AQUACULTURE CARRYING CAPACITY OF LAKE JATIGEDE, INDONESIA
Lake Jatigede is currently under construction in the eastern part of West Java, Indonesia. This reservoir will serve purposes of flood control, irrigation, hydropower and a domestic (non-potable) water supply. The dam has been completed, but the current occupants of the to-be-inundated land have not yet moved from the area. A critical question is whether aquaculture will be allowed in the waters of Lake Jatigede. Local residents want it as an alternative source of income, but some government agencies, appalled by the environmental degradation caused by aquaculture over-production in the nearby Lakes Saguling, Cirata and Jatiluhur, want to prohibit aquaculture in Jatigede. The question has broader implications, because Indonesia plans to build scores of similar new dams in the coming decades. One traditional way to decide on aquaculture acceptability in freshwater lakes and reservoirs is to model Aquaculture Carrying Capacity (ACC) based on some agreed-upon water quality criterion. The most common method in fresh water is mass-balance modeling of phosphorus (P), based on the Dillon-Rigler model. A necessary first step is the decision on acceptable [P] in the water. Maintenance of oligotrophic water for tourism requires a [P] of 10 mg/m3, maintenance of ecosystem function and fisheries requires a [P] of 50 mg/m3, whereas maintenance of water quality for tilapia aquaculture production allows [P] of up to 250 mg/m3.
As part of a larger project on modeling of ACC in Indonesian lakes and reservoirs, we investigated the potential ACC of Lake Jatigede. Modeling ACC based on [P] requires an estimate of ambient [P], which of course does not yet exist for Jatigede. Our colleagues at Institut Pertanian Bogor are currently determining [P] in the Cimanuk River, whose damming will fill Lake Jatigede. The P levels in this river are extremely variable. In addition, during the filling of the reservoir, the P levels will be elevated due to the decomposition of terrestrial plant material in the inundated area. We have used satellite imagery to assess the current land usage in the prospective inundated area in order to estimate the spike in [P] of the water following dam construction. Based on literature about similar [P] spikes during dam construction, we recommend measuring [P] for at least one year following inundation in order to allow the [P] spike to return to a stable level before finally calculating ACC for Lake Jatigede. Land use and activities within the watershed should also be considered during/ following the initial spike and for calculating ACC.