability and use of proteins shall be increased.

Thus, in addition to the more technical benefits, the lakes and reservoirs may play an important  role in the production of fish proteins and in sustaining the rice self-sufficiency of the country. For this  end, fisheries and aquaculture in the lakes and reservoirs may also be intensified. The lakes and reservoirs are also important in reflecting some environmental trends.

Such lakes as e.g. Batur, Bratan, Maninjau, Singkarak and Toba have especially great value for recreation and tourism, as well as some of the reservoirs in Java.


The project area stretches from the northern tip of Sumatra to the northeastern corner of Irian  Jaya (see Figure 1). For comparison, it is very similar to the distances between Brest in France to  Sverdlovsk in Russia in Europe, or from Monrovia in Liberia to Addis Ababa in Ethiopia in Africa.

Figure 1. The project area shows the approximate locations of the natural lakes and reservoirs in the program.

West of Sulawesi and between Bali and Lombok lies the Wallace’s line, one of the sharpest  zoogeographical frontiers in the world (see Collins et al. 1991). However, this may not have much effect on the lakes and reservoirs.

1.3.1. Geography

The natural lakes are situated at elevations from close to the sea level, Limboto, Lindu, Sidenreng,  Tempe and Sentani lakes, to as high as 2,008 meters above sea level, Lake Segara Anak in Lombok, and   their surface areas are from 0.4 km2, Tigawarna lake in Flores, to 1,130 km2, Lake Toba in Sumatra.

The smallest so far visited lake is 1.9 km2 , Tamblingan in Bali. The reservoirs are located at elevations from near sea level, Palasari in Bali, to moderate altitudes of 670 meters (see Table 1).

The depths of the natural lakes vary from 2.5 meters, Lake Limboto, to 590 meters, Lake  Matano, both in Sulawesi. The depths of the reservoirs are from 6 meters, Wlingi, to 136 meters, Gajah Munkur. They both are situated in Java.

Morphometry of the lakes and reservoirs is presented in Table 1. The data has been compiled  from various sources, and it has to be noted that there are pieces of information, which do contradict  each other in the literature (see e.g. Ruttner 1931, Hutchinson 1957, Nontji 1990, 1994, Hehanussa 1994, Giesen 1994, Tjetjep 1994).

Kaul (1987) has compiled information on tropical mountain lakes. From Indonesia, he mentions  only two lakes, Bratan and Diatas, and only the altitudes are given. The most appropriate data, verified  by observations during visits to the lakes and reservoirs, have been taken in Table 1. This background  data shall be carefully reviewed and, when necessary, supplemented, revised and corrected.

1.3.2. Climate and weather

Climate in Indonesia is generally tropical and humid. It is governed by the wet southwesterly  monsoon from May to September and dry northeasterly monsoon from December to March, and the  main rainy season usually falls during the transition period after the southwesterly and before the  northeasterly monsoon. A rainy period occurs also in about April, after the northeasterly monsoon.  Rainfall varies from 6,000 mm/year to 600 mm/year in Palu Bay in Sulawesi being the driest location in the country.

The temperature variation is from 27 to 32 centigrade, although temperatures from 10 to 17 centigrades are common in the mountain areas.

The general hydrological patterns are connected to the rains. However, detailed information is  only available for some individual natural lakes. Water balances have been prepared e.g. for Sentani and Tondano lakes. Naturally, this information shall be available for the reservoirs.


The state of rivers in Indonesia has been studied for decades mainly by RIWRD, and there is a  hydrological monitoring network having a good coverage maintained also by RIWRD. A good example  is also the Brantas river basin monitoring network operated by the Jasa Tirta Public Corporation in Malang, Java. Lakes and reservoirs have been sampled on a sporadic basis.

The studies made by the German Sunda-Expedition in 1928-1929 (see e.g. Ruttner 1931), which  was a great exercise at that time, gave a good impetus for the studies of the natural lakes in Indonesia.  Franz Ruttner, August Thienemann and Heinrich J. Feuerborn, all early developers of limnology, were  members of the study team. Large reservoirs were not yet constructed at that time. However, time- relationships and rate-measurements were little represented (Talling 1995), and most of its contributions are on hydrobiological issues.

As the result of the Sunda-Expedition, a voluminous amount of information, mainly on biology,  was collected from almost all the larger natural lakes in Sumatra, Java and Bali, and also from many  various kinds of small water impoundments. The latest publications based on the material of the  expedition are from the fifties. There is a time gap of more than 65 years between the German Sunda-Expedition and the present nationwide Indonesian-Finnish Expedition Indodanau.

After the Sunda-Expedition some of the lakes and reservoirs in Indonesia have been studied only  sporadically, as already mentioned, by the Inland Fisheries Research Institute in Bogor to support  fisheries (Nontji 1994), by RIWRD (mainly in 1983-1988), by many of the other government institutions  and universities and in some occasions also by environmental consultants in relation to environmental impacts, and by individual researchers.

Giesen (1994) states that basic inventory information is available for most of the lakes in the  western part of Indonesia, and that at least half of the lakes he mentions have been studied relatively  comprehensively, while the others are virtually undocumented and still poorly understood. However, the  earlier physical and chemical data are of limited value, since the sampling locations cannot be accurately  recalled, the samplings with no consistent patterns do not allow any closer look at the stratification, and  no temporal and spatial long-term data exists for evaluation of any trends. Usually the surface waters or  the very upper epilimnion (30-50 meters) have been studied. The Whitten’s books on the ecology of  Sumatra and Sulawesi (Whitten et al. 1987 a, b) clearly reflect the lack of information, and e.g. Green et  al. (1995) the neglect of depth wise studies. In addition, except the study of the Balinese caldera lakes in  1977 by Lehmusluoto & Machbub (1989), the results of RIWRD cannot yet be publicly utilized to  evaluate the possible temporal changes in some of the lakes and reservoirs. The previous data exhibit  values, which cannot be calibrated against the present sampling procedures and analytical methods.  Thus, Expedition Indodanau is the only research project giving, in this sense, calibrated nationwide  information on the Indonesian lakes. For the biology of the Indonesian lakes, the earlier studies are  giving more background data.

Widjaja (1980), Nontji (1994) and Giesen (1994) mad compilations of research and studies on the  Indonesian lakes, and the most recent studies were reviewed at the International Conference on Tropical  Limnology in Commemoration of the 65th Anniversary of the Ruttner-Thienemann Limnological  Sunda-Expedition held on 4-8 August 1994 in Salatiga, Indonesia. They indicate that the studies have  generally been “task oriented” rather than synoptic. They all demonstrated the immediate need of such a  nationwide inventory of especially the physical and chemical properties and processes of the Indonesian  natural lakes and reservoirs, as the present Expedition Indodanau, and integrated studies of lakes and their respective catchments areas.

The main problems have been the meager funds, inadequate communication with other authorities  and agencies (sectoral egotism as pointed out by President Soeharto 1992), inadequate communication with regional research institutions and channels for publication (Nontji 1994).


As Nontji (1994) has stated, a detailed inventory of all Indonesian lakes has yet to be drawn up.  The Expedition Indodanau, a joint study of the Research Institute for Water Resources Development  (RIWRD) at the Agency for Research and Development of the Ministry of Public Works of Indonesia  and the Department of Limnology and Environmental Protection at the Faculty of Agriculture and  Forestry of the University of Helsinki, Finland, which was commenced in 1991 after a two-year planning  period, is attempting to be an exercise to widen the knowledge and enhance the activities of regional  data acquisition. In addition, the Research and Development Center for Limnology of the Indonesian  Institute of Sciences (LIPI) in Bogor was interested in participating in some parts of the study.  According to LIPI, the study is of great importance and fulfills the needs of “Post-Ruttner Study” (Badruddin-Lehmusluoto Study).

The aim of the study is to produce basic physical and chemical information, and that on  phytoplankton, on the selected lakes and reservoirs for all interested user groups, and compare the  results with the limited earlier studies. A database was established for this purpose in cooperation with Mr. Robert Fortin, a Canadian data base expert assigned to the RIWRD.

It is necessary to carry out both comparative (regional and lakewise), and at a later stage,  integrated studies. Presently, it is also necessary to ascertain the trophic status of the lakes, provide  baseline data for ongoing evaluations establish programs for continuous monitoring and plan programs for lake and reservoir management, including their watersheds.

It is also necessary to be aware of the ecological rules governing the lakes and reservoirs in order  to maintain their ecological health. Long-term monitoring is a prerequisite for action, sustainable development and self-sufficiency.

Otherwise, there may be a risk of encountering the lake and reservoir responses beyond our  experience and theoretical background, and important environmental decisions may be faced with great  uncertainty. Without basic geographical and hydrological information, hydrological budgets and new  inputs of nutrients (precipitation and rivers) cannot be calculated. Without vertical water column physics  and chemistry thermal dynamics (seasonal and short-term vertical mixing), deep- water “ventilation”  (hypolimnetic turbulence) and renewal, and nutrient balances (N, P, S and Si) for e.g. productivity drive  cannot be evaluated. In this respect diel, seasonal and long-term observations are needed. An  economically “unproductive” lake may be assigned to a shortsighted use, if the ecosystem is not  thoroughly known. In this respect, we have to remember that e.g. also lakes are fundamentals of the  people’s prosperity. Therefore, they and their use should be controlled by the state (see Republic of Indonesia 1988).

The first field phase was in February-March 1992 and the fourth in July-August 1993. The  information in this report is based on the results obtained during the four field phases of the expedition.  Altogether 38 natural lakes and reservoirs were included in the study program. They are situated in  Sumatra, Java, Bali, Sulawesi, Lombok, Flores and Irian Jaya (see Figure 1). By now 36  (92 %) of the lakes and reservoirs have been visited, some of them three times (Table 2). Attempts to  visit Lake Segara Anak in Lombok, Lake Lindu in Sulawesi and Tigawarna Lake in Flores were made,  but due to logistical hardship, it was not possible. Two sites, Lake Tawar Laut in Sumatra and  Tigawarna Lake in Flores were later rejected, and Sengguruh reservoir in Java was added in the program, but has not yet been visited.

According to the original project plan, each of the lakes and reservoirs would have been visited  twice, once in the rainy season and once in the dry season. Visits that are more frequent would have  been economically unfeasible. The planned visits took some 15 months due to the long distances and  logistical hardships in Indonesia. The large areas and great depths of the water bodies also cause  logistical difficulties, when there are no suitable vessels available. The 39 variables, which were  measured either in situ or from the sampled water in the field and in the laboratory of RIWRD in  Bandung, and analytical methods used, are shown in Annex 1. The phytoplankton identification and enumeration were made in Finland.

Unavailability of a specifically equipped boat caused some nuisance. The boats used varied from  dug-outs and outriggers to large diesel powered vessels, and in each case, the hoisting gear and other  equipment had to be separately prepared and adjusted. On the other hand, transportation of such a vessel could be just another problem.

Lack of detailed bathymetric information is perhaps the most serious deficiency in the  morphometric knowledge (Herdendorf 1982, Nontji 1994). The great depths of some of the lakes  created also some problems in sampling and measurements in the field. The major equipment prepared  especially for this expedition were the water sampler with a 500 meter long cable and the Marvet AJ90  RS temperature and oxygen analyzer having a probe with a 500 meter long electrical cable. The probe is  constructed to stand water pressures up to 500 kPa. It would have been desirable to have thermistor and  dissolved oxygen sensor strings in some of the lakes to monitor the mixing depth and possible circulation, and oxygen replenishment of the lakes.

The limnology of the natural lakes and reservoirs, despite the overall geographical and  climatological similarities, has differences in morphometry, hydrology and land-use patterns of the  drainage areas. Therefore, the natural lakes and reservoirs are dealt with separately, when necessary.



These studies of the water resources are important to limit the harmful activities, which may  reduce the availability and to deteriorate the quality and production capacity of water. The information is  necessary to avoid unnecessary stress on water and water resources. In the following only the main issues are dealt with.


The natural lakes have usually been formed by volcanic or tectonic activity (see Table 1). Caldera  lakes were formed in the depressions of the collapsed walls of volcanoes. Good examples of caldera  lakes are the Batur, Bratan, Buyan and Tamblingan lakes in Bali. Crater lakes were formed in the extinct  craters. Maninjau and Ranau lakes in Sumatra and Lake Segara Anak in Lombok are also typical crater lakes, and many of the small lakes in Java and Tigawarna Lake in Flores.

The history of Lake Toba, situated in North Sumatra, has various stages, volcanic, tectonic and  volcanic. The lake is in the collapsed “Batak tumor”, which is one of the most magnificent volcanic  formations in the world. The elevation of the formation is about 2,000 meters, and it is about 275 km  long and 150 km wide. After the volcanic eruption some 75,000 years ago, in which about 2,000 km3 of  soil material was blown off, the dome collapsed and the 90 km long and 30 km wide Lake Toba was  formed in the center of the 2,269-km2 basin occupying a total area of 1,786 km2, including the Samosir “island”. The water level is at an elevation of 905 meters (Bemmelen 1930, Ninkowich et al. 1978).

Lake Toba is the largest volcanic caldera type lake in the world, having a water area of about  1,130 km2. The Samosir “island” was formed afterwards by tectonic activity. It is at present about 40 km  long and 20 km wide, it rises about 1,630 meters above sea level, and is connected by a small neck of  land, formed much later, to the western shore of Lake Toba. In this area there are so called solfataras,  which actively release sulphuric vapors. Samosir is thus actually not an island. It divides the lake into the southern and northern basins, which are connected by a long strait.

Lake Toba is the ninth deepest lake in the world (although in the literature other information is  found), having a depth of about 529 meters (northern basin). This could not be verified since our cable  and wire did reach only 500 meters, and no appropriate echo sounder was available. The southern basin  has a depth of 433 meters. It is a great lake, in various meanings, but is it a Great Lake worth special attention?

The tectonic Lake Singkarak in Sumatra is 268 meters deep and graben fault lake Matano in  Central Sulawesi has a reported depth about 590 meters (Bemmelen 1949, see Herdendorf 1982). We  can confirm that the depth is more than 500 meters. With the depth of 590 meters, it shall be the seventh  deepest lake in the world. Lake Poso should, according to the present information, be the seventeenth  deepest and Lake Dibawah the forty-first deepest in the world, although they are not included in the list  prepared by Herdendorf (Herdendorf 1982). Similarly, Lake Towuti is not included in the list of lakes  larger than 500 km2, and Lake Toba is not included in the list of lakes with great volumes, although it may have an evaluated water volume ranking approximately twenty-fifth.

There are only a few shallow lakes originating because of minor tectonic movements of the earth’s  crust, such as Lake Sentani in Irian Jaya, which was formed by a landslide dam on the Jafuri River. Sidenreng and Tempe lakes in Sulawesi are floodplain lakes.

In Sumatra, there are also a number of shallow dystrophic lakes in the peat areas, as well as in  Kalimantan and Irian Jaya. In Java and Irian Jaya, there are also solution lakes in the karst areas (see also  Giesen 1994). Many of the natural lakes have control dams at their outlets, e.g. Toba, Maninjau,  Singkarak, Rawa Pening, Poso and Matano.


The geography and especially hydrology of the natural lakes is not well known, except e.g. of  Rawa Pening in Central Java, Tondano in North Sulawesi and Sentani in Irian Jaya. The geological  formations of the reservoirs, with many fjords like bays, are more complex than the formations of the  natural lakes. All the reservoirs are about 100 meters or less at the deepest. The drawdown of the  reservoirs varies from 1.5 meters in Wlingi to 32 meters in Jatiluhur reservoir. Available geographic  information of the lakes and reservoirs has been compiled separately in the data volume (see also Table1).

The surface areas of the lakes vary greatly. The smallest lake studied is Tamblingan in Bali, 1.9  km2 and largest Lake Toba, 1,130 km2 in Sumatra. The smallest lake in the programme is Tigawarna in Flores, 0.4 km2.

Most of the natural lakes are rather deep, from 200 to 590 meters (Lake Matano in Sulawesi), but  there are also lakes with depths from some tens of meters to depths of only one or two meters, Sidenreng, Tempe and Limboto, depending on the season.

There is only one lake having a cryptodepression (the deepest part is below sea level), Lake  Matano in Sulawesi. The cryptodepression is 208 meters, and total depth 590 meters.  In addition, the relative depth (the greatest depth in percentage of the mean diameter of the basin,  Zr) of the natural lakes may vary greatly. However, compared to the lakes of the temperate region the  relative depth is greater. However, the relative depth of the reservoirs is usually small, corresponding to the temperate lakes.

Due to the inadequate geographical information, also the drainage areas of the lakes have not been  accurately defined. However, this should be a prerequisite for the hydrological water balance calculations. For the reservoirs, this information is generally available.

The volumes of most of the natural lakes are unknown. For that purpose, detailed echo sounding  is necessary (an appropriate echo sounder has already been designed in connection to the Expedition  Indodanau). Residence times and flushing rates of the natural lakes cannot be calculated, because the  information on inflow and outflow rates, water volumes, and other necessary variables are lacking.  However, it may be suggested that the average residence times of most of the large and deep natural  lakes are quite long, longer than the world’s average water residence time of 17 years, making them rather vulnerable and sensitive for the effects of loading.


The ranges of the measured limnological variables of the abiotic environments of the natural lakes  and reservoirs are compiled in Table 3. General conditions, marked deviations and exceptional  concentrations are pinpointed in this chapter, and the results are discussed in more detail. Comparative  information (Figures) and detailed information on each individual lake and reservoir (Tables and Figures) are in separate data files. In addition, all data are stored in the computer database in RIWRD.

2.4.1. Thermal properties

In the natural lakes, the annual dynamic patterns of temperature may differ slightly from year to  year, and annually by the northeasterly and southwesterly monsoon, and transitional periods between  them. During the southwesterly monsoon from May to September, bringing rain, the clouds hinder the heat transport from the ground and the lakes.

Thus, the lakes have higher temperatures at the surface and in the epilimnia during the  southwesterly monsoon than during the northeasterly monsoon in December-March. In none of the  lakes, deep hypolimnetic temperature rise (adiabatic or volcanic), dicothermic temperature curves, could  be observed. In the following the major physical and chemical variables, and phytoplankton of the lakes,  are dealt with in more detail.

Temperature of the natural lakes in Indonesia varies somewhat due to location, but mainly by altitude. Annual seasonality may also cause some differences.

In the natural lakes, a primary thermocline can be found, which may coincide with the euphotic  zone (Payne 1986), and in many of the lakes also one or more secondary thermoclines. The secondary  thermoclines close to the surface are often daily microstratifications, which may break down by  nocturnal cooling. In many of the lakes, the uppermost distinctive secondary thermocline was usually  very sharp, as in the Bratan and Tamblingan lakes (see page 18). This stratification may often have its daily changes due to nocturnal cooling and daytime superficial warming.

However, in the shallow Lake Rawa Pening, there was no epilimnion, and the thermocline began  at the surface. The temperature curve resembled the light extinction curve. Unfortunately, it was not possible to make any 24-hour studies to observe the extent of nocturnal cooling.

In the large and deep lakes the primary thermoclines may be at considerable depths, e.g. in Lake  Toba at least at a depth of some 160 meters. Several secondary thermoclines, representing the younger history of the lake, were located above it (see page 17).

During the warm southwesterly monsoon, the location of the thermocline seems to move  downwards indicating some heat accumulation especially in the lakes at higher altitudes.  As the data are rather limited, and no 24-hour and long-term observations were available, it is not  possible to conclude further about stratification, mixing and circulation. It can only be said, that some of  the lakes seem to be permanently stratified, and some may be annually stratified for longer periods.  However, the oxygen concentrations in the hypolimnion indicate, that some of the lakes are receiving  oxygen replenishment sometimes. One of the possible times is during the cooler hemispheric winter before the northeasterly monsoon in December-March.

The general temperature patterns of the reservoirs follow the same trends as in the natural lakes.  However, the draw-down and filling-up periods may cause some disturbances in the form of hydraulic  stratification in the temperature regime (see Gavilan-Diaz & Matsumura-Tundisi 1995). This may be  dependent on the turbine intake depth, and can be demonstrated by the very steep stratification in most  of the reservoirs, and with relatively shallow epilimnia. In addition, the cooler river water entrainment in the beginning of the filling-up period may be the cause.

In the reservoirs, there are annual variations in the water level caused by draw-down, and in the  water flow, relatively great water flow in the main stream and high silt concentrations during the  filling-up periods. Because of these, stratification may be disturbed during the draw-down and filling-up  periods, and it may not develop in the similar way as in the natural lakes. The relative depth of the reservoirs is usually small and of the natural lakes great.

If the stratification breaks down nightly in the reservoirs during the low-water period and north-  easterly monsoon, it will be quickly reestablished in the beginning of the high-water season. During the high-water season epilimnia are also getting thicker by the action of wind.

Relative thermal resistance (RTR) may be used as an indicator for the stability of lakes. The  majority of the lakes were at least weakly or moderately stratified, and occasionally strongly stratified.  For example, in the deep Lake Toba RTR was in September 1992 only 25.0 in the south basin, and in  the north basin 31.1. In March 1992, it was 61.1. The oxygen concentrations in the northern basin in  Lake Toba may indicate the possibility of circulation in contrast to the southern basin. Lake Tempe was  practically not stratified (RTR 3.5). The greatest RTR was in Batur in March 1993, 126.9. In the  reservoirs, stratification was generally stronger than in the natural lakes. The lowest RTR was 13.7 (Jatiluhur) and highest 156.2 (Selorejo).

Density currents and seiches were not actually observed in this study, although there were some  indications of their presence in some of the lakes, but especially in the large reservoirs as tilting of the thermocline.

Temperature and dissolved oxygen are two of the major characteristics depicting the dynamics of  the water bodies. However, in the Indonesian lakes the hypolimnetic anoxia will develop quickly   after a possible overturn. Within 3-5 weeks, a pronounced oxycline may develop also in oligotrophic  shallow lakes and reservoirs (see Townsend 1995), and hypolimnia may become oxygen depleted. This  is mainly due to temperature dependent chemical processes and not because of high organic load (see  Ruttner 1940). The hypolimnetic oxygen depletion rate may thus not be used as a criterion in assessing  the eutrophication status, as suggested by Cole (1983) and Rast et al. (1989), but the depth at which  oxygen is totally depleted (zero oxygen depth, Zzo) may be used to follow the overall situation in the  lakes (see also Ruttner 1931). The density structures of the natural lakes may vary from slightly stable to  moderately stable and strongly stable, but metastable lakes (small temperature gradients and high carbon  dioxide concentrations) were not found (see Kling 1988). Relatively great gradients in total dissolved  solids may affect the stability in some of the lakes. In Table 4 below the ranges of temperature, dissolved  oxygen and concentrations of dissolved solids in the natural lakes and reservoirs are shown for comparison.

In some of the natural lakes, the primary stratifications may be decades old and at considerable  depths, which can be seen from the temperature and oxygen curves. The deep water has a certain  “memory” lasting for years, which may be eroded sporadically due to temperature variations and wind.  The time elapsed since the deep water has been in contact with atmosphere may be computed (see  Imboden et al. 1993). Some quite recent secondary thermo- and oxyclines, and diurnal  microstratifications, were observed in some of the deeper lakes, e.g. in Toba and Batur. This may be  partly due to surface warming and low wind stress, causing gradual increase in deep water temperature  because of several years gradual warming (Kling 1988). The “sawtooth” warming and cooling is  important for deep-water oxygen replenishment (see Livingstone 1993, 1995). There may also be daily  inverse and superficial variations at the very surface. Examples are shown in Figures 2 and 3.

Figure 2. Stratification patterns in the main basins of Lake Toba, North Sumatra. The upper figures are in full scale, and in the lower figures the scale has been stretched to show only the upper 100 meters and the minute variations in the temperature and oxygen values. Note also the difference between the north and south basins (see also Lehmusluoto et al. 1995).

By using the vertical sampling intervals applied by Ruttner in April 1929 (Ruttner 1931), these  would have been left unobserved. The resolution and precision of the temperature measurements made  by a mercury thermometer (usually 0.1-0.05 centigrade) are not accurate enough to record all the  minute changes in temperature. Therefore, e.g. the temperature values of Lake Toba measured in April  1929 (Ruttner 1931) and in March 1992 seem to decrease stepwise by depth in contrast to the curves measured in September 1992.

In March 1992, the temperature was in the epilimnion almost the same as in April 1929, but the  hypolimnetic temperature was about 0.5 centigrade higher in 1992. However, in September 1992 the  upper epilimnetic temperature was about one and a half centigrade lower than in March. The  temperature difference in the whole water column was in September 1992, without daily changes at the  near surface, only less than one centigrade in both of the basins, when the difference in March was about  two centigrade in the northern basin. The heat gain of the hypolimnion during the past 65 years may  have been the result of the solar radiation or turbulent conduction and by the currents (Wetzel 1975).

Figure 3. Vertical stratifications in the Batur, Bratan, Buyan and Tamblingan lakes in Bali on 1-3 September 1992. Lake Tamblingan is well sheltered from winds by rather high crater rims. In March 1993 in Batur and Bratan lakes hypolimnia were almost anoxic. Buyan and Tamblingan lakes were not visited then.

The annual epilimnetic temperature and oxygen differences of Ranau, Singkarak and Toba  (northern basin) lakes may be explained by the annual variations in climatological and weather  conditions (north-easterly and south-westerly monsoons). The northeasterly monsoon brings dry  weather while the southwesterly monsoon is moisture-laden bringing rain. The clouds reflect back the  solar radiation, but the cloud cover also prevents from heat transport from ground and water, thus  making the epilimnetic temperature raises possible in the beginning of each year. The cooling effect of  clouds is dominant in the temperate regions, but it is not clearly known what is the effect in the warmer  climates. The depths at which hydrogen sulfide was first found (detectable hydrogen sulfide depth, Zhs) were also observed. It usually coincided with the zero oxygen depth.

Because of the differences in size and form of the lake basins the transport of heat and oxygen into  the deeper water layers, into which no light is penetrating, is more efficient in Toba and Ranau lakes  than in Lake Singkarak. As mentioned, in Lake Toba there seems to have been some heat accumulation  into the deeper layers since 1929. The area of lake Toba is 1,130 km2 (excluding the Samosir “island”),  of Lake Ranau 125 km 2 and of Lake Singkarak 107 km2. Lake Toba is thus more favorable for wind  induced heat and oxygen transport, although the large Samosir “island” may reduce the effects of wind.

In the reservoirs, the stratification is usually rather distinct. It can be clearly seen from the  steepness of the temperature and especially of the oxygen curves. For example, in the Saguling reservoir  the oxygen depleted layer began during the high-water season at the depth of 5-7 meters, and during the  low-water season at the depth of 4-5 meters. The stratification of Cirata and Selorejo reservoirs are  shown in Figure 4 as examples. In Selorejo the effects of inflowing cooler river water can be clearly  seen. The steep stratification in the reservoirs may also be partly caused by hydraulic stratification  depending on the turbine intake depths (see Gavilan-Diaz & Matsumura-Tundisi 1995). In addition, inflowing waters into reservoirs prevent from destabilization (Lewis & Weibezahn 1976).

Figure 4. Vertical stratifications in Cirata and Selorejo reservoirs in Java. Note the changes in Selorejo reservoir in the hypolimnion caused by the inflowing cooler river water.

The Bongas Bay in the fertile Saguling reservoir (see Widjaja & Adiwilaga 1995) is an area,  where fish cage cultures have been intensively developed. The sudden fish kills have caused great  economical losses. The reasons for the fish kills are still under survey and discussion. The local and  national economic value of the Saguling reservoir (maximum area 56 km2  and maximum depth 99 m) is  great. The hydropower production is 700 MW/year and fish production 160 tons/year (400 nets producing each 400 kg/year).

However, the Saguling basin, as well as the basins of Cirata and Jatiluhur reservoirs, and  especially the Wlingi reservoir, are silting faster than expected. This is said to be due to the acts of the  people living in the drainage areas, which have not been controlled by the authorities (Jakarta Post 24.12.1989).

The steepness of the temperature and oxygen curves in the reservoirs is affected, as already noted,  by the great annual variations in water level and water flow, and in concentrations of silt. Silt does not  necessarily affect the layering of the inflowing water but the water temperature (Faithful & Griffiths  1994). Silt accumulates heat and hinders the light penetration into the deeper layers of water. Although  the Saguling reservoir is large, its relative depth is small. This may suggest, that mixing of the water  layers would have been more efficient than it has been. Ruttner (1940) states that oxygen deficit is  dependent on temperature in the tropical lakes, as opposed to oxidizable matter in the temperate lakes,  and in meromictic lakes also on the duration of stagnation.

2.4.2. Circulation and mixing

The very shallow lakes situated at low altitudes are primarily polymictic, shallow lakes and  reservoirs (< 100 in depth) are oligomictic, which means that they may seldom have a complete mixing  even during the north-easterly monsoon period and hemispheric winter, and high-altitude lakes may be  monomictic. During the low-water period the very shallow (water depth about 5-7 meters) reservoirs may circulate, because of wind.

Lakes situated at high altitudes may circulate during the cool north-easterly monsoon period at  night time, when the surface water is cooling causing convection, down to the depth of some tens of  meters or even deeper, depending on the temperature differences and wind. Full circulation is unlikely,  because the total and relative depths are usually relatively great. The high-altitude lakes may thus be monomictic, and polymictic to a certain depth in the suggested mixolimnion (see also page 35).

It is important to be able to measure accurately vertical temperature and oxygen profiles in the  entire water column with adequate intervals to see the possibility of circulation, and the depth of mixing  which is vital in maintaining productivity. The depth of mixing may be the major determinant of  interannual variation in primary production (see Lewis 1995). The mixing depth may be taken to be the  maximum point at which an upper layer of approximately uniform nitrate concentration meets a lower  layer of rapidly increasing nitrate concentrations. The mixing events may also inoculate the surface waters with viable phytoplankton (see Goldman 1988, 1990, Goldman & Jassby 1990).

If a complete circulation for a reason or another would happen in some of the deep meromictic  lakes in Indonesia (which is unlikely), the released amount of carbon dioxide and hydrogen sulfide,  among others, could be harmful in and around the lakes. For example in Lake Singkarak, having a  relative depth of about two, the accumulated carbon dioxide amounts to 60,000 tons and hydrogen  sulfide to 18,000 tons. When Lake Nyos in Cameroon accidentally circulated in 1986, 1,700 people died  of asphyxiation. The relative depth of Lake Nyos is 15. The reason for the circulation is still under  consideration. The main cause under discussion is the unfortunate simultaneous timing of all the causes  weakening stratification. It has been also shown that in the hypolimnion of Lake Nyos temperature  increases towards the bottom of the lake (Kling 1987). Such a phenomenon was not observed in any of the studied lakes in Indonesia.

Some of the shallow high-altitude lakes, such as Lake Bratan in Bali and Lake Diatas in Sumatra,  may annually have a complete overturn. This is indicated by the oxygen curves. Lake Bratan is 20  meters and Lake Diatas 44 meters deep. There are also indications that such relatively deep and deep  lakes as Batur and Toba may have complete overturns annually, possibly during the hemispheric winter  in July-October. To verify these, there should be studies involving also climate and meteorological information.

2.4.3. Temperature variations

Füllerborn discovered the first tropical thermocline in Lake Nyassa (Malawi) (1900). As late as in  the thirties Welch (1935) presented that the tropical lakes do resemble in their temperature regimes  temperate lakes, and that the hypolimnion water has in most cases temperatures close to four centigrade.  Only in his third order lakes “the bottom water temperature is very similar to that of surface water, and  circulation is practically continuous throughout year”. However, Ruttner (1931) gave the present view  already in 1931. As we can see from our observations (Table 4, page 15), the greatest observed vertical  temperature difference was in a single natural lake less than 100 meters in depth 3.88 centigrade (Batur),  in a lake deeper than 100 meters 2.30 centigrade (Singkarak) and in a reservoir 4.35 centigrade (Cirata). The smallest differences were 0.10 (Tempe), 0.40 (Poso) and 0.32 centigrade (Jatiluhur), respectively.

Albeit the actual differences are small, the stability may be great due to the rather high  temperatures. This statement made in several textbooks needs some further verification. For example  in Lake Toba the temperature difference between the surface and the depth of 450 meters was in  September 1992 during the hemispheric winter only 0.85 centigrade, and the relative thermal resistance  for the entire water column was only 25. The lowest RTRs for the entire water columns were 3.5  (Tempe) and 8.9 (Batur). In Poso Lake, the RTR was for a water column of 400 meters only 13.3 and  in Lake Matano for 500 meters 34.3. The highest RTR was in Batur Lake, 126.9. In the reservoirs, the respective values were 13.7 in Jatiluhur and 156.2 in Selorejo.

If we compare the RTR values of the entire water columns with a theoretical temperate lake  during a summer stagnation (epilimnion temperature 18 and hypolimnion temperature 4 centigrade) the  RTR for the whole water column is 170. None of the studied lakes and reservoirs did reach this value  during any of the observation days in various seasons. The relationship of RTR and meromixis was not  clear. Therefore, the additional chemical stability of the lakes, where it may have some importance, is yet to be evaluated.

The fall in the temperatures within the regions referred to as the thermoclines fail to qualify as the  originally defined thermoclines under the Birge’s rule (Birge 1897), that temperature decline shall be one centigrade per meter (Welch 1935, see Talling 1995).

The highest observed temperature in the epilimnion of the natural lakes was 30.20 in lakes less  than 100 meters in depth (Sentani), 28.40 in lakes deeper than 100 meters (Singkarak) and in the  reservoirs 30.82 centigrade (Jatiluhur). The respective lowest epilimnetic temperatures were 21.42  (Buyan), 25.00 (Toba, northern basin) and 25.89 centigrade (Palasari). The highest near bottom  hypolimnetic temperature was in the lakes less than 100 meters in depth 28.80 (Sentani), in the lakes  deeper than 100 meters 26.80 (Matano) and in the reservoirs 29.21 centigrade (Jatiluhur), and the lowest temperatures 20.11 (Buyan), 24.03 (Toba, northern basin) and 22.62 centigrade (Selorejo).

The temperature of the water mass seems to be dependent on altitude (Ruttner 1931,  Lehmusluoto 1995 a, e). The temperatures of epilimnia were at sea level about 29-30 centigrade and at  the altitude of 1,500-1,600 meters about 21-22 centigrade, and the respective temperatures of the  hypolimnia were about 28-29 and 20-21 centigrade. The average temperature difference between epilimnion and hypolimnion was 1.5 centigrade.

As earlier noted, the natural lakes, with the exception of the few shallow lakes, which are  polymictic (depths not more than 40-50 meters), are oligo- or monomictic (or atelomictic), although the  vertical temperature differences are small, and many permanently chemically stratified, meromictic.  Generally, RTR was in the high-altitude lakes lower in August-September than in February-March.  The incomplete circulation is more obvious the deeper the lakes are and the less persistent the  wind or other weather conditions are. This can be well demonstrated by the earlier mentioned relative  depth, or by the ratio of maximum depth and area. It is also obvious that at the lower altitudes the  nocturnal cooling of the surface layers is not effective enough to cause partial or complete mixing, as  may happen at the higher altitudes during the cooler period of northeasterly monsoon.  Although the stratification is annually disturbed in the reservoirs because of the great changes in  the water level and flow, it obviously does not often break down. The hypolimnion may also remain  anoxic. However, the stratification may be very steep due to e.g. hydraulic stratification. The deepwater abstraction may also increase the temperature reserves.

2.4.4. Dissolved oxygen

Dissolved oxygen is in the Indonesian lakes one of the indicators of the effects of temperature and  mixing, but not necessarily of the trophic state. It also indicates the effects of lake morphology and wind  in the vertical distribution of oxygen. Unfortunately, due to the fact that the oxygen analyzers (Marvet  and WTW) were at times out of order in the demanding tropical conditions no oxygen measurements  could be performed in some of the lakes. However, this did not restrict the overall evaluation of the state of the lakes.

The primary oxyclines followed in many lakes the temperature stratification patterns. However,  the oxyclines may also be located at different depths, and secondary oxyclines could also be found.  Oxygen depletion is a common phenomenon in the hypolimnia of the Indonesian lakes. There are  several, usually large and deep lakes, which are permanently stratified and in which hypolimnia are  permanently oxygen depleted. Such lakes are for instance Ranau (depth at which anoxia begins 100  m/total depth 229 m), Singkarak (50 m/268 m), Buyan (40 m/88 m) and Tamblingan (29 m/90 m?). In  only two of the natural lakes hydrogen sulfide could be observed, in Lake Singkarak the detectable  hydrogen sulfide depth was 50 meters and in Lake Ranau 100 meters, and in Sentani there is a possibility for its occurrence.

In the reservoirs, dissolved oxygen was depleted in the hypolimnion of the main basins, except in  Darma and Wlingi reservoirs, which are rather shallow (9-14 meters deep). Only in the shallower parts,  closer to the inlets, oxygen could be found in the hypolimnia in e.g. Jatiluhur, Kedung Ombo, Lahor, Mrica, Selorejo at Konto inlet and Sutami reservoirs.

It has to be noted that the epilimnia were in most cases very thin and the stratification sharp. Thus,  the oxygen rich water volume was usually small in the reservoirs. During the high-water season  (August-November) the epilimnia may, however, be several meters deeper than during the low-water season.

Deoxygenation of hypolimnion may result in a few days or weeks, and hydrogen sulfide has been  formed in the hypolimnia of some of the reservoirs. The stratification patterns of the reservoirs seem to  resemble much each other, a shallow epilimnion and a very sharp thermo- and oxycline. Hydrogen  sulfide was present in the anoxic lower parts of the hypolimnia in Kedung Ombo, Palasari, Saguling and Sempor reservoirs.

In the Indonesian lakes monitoring of the depth, where the anoxic layer begins, gives a good idea  of the development of the state of the water body. Monitoring of the short-term changes in the oxygen  concentrations in the hypolimnia and hypolimnetic oxygen depletion rate are not good indicators,  because the annual stagnation and circulation periods may not be reliably frequent, as in the temperate regions.

In Batur and Bratan lakes in Bali it can be estimated, that a complete circulation may have  happened during the northeastern monsoon (probably in September), because the oxygen reserves have  been replenished. However, during the warm southwesterly monsoon the oxygen reserves near the bottom were almost entirely consumed.

During the German Sunda-Expedition the depth, at which 1 mg/l oxygen was present, was  determined in some of the lakes (see Ruttner 1931). It is a good starting point for the monitoring. The  other is the monitoring of the Secchi disc reading, if the waters are not turbid by inorganic matter. The  oxygen conditions of the lakes less than 100 meters deep, deeper than 100 meters and in the reservoirs are compared in Table 4 .

According to the zero oxygen depth level it could be seen, that only in Lake Singkarak the level  had markedly moved upwards. The anoxic water volume of the lake had thus increased. Simultaneously,  its transparency had decreased, as also in Lake Maninjau. In Lake Ranau the zero oxygen depth had  moved some meters downwards. It is important that the oxygen sensor operates in this kind of situations  also after having been in contact with hydrogen sulfide rich water, as does the Marvet AJ90 RS probe.  The increasing and expanding mats of floating vegetation, which already cover large areas of  many of the lakes and reservoirs, may also be a problem by hindering oxygen transport from air into water, and by their nocturnal use of oxygen.

The vertical movements of water layers caused by heavy rains or strong winds (e.g. seiches), of  which the Expedition Indodanau in some of the reservoirs observed indications, may cause especially in  the fjord like bays upwelling of the anoxic and hydrogen sulfide rich waters. When the major cage  cultures of fish are situated in the bays, in which water exchange is restricted and in which epilimnion is  usually shallow, upwelling may cause great economic losses in the form of fish kills. The fish cages have  been generally moved to the bays, because it has happened that during the rainy season harmful chemicals have caused fish kills when transported into the reservoirs in the main stream of water.

2.4.5. Supplementary abiotic environments

Carbon dioxide values were low, and did not generally exceed 10 mg/l in the hypolimnia. Only in  Maninjau and Singkarak lakes, the concentrations were 18.0 and 14.0 mg/l. In the shallow floodplain  lakes and semi-natural lakes receiving great amounts of allochthonous matter from the catchment, such  as Tempe and Rawa Pening, the concentrations exceeded 30 mg/l. Clear vertical gradients were in  Maninjau, Ranau, Rawa Pening and Singkarak lakes, but they were not confined to the deeper lakes. In  the reservoirs, the concentrations exceeded 10 but not 20 mg/l in Kedung Ombo, Saguling, Sempor,  Sutami and Wlingi, and only in Selorejo it was higher, 21.8 mg/l. Vertical gradients could be found in all the reservoirs, but releases of great amounts of carbon dioxide are unlikely.

The carbon dioxide regimes seem to be rather conventional in the lakes and reservoirs, being  reciprocal to the oxygen and pH values. Concentrations were usually low also in deep and meromictic  lakes. The high concentrations of 30-40 mg/l in Lake Toba referred to by Hehanussa (1994), were  observed by Ruttner (1931) only in the small and isolated Pangururan basin, and not in the entire lake. In  the main north and south basins, the concentrations did not exceed 10 mg/l (see also Ruttner 1931). The Nyos-type catastrophe caused by “explosive” gas outburst is unlikely in the studied major natural lakes.

The majority of the lakes and reservoirs were alkaline. Alkalinity values did range from 0.19 meq/l  (Bratan) to 3.70 meq/l (Batur) in the natural lakes, but generally, it was some 1 meq/l. In Buyan Lake, it  was about 2.40 meq/l, in Ranau 2.10 and in Singkarak about 2 meq/l. Only in Matano Lake, there was  clear difference between epilimnion (1.30 meq/l) and hypolimnion (2.60 meq/l). In the reservoirs, the  values were from 1.00 (Saguling) to 3.20 meq/l (Sutami). The lakes and reservoirs do thus have a rather reasonable acid neutralization capacity, which also affects the carbon dioxide regime.

In the natural lakes pH of the epilimnia were at or above neutral, from 6.8 (Kerinci) to 8.8  (Batur), and in the hypolimnia of lakes less than 100 meters in depth 8.9 (Limboto) to 6.8 (Diatas), and  in the lakes deeper than 100 meters from 6.1 (Maninjau) to 7.6 (Ranau). The usually high pH values  affect also the carbon dioxide equilibria. In some of the reservoirs, the daytime pH was quite high.  Especially in Darma, Lahor and Selorejo reservoirs, in which it was about 9 probably due to rather high daytime algal production.

Electrical conductivity was usually at the range of 80-300 µS/cm in the natural lakes. However, in  the two Balinese lakes, Lake Batur and Bratan, the two extremes were observed, 1,747 and 22 µS/cm,  respectively. In many of the lakes, which may be permanently stratified or meromictic, such as Buyan,  Maninjau, Matano, Poso, Tamblingan and Tondano there was a gradient between surface and bottom.  In the reservoirs, the general conductivity was some 200 µS/cm. No great deviations were found.  Distinct vertical gradients were in Darma, Kedung Ombo, Mrica and Sutami indicating either that they  may not mix to the bottom or the inflowing water may be chemically denser and is layering close to the bottom.

In the lakes, the concentrations of dissolved solids were usually from 90 to 130 mg/l, Batur (about  1,540 mg/l) and Bratan (about 20 mg/l) being the great exceptions. Vertical gradients were found only  in Poso, Tondano and Towuti. In the reservoirs the dissolved solids concentrations were roughly 100-  130 mg/l. Vertical gradients were found in Darma, Mrica and Saguling reservoirs. Concentrations of  suspended solids were generally less than 10 mg/l. However, the concentrations were in Limboto 36.0-  40.0 mg/l, in Poso 16.0-30.0 mg/l, Tempe about 20 mg/l, Tondano 6.0-176.0 mg/l and in Towuti 10.0- 18.0 mg/l. In Matano, the concentrations were from 15.0 to 58.0 mg/l, but the lowest concentration was  at the surface, the highest at the depth of 200 meters decreasing to 16.0 mg/l at the depth of 500 meters.  In the reservoirs the suspended solids concentrations were generally in the epilimnia less than 10 mg/l  and in the hypolimnia less than 30 mg/l. The greatest gradient was in Palasari, from 3 to 210 mg/l, because there had been no water abstraction for a long time due to shortage of water.

Chemical oxygen demand was in all the other lakes less than 10 mg/l, except in Limboto in which  it was 17-18 mg/l. No great vertical differences were found. In the reservoirs the chemical oxygen  demand was also generally less than 10 mg/l, the only exceptions being the hypolimnia of Palasari, Saguling and Selorejo. In Palasari, the value was 36.0 mg/l.

The lakes were usually not turbid, the turbidity values being less than 10 NTU, with the  exceptions of Limboto (22 NTU), the depths between 100-300 meters in Matano, and hypolimnia of  Poso and Tempe lakes. In the reservoirs, turbidity was also generally less than 10 NTU, but in the  hypolimnia of Lahor, Mrica, Palasari, Saguling, Selorejo and Sutami it exceeded 10 NTU. The greatest vertical gradient was in Palasari, from 1.6 to 210 NTU.

The transparency observations of the Expedition Indodanau were in the lakes from 0.4 m in  Limboto to 20.0 m in Towuti, and in the reservoirs from 0.5 m in Saguling to 5.0 m in Kedung Ombo.  In addition to Lake Towuti, transparency was more than 10 meters in Toba and Matano lakes. For  various correlations, see Figure 5 (page 28). The German Sunda-Expedition made also Secchi disc  observations. Our observations showed, that the Secchi disc readings varied in a single water body from  some tens of centimeters to about five meters (Lake Toba, Sumatra). Only in Lake Maninjau and  Singkarak in Sumatra, the Secchi disc depth has markedly decreased during the past 65 years. From Lake Toba, there were no earlier values available.

The oxydation-reduction potential was in the natural lakes from -300 at the bottom of Lake  Ranau to 142 at the surface in Lake Buyan. Negative ORP was found in the hypolimnion of Tamblingan  Lake, and negative values in the entire water columns were found in Kerinci, Limboto, Maninjau,  Matano, Poso, Ranau, Sentani and Singkarak lakes, and in Buyan lake ORP were during different  observation days either negative or positive. In the few reservoirs, in which ORP could be measured  before the sinking of an outrigger and drowning of the instrument, it was positive in Lahor, Selorejo  (except at Kwayangan inlet) and Wlingi. In Kedung Ombo, it was positive in the epilimnion and negative in the hypolimnion, as in Sutami.

Because of the volcanic surroundings, the sulfate concentrations of the natural lakes could have  been expected to be rather high. Sulfates are constantly introduced into the lakes through minor  eruptions and from hot springs. In the natural lakes, the concentrations were usually from 1.0 to 3.0  mg/l. The highest concentrations were in Batur (650.0-670.0 mg/l) and lowest in Bratan and Poso  (0.35-0.50 and 0.30-0.50 mg/l). In the reservoirs, the sulfate values varied from 1.2 (Darma) to 36.0  mg/l (Kedung Ombo). They were generally less than 10 mg/l, but in Sutami and Wlingi they were some  15.0-19.0 mg/l in the whole water column. The hydrogen sulfide concentrations of the anoxic  hypolimnia in some of the lakes (Ranau, Sentani and Singkarak) may be as high as 1.5 mg/l. In the reservoirs, hydrogen sulfide was traced in the hypolimnia of Palasari, Saguling, Selorejo and Sempor.

Calcium concentrations were in the lakes from 1.9 to 35 mg/l. The lowest concentrations were in  Bratan and the highest concentrations in Batur and Ranau lakes. Generally, there were no gradients,  except that in Matano Lake, in the epilimnion 15.0-16.0 mg/l and in the hypolimnion from 200 meters  on 23.0-25.0 mg/l. In the reservoirs, the calcium concentrations were from 8.1 (Darma) to 36.0 mg/l  (Kedung Ombo and Sutami), and no vertical differences were found. In the lakes, the chloride  concentrations were from 1.5 to 225 mg/l. The low values were in Diatas and Bratan lakes, and the high  concentrations in Lake Batur. Chloride concentrations varied in the reservoirs from 4.0 (Darma) to 27.0  mg/l (Selorejo). There were no great vertical gradients in the reservoirs. Potassium ranged from 0.45  (Bratan) to 22 mg/l (Batur), but usually it was 0.5-3.0 mg/l. There were no distinct vertical differences  except in Lake Matano, in which the epilimnetic concentration was about 1.0 mg/l and hypolimnetic  concentration 0.1-0.2 mg/l. In the reservoirs, the concentrations were from 1.1 (Sempor) to 4.1 mg/l (Saguling).

Sodium levels did range from 1.3 (Diatas and Bratan) to 350 mg/l (Batur), but usually it was  about 1.5-18.0 mg/l. In Lake Matano the epilimnetic concentrations were 3.0-3.2 mg/l and hypolimnetic  concentrations 1.0-1.1 mg/l, but otherwise vertical gradients were not found. In the reservoirs, the  values were from 2.3 (Sempor) to 44.0 mg/l (Cirata). Only in Cirata, there was an observable vertical  gradient in February 1992, from 10.0 to 44.0 mg/l. In Saguling, the concentrations were also higher,  from 11.0 to 32.0 mg/l. Magnesium concentrations in the lakes were from 0.85 (Bratan) to 62 mg/l  (Batur). Generally, the concentrations were from 4.0 to 10.0 mg/l. In Matano Lake, there was a clear  gradient, in the epilimnion 7.3-7.5 mg/l and in the hypolimnion 18.0-21.0 mg/l. In the reservoirs, the concentrations were from 2.2 (Darma) to 13.0 (Sutami) and 14.0 mg/l (Wlingi).

Iron concentrations were moderate, and did not usually increase much even in anoxic  hypolimnetic conditions. In the lakes, the iron concentrations were usually less than 0.5 mg/l, but in  Maninjau, Poso and Rawa Pening they were elevated even up to 2.50 mg/l. In Matano Lake, the  epilimnion concentrations were from 0.12 to 0.32 mg/l, and from 200 to 500 meters 4.44-5.10 mg/l. In  the reservoirs, the concentrations were also generally less than 0.5 mg/l, but in Cirata, Saguling and  Sempor reservoirs the near bottom concentrations were slightly elevated, highest being 1.40 mg/l. The  manganese concentrations were in the lakes from undetected to 0.14 mg/l. In Matano Lake, it was  undetectable in the epilimnion, and in hypolimnion, the concentrations were from 0.22 to 0.24 mg/l in  the water layer of 200-500 meters. In the reservoirs, manganese was undetectable in the epilimnia of Darma and parts of Saguling reservoir, and usually the concentrations were 0.02-0.15 mg/l.

Heavy metals (cadmium, chromium, copper, nickel and lead) were not detected in the lakes and  reservoirs, but zinc concentrations varied in the natural lakes from undetectable to 0.34 mg/l in Lake  Batur, and in the reservoirs from undetectable to 0.25 mg/l in Saguling and 0.30 mg/l in Cirata. Results of agro-chemicals are not yet available.


In the natural lakes, ammonia (NH4+NH3-N) concentrations were very variable. Usually the  concentrations were lower in the epilimnion, from about 0.025 to 0.300 mg/l N, increasing towards the  bottom, but e.g. in some occasions in Bratan and Diatas it was considerably higher at the surface, even  1.117 mg/l in Bratan, and undetectable in Toba. The highest hypolimnion concentration was 1.450 mg/l  in Matano Lake. There was a clear gradient between 50 and 200 meters. In the reservoirs, the values  were often undetectable both in the epilimnia and hypolimnia. The highest epilimnion value of 0.100 and the highest hypolimnion value of 0.420 mg/l were both found in Saguling reservoir.

The nitrite concentrations were usually undetectable in the epilimnia of lakes, and in the cases it  could be detected it was 0.018 mg/l N in Diatas Lake. The hypolimnia concentrations were also  generally rather low, the highest being 0.037 mg/l in Buyan Lake. In Towuti Lake, it was entirely  undetectable. In the reservoirs, nitrite was generally undetectable, especially in the epilimnia. The highest  concentration found, 0.043 mg/l, was in the hypolimnion of Saguling reservoir. The nitrate  measurements showed that the concentrations were as a whole low. The epilimnetic values were from  undetectable (Toba and Poso) to 0.270 mg/l N in Diatas. Concentrations increased towards the bottom,  being highest in Ranau (0.760 mg/l) and Singkarak (0.590 mg/l). In Matano Lake, it was undetectable at  the surface and at the bottom, but between the depths of 20-200 meters, the concentrations varied from  0.050 to 0.330 mg/l. In Towuti nitrate was not detected in the basin visited by the Expedition Indodanau  team. In Darma reservoir nitrate was not found, and at times not in the epilimnia of Cirata and Saguling  reservoirs. The highest concentrations were in Sutami and Wlingi (1.200-1.500 mg/l), and in Selorejo  (0.880 mg/l). In some of the reservoirs, the concentrations did increase somewhat towards the bottom.  Organic nitrogen was at its lowest 0.020 mg/l N (Diatas and Maninjau) and at its highest 0.330 mg/l in Ranau lake. Exceptionally high value of 1.042 mg/l was observed in Lake Toba in March 1992.

No notable vertical differences were observed, but it is noteworthy that organic nitrogen was  undetectable at times in the hypolimnia of the Diatas, Dibawah and Singkarak lakes, indicating that the  decomposition processes may be rapid in the epilimnia of the lakes. In the reservoirs, the organic  nitrogen concentrations were generally from 0.010 to 0.400 mg/l. There were some increases near the bottom of Cirata and Saguling reservoirs, 0.760 and 1.180 mg/l, respectively.

The lowest epilimnetic total nitrogen values were at the range of 0.180-0.380 mg/l N in Dibawah,  Maninjau, Ranau, Sentani, Toba and Towuti lakes, and the highest 1.310 mg/l in Bratan. Generally, the  values did not exceed 1 mg/l, except the exceptionally high value in Lake Toba in March 1992 because  of high organic nitrogen concentration. In Matano Lake, the epilimnetic concentrations were less than  0.620 mg/l, but from 300 to 500 meters they were some 1.740 mg/l. In Ranau and Singkarak lakes, a  hypolimnetic increase could be observed, from 0.158 to 0.893 and 0.381 to 1.199 mg/l, respectively. In  the reservoirs, generally, the total nitrogen concentrations were below 1 mg/l. In the hypolimnia of some  the reservoirs the values were at times higher, in Cirata 1.220 mg/l, Lahor 1.080-1.300 mg/l, Mrica  1.100 mg/l, Palasari 1.720 mg/l, Saguling 1.780 mg/l and Selorejo 1.020 mg/l. In Sutami and Wlingi the entire water column had a concentration of 1.500-1.600 mg/l

The phosphate phosphorus values were in the epilimnia of the lakes often undetectable, and the  highest value was 0.080 mg/l P in Ranau lake, in which phosphorus was mostly in dissolved form (see  below). Vertical differences were not notable even if the hypolimnia were anoxic; the highest  hypolimnetic value was 0.256 mg/l (Ranau). Phosphate phosphorus was not found in Tamblingan,  Tondano and Towuti. The phosphate phosphorus was in the reservoirs in the majority of cases  undetectable. The highest concentration found was in the hypolimnion of Saguling reservoir, 0.015 mg/l.  Similarly; total phosphorus values were low and often undetected (Tamblingan, Tondano and Towuti).  The highest epilimnetic values were in Diatas (0.050 mg/l P), in Dibawah (0.070 mg/l), in Ranau (0.085- 0.056 mg/l), and in Tempe (0.050 mg/l). No notable increases in total phosphorus concentrations were  observed in the hypolimnia. The total phosphorus concentrations in the reservoirs were also very low. The highest concentration, 0.018 mg/l, was found in the hypolimnion of Saguling reservoir.

As already noted in the previous chapters, the total nitrogen and nitrate nitrogen concentrations  were both in the natural lakes and in the reservoirs low. This could have been expected for the natural  lakes, because their drainage areas are small and most of their new inputs are from rain. But the  relatively low values in the reservoirs need more clarification, because the drainage areas are large, and  there are various activities (agriculture, population, fisheries, industry, etc.) loading the waters. In  addition, the nitrogen circulation needs to be studied. Wetlands and rice paddies may, however, play  certain roles and act as good nitrogen traps. Gradients of nitrates have been used as an indication of mixing depth (see Goldman & Jassby 1990).

For phosphorus applies the same as for nitrogen, both the total phosphorus and phosphate  phosphorus values were very low. This could be expected for the Indonesian lakes, because the soil is  low in phosphorus. The correlations of transparency, electrical conductivity, total nitrogen, total  phosphorus versus chlorophyll a, and transparency versus total nitrogen and total phosphorus in the natural lakes and reservoirs are in Figure 5.

Figure 5. Correlations of transparency, electrical conductivity, total nitrogen, total phosphorus versus chlorophyll a, and transparency versus total nitrogen and total phosphorus.

The N:P-ratio for total nitrogen and total phosphorus in the natural lakes showed in many cases  rather normal relations, although the range is from 1.9 to 485. For the reservoirs, the ratio was generally  high, and the range was wide, from 3.3 to 665 (see Figure 6). The low phosphorus concentrations made  the ratio high (see Bomchul 1995). The N:P-ratio for nitrate nitrogen and phosphate phosphorus was  distorted. The nutrient regimes need necessarily more attention and studies. Total nitrogen and  phosphorus are good guides for nutrient status evaluations (see Payne 1986).

Figure 6. N:P-ratio of total nitrogen and total phosphorus in epilimnia and hypolimnia of the natural lakes and of the reservoirs.

Silicate levels were generally from about 10 mg/l to 40 mg/l in lakes. The lowest concentrations  were in Bratan lake, 3.6 mg/l, and highest in Buyan, 68 mg/l. No vertical differences were observed. From the reservoirs, silicate measurements were not made.

The nutrient regimes in the Indonesian lakes and reservoirs may differ from the temperate lakes. In  the natural lakes the nutrient concentrations are usually low, and the annual productivity is mainly  dependent on the new inputs by rains, rivers and drainage, rains being the major sources for nitrogen and  drainage areas for phosphorus. In addition, the “ventilation” of hypolimnetic waters by turbulence and  deep mixing may be a major source of nutrients. The normal sediment-water -interface may be important only in the very shallow natural lakes and in some of the reservoirs.

The lakes in Indonesia are, like many other inland waters in the tropical region, quite nutrient poor  (see Ruttner 1931, Anton 1994, Booth et al. 1994). Consequently, eutrophication is not a very common  feature. The wetlands and rice paddies surrounding the lakes and reservoirs may have their implications  in reducing the nutrient load entering the water bodies. Phosphorus accumulated in the hypolimnia  during stagnation is a measure of the release of phosphorus by the sediments in anaerobic conditions  (Gächter & Wüest 1993). Only when oxygen penetrates into the sediment, not that deep water is  oxygenated, and can an oxic layer develop in which oxidized iron and manganese, minerals become  enriched, and gives rise to phosphorus adsorption capacity (Wehrli et al. 1993). However, rapid  mineralization in the Indonesian lakes prevents from the accumulation of organic nutrient loaded sediments making eutrophication less irreversible than in temperate lakes (see also Graneli 1987).

The anticipated low production of the deep natural lakes, similar to the arctic lakes, may allow  pronounced increases in productivity when minute quantities of nutrients are increased. It applies also to  small amounts of deleterious substances, which could possibly produce equally pronounced reductions  in the rates of production since the total biological matter present is so small that there is little capacity for the system to absorb such material and render it harmless.

The increasing floating vegetation, such as water hyacinth (Eichhornia crassipes), water fern  (Salvinia molesta) and water cabbage (Pistia stratiotes), which already cover large water areas in the  lakes and reservoirs, and the submerged plant Hydrilla verticillata, may also create problems. In  addition to the mechanical obstructions, the vegetation hinders oxygen transport from air into water, and  the plants utilize large amounts of oxygen during nighttime. Due to the evapotranspiration, especially of  the water hyacinth, water is lost 3.5 times more than through natural evaporation. It causes unnecessary  loss of water. The plants are increasing rapidly. The biomass may be doubled in a week or two, and the  means for their removal are scarce. Eradication would need incentives and income generating methods.


The detailed data of chlorophyll a, algal biomass and phytoplankton composition are in the separate data files, and in the data bank of RIWRD.

Chlorophyll a measurements in the natural lakes showed rather low values, from 0.15 in Matano  lake in Sulawesi to 7.33 mg/m3  in Bratan Lake. For Lake Kerinci a value of 17.17 mg/m3  was measured  at the outlet, but this may be due to sampling error. In addition, in the reservoirs chlorophyll a  measurements showed similarly low values, from 0.79 mg/m3  in Jatiluhur reservoir to 2.69 mg/m3  in the  Saguling reservoir, although they were generally somewhat higher than in the lakes (see Table 3, page 13).

Phytoplankton of the natural lakes and of the reservoirs was identified and enumerated from  samples taken from the epilimnion water as water samples, and in some occasions with 10 micrometer  net by pouring 30 liters of water through it. The cell densities were low. According to the chlorophyll a  results all the lakes were oligotrophic in the temperate lake scale, but according to the phosphorus  concentrations at least some of the studied lakes could be more productive, nitrogen allowing. However,  the vertical distribution of phytoplankton is obviously uneven and the higher biomasses could be below the sampling depth. This is caused by the supraoptimal high solar radiation in the surface layers.

The samples were investigated either by using the inverted light microscope or by interference  contrast optics in light microscope but due to low densities, preservation and changes during the storage  many taxa were not possible to identify to the species level. Further, the keys made for temperate lakes  are not necessarily valid for the Indonesian waters. The results are shown in Figures 7 and 8, and the detailed results are in the separate data volume.

Algal biomasses varied in the lakes from 0.002 mg/l in Matano and 0.016 mg/l in Towuti to 4.7  mg/l in Limboto and 4.4 mg/l in Bratan Lake. In the reservoirs, the biomasses were from 0.052 mg/l in  Jatiluhur and 0.061 mg/l in Wlingi to 0.565 mg/l in Darma (see Figure 7). Contrary to the chlorophyll values the biomasses were generally smaller than in the lakes.

Figure 7. Number of algal taxa and biomass in the studied natural lakes and reservoirs in Indonesia. In some cases, also the ranges of numbers are shown as the result of enumeration of taxa from various dates.

Although the numbers of identified taxa of the lakes are at this stage not statistically comparable,  they give an estimate of the differences in species diversity between lakes (see Lewis 1995). The  phytoplankton of Lake Toba was dominated by one diatom, Denticula tenuis. It is known to prefer  alkaline waters and waters with low concentrations of organic matter (oligosaprob). Besides the  mentioned diatom the lake had rather diverse phytoplankton community with many species of coccal  green algae (50 % of taxa), some desmids, tribophytes, other diatoms, cryptophytes, dinoflagellates and  even some chrysophytes, such as Dinobryon divergens, Kephyrion sp. and obviously Uroglena sp. cells.  Additionally, this last mentioned algal group was only found in Maninjau and Rawa Pening lakes and in  Kedung Ombo reservoir. In September 1992 the phytoplankton was dominated by green algae, in the  northern basin by Monoraphidium and Oocystis, and in the southern basin by Monoraphidium and Lagerheimia.

In Lake Maninjau in early 1992 about 50 % of the observed taxa were coccal green algae  representing several common genera (Oocystis, Lagerheimia, Treubaria and Scenedesmus) without any  strong dominant genus, but Oocystis was strongly dominating during the hemispheric winter in August  1993. Some taxa would need more detailed investigations to ensure if they belong to green algae or to Tribophyceae.

One of the poorest communities was observed in Lake Diatas. Phytoplankton was strongly  dominated in March 1992 by colonies of small-celled blue-green alga, similar with Cyanodictyon  imperfectum. The others were small coccal green algae, mostly Oocystis spp., and one diatom species,  but all of them were represented by low densities only. However, Oocystis sp. dominated in August 1993, and the diatom Aulacoseira granulata was abundant.

A relatively low number of taxa were also found from Lake Dibawah. The phytoplankton  community of this lake was dominated by coccal green algae in early 1992. The most abundant taxa was  species with cell size 3.5–4.5 micrometer and which mother cell wall divided into two halves and  remained in the sample after autospore release like in the genus Coenochloris. The cells were not,  however, in colony mucilage but free. The other taxa were less abundant but typical algae in all the  lakes. In August 1993, the dominant genus was conjugatophyte Spirogyra sp., and chlorophytes Didymocystis bicellularis and Oocystis cf. solitaria were rather abundant.

A high number of taxa were recorded in March 1992 from Lake Singkarak, which is downstream  Lake Dibawah. However, the chlorophyll a concentration in this lake was one of the lowest from the  lakes in these phytoplankton community studies. The main groups were also in this lake Chlorococcales  and desmids from the group Conjugatophyceae. One of the coccal green algae species occurred as  single cells but those cells were similar with Coelastrum cells with blunt appendages in three corners. In  August 1993, the majoring algae were conjugatophytes (Spondylosium planum and Cosmarium  punctulatum), dinophytes (Peridinium umbonatum) and chlorophytes (Tetraedron minimum). This lake  was one of the few where also filamentous blue green algae were observed. Their proper taxonomical identification would need more fresh and rich samples.

In Lake Kerinci the algal composition was rather diverse in August 1993. The dominant species  was raphidophyte Gonyostomum semen, and other well-represented algae were dinophyte Peridinium  cf. gutwinskii, conjugatophyte Spondylosium secedens, and chlorophytes Monoraphidium contortum and Crucigenia tetrapedia.

In Lake Ranau, which locates in the southwestern end of Sumatra, total phosphorus  concentrations were among the highest in the lakes, but the total nitrogen was relatively low. The N:P- ratio of 1.8 in the epilimnion means that the phytoplankton growth was strongly limited by nitrogen.  This is, however, likely an exception among the Indonesian lakes. The most abundant taxa were in  March 1992 Chodatella spp. (C.ciliata, C.citriformis) and filamentous Planktonema lauterbornii. In  August 1993, the dominant algae were chlorophyte Botryococcus braunii and diatom Aulacoseira granulata.

In Rawa Pening lake in Central Java the strongly dominant alga was dinophyte Peridinium umbonatum, but also euglenophytes were rather well represented in August 1992.

In the Bratan Lake in Bali, the highly dominant alga in September 1992 was conjugatophyte  Staurastrum cf. tetracerum, 97 % of the biomass. Additionally some dinophytes were found. In Buyan  lake the conjugatophytes Cosmarium bioculatum and Staurastrum chaetoceros were outstandingly  represented, 74 % of the biomass. The phytoplankton composition was in Lake Batur more diverse,  dominant alga being diatom Synedra acus v. angustissima. In addition, cyanophytes, dinophytes,  chlorophytes and conjugatophytes were well and rather evenly represented. In Lake Tamblingan, the  composition comprised rather evenly of dinophytes, chlorophytes and conjugatophytes, the dominant species being diatom Synedra acus v. angustissima.

In Lake Tondano in Sulawesi, the phytoplankton composition was dominated in August 1993 by  cyanophytes and diatoms, and the dominant species was the diatom Aulacoseira granulata, but  chlorophytes were also well represented in the lake. The composition was strongly dominated by  chlorophytes (98 % of the biomass) in the very shallow Limboto lake, the dominant species being  Oocystis sp. Lake Poso was strongly dominated by diatoms (Aulacoseira sp.). In the shallow floodplain  Lake Tempe, the phytoplankton composition was diverse, without any distinct dominant alga. In Lake  Matano in Central Sulawesi, only seven algal species were found, lowest number in the study,  chlorophyte Staurastrum furcigerum dominating, but also the dinophyte Peridinium sp. was prominent.  In the downstream Lake Towuti the number of species was second lowest, only nine. The strongly dominating species was the diatom Peridinium cf. baliense.

The only lake in Irian Jaya included in the study was Sentani. It was dominated by diatomophyte  Aulacoseira granulata, which comprised of 47 % of the biomass. In addition, cyanophytes and chlorophytes were found.

The species richness of phytoplankton in the studied reservoirs was rather similar with that in the  studied lakes (see Figure 8). However, the relative contributions of different algal groups in the  phytoplankton communities were more equal. Biomasses were according to densities and according to  the chlorophyll a results somewhat higher than in the lakes. This was surprising when compared to the  nutrient concentrations (N and P), which were very low. Filamentous blue green algae and several large  dinoflagellates were also characteristic. Coccal green algae and desmids were mainly the same taxa as in the lakes.

Figure 8. Relative contribution of different algal groups to the total biomass in the phytoplankton communities. For Lake Kerinci (7), the bargraph symbol for Chrysophycaea represents Raphidophyceae.

Saguling, Cirata and Jatiluhur in West Java form a chain of reservoirs, but only Cirata and  Jatiluhur were sampled in March 1992, and Saguling and Jatiluhur in August 1993. In the upstream  Saguling, the dominant algae were cyanophytes and cryptophytes, and the strongly dominant species  was Cryptomonas sp. (48 % of biomass). In the deeper layers of Cirata Reservoir, oxygen was depleted  causing increase in iron concentrations, which facilitate the abundant occurrence of iron bacterium  Planktomyces bekefii. In Jatiluhur, the dominant algae were dinophytes (Peridinium spp.), conjugatophytes (Spondylosium spp. and Staurastrum spp.) and chlorophytes (Tetraedron minimum).

In the Darma reservoir the algae were outstandingly dominated in August 1992 by  conjugatophytes. The dominant species was Staurodesmus mucronatus, but also other species were  represented. The strongly dominant species was in Sempor the conjugatophyte Cosmarium  sphagnicolum. Other algae were practically non-existent. In Mrica, the dominant algae were Peridinium  spp., otherwise the species composition was sparse. The Kedung Ombo reservoir was dominated by  conjugatophytes, mainly by Staurastrum spp., but also Cosmarium spp. were found. In addition, dinophytes and chlorophytes were represented.

Lahor, Sutami, Wlingi and Selorejo reservoirs are situated in the Brantas River system. In the  upstream Lahor reservoir dinophytes did dominated in August 1992, mostly Peridinium spp., but also  chlorophytes (Oocystis spp.) were strongly represented. In Sutami, the highly dominant algae were  diatomophytes, and the dominant species was Aulacoseira granulata. In addition, chlorophytes were  present, such as Tetraedron minimum and Oocystis spp. Wlingi was dominated by diatomophytes  (Aulacoseira granulata, Synedra acus v. angustissima) and chlorophytes (Tetraedron minimum). The  Selorejo reservoir was dominated by conjugatophytes Cosmarium punctulatum and Spondylosium pygmaeum, but also diatoms were found in some degree (Achnanthes sp.).

The only reservoir in Bali, Palasari, was strongly dominated by dinophyte Peridiniopsis cunningtonii (87 % of the biomass).

Although the biological components of an ecosystem can be characterized by simplistic  descriptions (e.g. taxonomic lists, diversity indices, etc.), the species composition of many Indonesian  lake communities is often poorly documented, diversity is unknown, and may well include previously  undescribed species. Giving organisms the correct names, therefore, is important. The diversity of the  aquatic communities in Indonesia should be carefully studied, because e.g. introduction of exotic fish species may affect the diversity in the long run.

It is quite evident that the majority of the studied natural lakes and reservoirs were oligotrophic,  at most mesotrophic. The fact is also indicated by the rather low chlorophyll a concentrations ranging  from 0.15 in Matano to 7.33 mg/m3  in Bratan. From the Sengara Agung River at the outlet of Lake  Kerinci the chlorophyll a concentration was 17.17 mg/m3. This high value may be erroneous due to  sampling. The reservoirs showed, generally, to have somewhat higher concentrations of chlorophyll a,  from 0.46 in Mrica to 6.08 mg/m3  in Selorejo, although the range is about the same. For correlations, see Figure 5.

Results of phytoplankton communities gave some general view over the communities in these  little known lakes (see Figure 8). The richness of coccal green taxa was connected to the neutral and  alkaline waters with moderate conductivity. The rather low abundance of blue green algae and lack of  euglenophytes indicated oligotrophy, which was expected also from very low nutrient concentrations  and low chlorophyll a concentrations. In the clear lakes, the vertical distribution of the phytoplankton  biomass may be rather uneven and the community structure in the surface may differ much from that in  the deeper layers. The phytoplankton sampling should reach from the surface to deeper layers (e.g. from  the surface to the depth of the Secchi disc transparency). For the taxonomical investigations, more rich  samples should be collected using varying preservations (Lugol’s solution and formaldehyde or  glutaraldehyde). The sample series available for this report were from six Sumatran lakes and two  Javanese reservoirs. For more comprehensive evaluation, identification and enumeration of  phytoplankton of samples from other areas and islands with a wider range of water quality should be  accomplished.

To develop useful information on the indicator value of different taxa in the Indonesian lakes a  computer file should be collected with the names, illustrations (good quality drawings, digitized photos  or video pictures) and similar ecological background information. With this type of file different  phytoplankton workers could collect more uniform data and the evaluation of lake quality becomes much more precise.

When compared with the phytoplankton results to those published by Ruttner (1952) and Green  et al. (1995) there were many similarities among dominants and subdominants, but also much  dissimilarity. In the Green’s publication only genus names were given, which makes the view of  comparison very general. It is obvious that the water quality varies greatly between and within the  islands. Thus, the samples from different types of lakes/reservoirs and from different seasons should be  analyzed using similar methods to develop phytoplankton studies a useful tool as part of competitive water quality monitoring.


Stratification of the Indonesian lakes is a complex issue (see also page 15). Diurnal patterns of  water temperature and density may be more significant than seasonal patterns (Tundisi 1984). This may,  however, be not applicable only to certain e.g. monomictic lakes which may circulate only during the  hemispheric winter in Indonesia, in July-September. Thus, e.g. daytime superficial warming and  nocturnal cooling may play important roles, together with occasional unusual weather changes, such as  cool torrential rains. The mixing depth of the epilimnion is a function of mean wind velocity and  frequency, and fetch (Tilzer & Bossard 1992). The maximum depth of mixing in (meromictic) lakes is  proportional to the fourth root of the surface area (Berger 1955). Thus, in the large lakes physical  forcing is becoming more important, and thus the mixing depth is greater in large than in small lakes.  Especially wind may accelerate mixing to greater depths. The smaller Indonesian natural lakes are, in  general, strongly sheltered from winds. Ruttner (1931) has also shown, that the greater the lake area, the  deeper the thermocline is located and the weaker the stability is in the water layer of 0-20 meters. He  indicates that in lakes with an area of 1-2 km 2 the depth of the thermocline is 4-8 meters, with an area of  about 100 km2  the depth is 12-15 meters and with an area of 1,000 km2 the thermocline would be at the

depth of 20-25 meters (area ratios 1:100:1000 correspond to thermocline depth ratios 1:3:6), but e.g.  the effects of altitude and rim height were not considered. In Lake Toba Ruttner (1931) did observe the  thermocline depth of 30 meters, which according to our measurements was in the northern basin at a  depth of 65 meters and in the southern basin at 160 meters, although it was not very sharp due to small  temperature differences, as also Ruttner observed. As from Table 5 (page 37) can be seen the ratios  given by Ruttner are not that straight forward, since the geographic formations around the lakes,  (especially the rim height), morphometry of the lakes and geology of the surrounding area may also affect the stratification patterns.

However, Ruttner (1931) also stated that stability and mixing are not regular, and the age of the  stability cannot be accurately defined. In some cases, the thermocline is at the very surface, and there is  no epilimnion. This kind of stratification may be temporary during calm weather. There may also be  diminutive stratification in very shallow lakes, caused by cool water and suspended matter. In the lakes  and reservoirs receiving large inflow of cool, heavy water and large outflow there may be partial or  complete absence of stratification (see Welch 1935). In addition, isolated basins in a lake may act as  separate entities. The increased abstraction of hypolimnetic water will increase the temperature reserves  of reservoirs. It was found that the vertical stability was usually lower in the high-altitude lakes during the hemispheric winter in August-September.

Amictic lakes are improbable at sea level in the equatorial regions and below 6,000 meter elevations (Hutchinson & Löffler 1956).

Oligomictic lakes are generally stable low-altitude lakes with a very slow or rare mixing. Lewis  (1973) has given a new name to this pattern, atelomixis. The lakes are characterized by irregular,  incomplete mixing caused by effective convection because of surface cooling, either nocturnally or by  cool rains. As the surface temperature decreases, water density increases sharply in these higher  temperatures, and vertical convection is cooling the lower layers. Together with the daytime warming  “sawtooth” stratification may form (see Järnefelt 1958, Livingstone 1995). Circulation periods tend to  be unusual, irregular and short-term in duration, and mixing may be incomplete (Hutchinson & Löffler  1956). Lakes that mix deeply every year and become isothermal, but do not homogenize completely may be included in the oligomictic lakes (Lewis 1973).

Monomictic lakes have one regular circulation sometime within the year (Hutchinson & Löffler  1956). Warm monomictic lakes circulate during the hemispheric winter period but are stratified during  summer. Thus, the expected mixing in this kind of lakes may take place in Indonesia in July-September. Polymictic lakes have many mixing periods or continuous circulation throughout the year.

Polymictic lakes are influenced by diel fluctuations in temperature, such as superficial warming and  nocturnal cooling (Hutchinson & Löffler 1956). Downwelling convection currents of cool upper layers  destroy the stratification, and nocturnal circulation takes place until terminated by the following day’s  warming (see Cole 1983). The lakes have more or less continuous mixing periods depending on the lake  morphometry and climatic conditions. Typical polymictic lakes are equatorial and high-altitude lakes without severe temperature and density gradients.

Large shallow lakes may represent a second type of polymictic lakes. At night and during the  morning hours some stratification occurs although the shallowness of the lakes. Each afternoon when winds arise, they reduce and erode the morning stability.

Another type of polymictic lakes are those in which, in addition to the diurnal variations, cooling  of the superficial layer is caused by evaporation throughout 24 hours. During day light solar warming surpasses the cooling effect.

In the meromictic lakes there are two distinct water masses, mixolimnion and monimolimnion.  Ectogenic or endogenic processes may cause meromixis. In ectogenic meromictic lakes the density  gradient may be caused by e.g. intrusion of saline waters. Endogenic meromixis may be the result of  increase in salt concentrations caused by biological processes. Biologically caused meromixis may be  temporary, and break down because of changed weather conditions (see e.g. Goldman & Jassby 1990).  Meromixis may also be caused by continuous inflow of chemically denser river water, or by cessation of inflow.

To understand the functioning of the Indonesian lakes also their stratification, mixing and  circulation patterns shall be known. In this respect, according to the present information, the Indonesian  lakes and reservoirs could be divided into several categories for the management purposes. This is  important because some of the lakes have large volumes, which may give a feeling that they are well  protected against effects of loading. Due to the high temperatures the stabilities of the Indonesian lakes  and reservoirs may be, only in some occasions, at the same level as in the temperate lakes in summer  time (see Järnefelt 1958). The main types are oligomictic, monomictic, polymictic and meromictic, with their sub-types (see above).

In considering the possibility for deep or complete mixing it has to be taken into account that the  mixing depth is dependent on many factors, such as daily, seasonal, annual, long-term and unusual  climate and weather phenomena (e.g. temperature, wind). Hydrological factors may affect the general  stratification, mixing and circulation patterns. In addition, such geographical factors as how exposed the  lakes are to the wind (e.g. rim height) are important. In the Indonesian conditions lakes and reservoirs  situated higher than 750 meters above sea level may be considered high-altitude lakes, well knowing that  in other connections this altitude is low. The average temperatures were at sea level at the surface 30.0  and at the greatest depths of lakes and reservoirs 28.5 centigrade, at 750 meters 25.5 and 24.0, and at  1,500 meters 21.0 and 19.5 centigrades, respectively.

The lake types are primarily descriptive, and deviations from the basic classifications can be found  (Ruttner 1940, Hutchinson & Löffler 1956, Löffler 1958). However, the main principles prevail, and they may be adapted to the Indonesian conditions.

From the management and conservation point of view much more frequent information flow  (diurnal and annual) shall be available for the better understanding of the stratification, mixing and  circulation patterns of the Indonesian lakes and reservoirs, and for their classification. For this purpose  some lakes shall be selected, in which e.g. continuous temperature and oxygen measurement programs  could be carried out (temperature and oxygen strings). The likelihood of mixing has been defined as the  ratio of maximum depth (Zmax) and area (A), instead of fetch (see also Kling 1988). The individual  values are in Table 5. There are, in general, various types of stratification, mixing and circulation  patterns in Indonesia affecting the oxygen conditions and utilization capacity of the lakes and reservoirs.  The likelihood of mixing may be evaluated from the above Zmax:A-ratios. In the following Table 5  the lakes and reservoirs have been ranked according to this ratio. The stratification types are only  tentative, and they may be defined in detail only after thorough studies. The deciding factors are depth,  area, altitude and how sheltered the lakes and reservoirs are from winds. Also the geological and  volcanological conditions may affect, especially the formation of meromictic conditions, and in the  confined lakes there may be underground hot springs and effective seepage adding to this. In the  reservoirs, deepwater outlets and the depth of turbine intakes may cause hydraulic stratification, thus affecting the overall stratification and mixing.

Wind may accelerate mixing to the greater depths. In the meromictic lakes the permanently  stagnant water volumes may represent small (Tamblingan) or large (Singkarak) proportions of the total  volumes of lakes (see Table 7, page 54). In the meromictic lakes the thermo- and chemoclines were usually at the same depth, but they may be also at different depths.

Patterns of stratification and mixing of Indonesian lakes and reservoirs are, as Table 5 shows, still  largely unknown. For the definition of shallow and deep, see also Rutner (1931). In the open natural  lakes and reservoirs, inflow (temperature and density) and outflow (velocity) may cause some  disturbances compared to the confined landlocked lakes. Based on Table 5 the relationship between the  ratio of Zmax:A and mixing type is presented in Table 6. Beadle (1981) and John (1986) have also indicated that the terminology for stratification in the tropics is not always appropriate.


The development of limnology in Indonesia has had various periods: Prior to 1950, between  1950-1970 and after 1970. Before 1950, the only notable study was the German Sunda-Expedition in  1928-1929. Little was done in 1950-1970. Growing concern over environmental matters since the  1970s distinguished the third period of limnology in Indonesia, when it began to support fisheries and, in  some occasions, to provide solutions to various environmental problems (see Nontji 1994). Various  government institutions, universities and consultants carry out basic and applied work. In order to  promulgate and advance the activities in Indonesia the Indonesian Society of Limnology and the  National Committee of the UNESCO/International Hydrological Programme (IHP) were established in  1991. However, the development has neither been coordinated nor integrated in proportion to the needs  of basic information of the natural lakes and reservoirs. Such an issue as the “hazardous lakes” did  emerge in 1994 with the resolutions to further study the situation in the lakes. However, for example,  the carbon dioxide concentrations referred to as alarming in Lake Toba were only from the small basin  of Pangururan, not from the entire lake where the concentrations were much lower. Expedition  Indodanau would have been, and still is, in the position to give information of the conditions in the lakes  included in its program. As a whole, it seems that the major lakes are not hazardous.

There seems to be an obsessive manner to state that there are very large gaps in our knowledge on  the tropical lakes and reservoirs, and that there is much to be done. Nevertheless, much is not done on a  regular basis to narrow of the gap. Institutions concerned with tropical lakes, reservoirs are not many,  and major research terms are infrequent. Individuals and small groups have made the greatest contributions. Unfortunately, their work has neither been coordinated nor integrated.

The overall quality of lakes and reservoirs in Indonesia shall be carefully documented based on the  available studies. This needs an effort to compile all the generally scattered material, including  background data, in a computer based lake basin data bank of Indonesia. This may be a great task,  which may be carried out by LIPI/Limnology or RIWRD, or in cooperation with both of the institutions.  Presently, there are practically no former reliable data (dates, sampling locations, depths and analytical  methods which can be calibrated against the present study) for the estimation of the development of the  state of the water bodies during the years. Now, it is extremely difficult. Without adequate information,  it is not easy to prepare a national strategy, such as the needed Indonesian lake basin management plan for the well being of the lakes and reservoirs.

It is evident that the outcome of the Expedition Indodanau is not, at all, complete. It has its  shortcomings in many respects, e.g. due to high mobility it was not possible to make diurnal, short-term  or long-term observations. This is the general deficiency in the studies made in Indonesia. However, it  gave reasonable amount of information about vertical physical and chemical characteristics, and of  surface water phytoplankton, of the studied major lakes and reservoirs in the Indonesian archipelago  with equal sampling and analytical methods. Therefore, the results of the Expedition Indodanau are  comparable between the lakes and reservoirs, and islands. The study left many open questions, such as  seasonality and predictability of stratification, mixing and circulation, oxygen and nutrient regimes, and  phytoplankton and productivity, among others, in addition to the necessarily needed secondary  information. Expedition Indodanau may have partly helped to fill the information gaps. If it may also  prove to be an impetus, as many other studies have earlier endeavored, for the onset of continuous  nationwide lake and reservoir research and monitoring activities for the sustainable utilization of the  water resources, it has met its objectives and reached its goals, beyond dispute.

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