Green warning gleaned from the depths of RI lakes

By Pasi Lehmusluoto

MEDAN (JP): If you paid attention in geography class, you might remember the highest mountain in the world and the longest river.

But if you were asked to name the biggest crater lake would you know the answer? It is Lake Toba, in North Sumatra.

Lake Toba is about 1,130 square kilometers and has a volume of 240 cubic km.

Indonesia is home to not one but two great crater lakes. The second is Towuti fault lake in Sulawesi which should be included in the great lakes category because it stretches more than 500 sq km.

The depths of lakes here vary from 1.5 meters in the low-water season in the Tempe flood plain lake to the deepest depths of Matano fault lake which measure in at 590 m. Both are located in Sulawesi.

Lake Matano scoops down to 208 m below sea level, making it the seventh deepest lake in the world. The expansive Lake Toba comes in at ninth with a depth of 529 m, 61 meters short of Matano.

Lake Toba was formed 75,000 years ago in the huge "Batak Tumour" explosion, outdoing the Krakatau eruption and relocating 2,000 cubic km of soil.

In its southern basin, the lake shows some signs of pollution and there is no oxygen in the bottom reaches, in sharp contrast to the northern basin.

But the condition "down below" most of the 521 natural lakes and 100 reservoirs in Indonesia remains a mystery, even to many scientists.

Indonesia and Finland carried out joint research on the condition of the country's lakes from Sumatra to Irian Jaya.

Expedition Indodanau, staged between 1991 and 1995, observed 38 lakes and reservoirs, delving into their great depths, many for the first time.

Lakes reflect changes in the environment. The state of a natural lake is generally a good indicator. Indonesian lakes are advantageous, partly because drainage areas are limited and there is minimal pollution from them.

The lake chains and reservoirs are more problematic, since dense population, intensive agriculture and industry may cause quite extensive pollution.

Reservoirs are silting up faster than expected, but lakes are not exempt.

Uncontrolled use of the drainage areas, clearing off ancient forests for human settlement, industry and agriculture, and slash-and-burn practices increase erosion and silting.

This has a flow-on effect for surrounding sea areas. It is unfortunate that both during economic growth and recession, forest areas are cleared.

Further threats are caused by development projects in transition from an agrarian society to an agro-industrial state when the properties and behavior of the lakes are not adequately known and accounted for.

Unknown species

Damming, for example, causes water level fluctuations, there is the impact of tourism and introduction of exotic plants and animals.

In Matano and Towuti lakes in Sulawesi there are endemic and, possibly, still unknown species. They may disappear because of pollution or by the introduction of exotic species. Unknown species may even disappear before they are discovered.

The deep Matano fault lake, a recreational park, has a great water volume, but its surface waters may be threatened by airborne contaminants and deeper waters by effluents of the nearby nickel mining industry.

No long-term data are available to follow the development of the state of the lakes.

To secure the sustenance of the ecological and species diversities requires integrated planning based on long-term environmental monitoring and pollution control measures.

The ability of the lakes to resist pollution and contamination is relatively weak because of their chemical quality, small amounts of biological material and because water in the lakes is changing very slowly.

The chemical composition of Batur and Bratan lakes in Bali, situated in different calderas but only 30 km apart, exhibit great differences.

The water in Batur is highly salty and in Bratan it is almost like distilled water. Tourism is a great threat to the small and very shallow Lake Bratan.

Citarum River in Java is a good example of how reservoirs act to reduce pollution downstream. The Brantas River, also in Java, with its several reservoirs is an example of a well monitored river system.

In general, the major natural lakes are in good condition, except Lake Singkarak and the semi-natural Rawa Pening, which is heavily infested by weeds.

Maninjau, Kerinci, Bratan, Batur, Buyan and Tondano lakes may be subject to eutrophication, Singkarak and Matano lakes to pollution -- and especially the sensitive lake Toba to pollution and eutrophication -- while the water bodies of Limboto, Sidenreng and Tempe to silting up, depending on the activities in their drainage areas. Limboto is in need of dredging.

The most polluted reservoirs are Saguling and Cirata in the Citarum River downstream of Bandung, West Java. Many of the reservoirs are prone to eutrophication, such as Mrica, Lahor, Sutami, Selorejo and Palasari, and to siltation, like Gajah Mungkur and Wlingi. Wlingi is continously dredged.

The total water volume of all Indonesian lakes is estimated to be some 500 cubic km. They may be of great economic value in the region, as water sources for their biological yield and for tourism for generations to come.

Maintenance of their healthy ecology and good water quality needs monitoring, adequate research, development of criteria and indicators, environmental action plans for timely investments to abate water pollution, and good governance for their sustainable utilization.

Dr. Pasi Lehmusluoto is the project coordinator of the Expedition Indodanau lake project and has participated in the development of the Indonesian lake water resources since 1974.