Technology Needs for Lake Management in Indonesia- Investigation of Rawa Danau and Rawa Pening


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<Technology Needs for Lake Management in Indonesia -
Investigation of Rawa Danau and Rawa Pening, Java>

J. Precedents for the Restoration of Tropical Lakes

In the case of Rawa Danau an effort should be made to restore the lake which once existed in the caldera and which disappeared gradually as a result of human interference starting about 200 years ago. The objective should be to recreate the original lake in a form which is as near pristine as possible. This is because the maintenance of the conservation value of the swamp forest is a prime consideration in management of the caldera as a whole. The lake if restored, will overlap with the swamp forest, and should be a vital factor in rehabilitation of the swamp forest.

Examination of the literature suggests that there are no precedents for a project such as the restoration of the lake in the Rawa Danau caldera.

Much of the information on lake restoration is based on the publication "Restoration of Aquatic Ecosystems". This relatively recent publication (1992) lists and details many cases of lake rehabilitation and restoration. It is referred to here as the "Restoration Manual". The lakes mentioned in it are mainly northern hemisphere lakes (non-tropical lakes) which have been polluted in a variety of ways. Consequently a range of methods have been employed for their restoration.

Lake restoration, as understood in the United States, is concerned primarily with making such polluted and degraded lake environments more acceptable to, and fit for, use by people. The desire to return a lake to its pristine condition is seldom an objective of such lake restoration projects. Possibly this is because of the desire by people to utilise lakes for recreation and the fact that funds are more easily available if an economic benefit is perceived at the end of the project. Thus, many of the so-called lake restoration projects are really better classified as rehabilitation projects. They have really been designed to minimise the adverse effects of human interference, often in the interests of recreation.

The majority of restoration efforts in the northern hemisphere have been directed towards lakes showing accelerated eutrophication. Restoration technologies employed in these cases involve control of catchment processes, suppression of soil erosion, limitation of nutrient input from the catchment, and extraction of nutrients from the water. Some of these technologies are relevant here, since the erosion levels and nutrient inputs from the catchment may be higher than those of 200 years ago. The various technologies developed in industrial northern hemisphere countries to deal with lake acidification, or to use soil blankets to cover toxic deposits of mercury and chlorinated hydrocarbons in the bottom mud of lakes, are not relevant here.

The problems of soil erosion and siltation in reservoirs coupled with the lack of adequate sewage treatment in many developing countries have had a major impact on water bodies in those countries, particularly when supplies of water for domestic and agricultural use are not adequate to deal with the demands of industrial development as well (Brown and Wolf, 1984; Postel, 1985)

Dredging may be suggested in Rawa Danau as a standard restoration technique, but is unlikely to add significantly to the volume of water held in the lake if restored as recommended here. Dredging is unlikely to be successful in Rawa Danau in view of the large number of poorly decomposed tree stumps buried in the sediments of the former lake.

Dredging methods suitable for lakes are listed and detailed in a publication by UNESCO (1985). The only method of dredging which might be useful here is the suction dredge and even this is of doubtful value. The question would be where to dispose of the clay and organic matter released from the lake bottom by the suction pipe. The outlet of the pipe would presumably be downstream in the valley of the Ci Danau river. This would reduce water quality at the intake for Cilegon. The valley is narrow and not particularly suitable for the construction of holding dams to retain sediment permanently.

The reason for the high density of tree stumps is, of course, that a much larger area of the floor of the caldera was once covered by forest, presumably much of it swamp forest, than is the case at present. The stumps from these trees have been buried in the sediments of the lake particularly during the cultivation of padi. The method of cultivation of padi means that for much of the year, the soil is waterlogged and air cannot penetrate. Under these conditions bacterial decomposition of wood is very limited and may even be non-existent.

Restoration of the lake may initially lead to outbreaks of aquatic plants, such as Eichornia and Salvinia. This problem could be dealt with in various ways, but aggressive control measures such as the use of chemicals or mechanical removal have seldom proved to be successful in the long term and often produce unforeseen side effects. The use of herbicides on Eichornia leads to the release of large quantities of nutrients from the dying plants which, together with the toxic residues in the lakes, can lead to outbreaks of algal blooms. These are really blue-green bacteria and may produce toxins which affect people, particularly children, mammals, fish, and birds. Mechanical removal of aquatic vegetation can actually prolong infestation in some cases (Smith and Barko, 1990). Many invasive species of water plants have unusually effective methods of vegetative or sexual reproduction in which the seeds or vegetative organs are disseminated more widely after mechanical harvesting. The introduction of Ctenopharyngodon idella the grass carp will have an effect in reducing populations of submerged water plants, and will eventually have an impact also on population of water hyacinth, which may initially spread over large areas of the lake.

Biological control methods could be used to limit the spread of Salvinia and Eichornia. The appropriate species of insects used in control of Eichornia (Neochetina eichorniae) and of Salvinia (Cyrtobagous sp.) are widely available. Information can be obtained from the CSIRO Division of Entomology in Brisbane, Australia. The creation of any new water body is likely to lead to imbalances in the populations of plants and animals which form the ecosystem. Those species which can reproduce most rapidly will tend to develop "population explosions" as they utilise what is, for the moment, an unlimited spatial resource. Thus populations of mosquitos may increase to the point where they are a perpetual nuisance and, if they transmit malaria or dengue may become a health hazard. Such outbreaks of mosquitos for other small insects with aquatic larvae will eventually decrease as the fish population expands and grows to utilise this new source of food. Health authorities have to be vigilant to ensure that some rare diseases carried by insects or molluscs do not suddenly spread through the local population or affect tourists. This could be the result of a temporary increase in the abundance of intermediate hosts or vectors in the lake area, or of the appearance of unexpected new intermediate hosts and vectors as a result of the creation of the lake. It would be expected that populations of aquatic plants and animals would be unstable during the first two years after the creation of the new lake but would stabilise during the latter part of this period.

Thus, one can conclude that there is little information to be obtained from the contemporary literature which would be of relevance to the restoration of Rawa Danau.

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