Does High Silicate Supply Control Phytoplankton Composition and Particulate Organic Matter Formation in Two Eutrophic Reservoirs in the Brantas River Catchment, Java, Indonesia?

The Brantas River catchment on the Indonesian island of Java is among the most densely-populated regions in the world. Damming of the river and intensive agriculture in the catchment are supposed to affect the biogeochemistry and ecology of the river, its reservoirs and coastal waters. We collected water, suspended matter and phytoplankton samples from the Sutami and the Selorejo lakes, two major reservoirs in the catchment, in May 2001 and June 2002. Water samples were analyzed for dissolved inorganic nutrients and suspended matter was analyzed for carbon and nitrogen contents and stable carbon and nitrogen isotope composition. Phytoplankton cells were counted, identified and grouped into four major classes. Both reservoirs displayed clear signs of eutrophication as shown by high nutrient concentrations and high phytoplankton abundance. Phytoplankton abundance was generally higher in the Sutami than in the Selorejo reservoir and in the Sutami reservoir it was much higher in June 2002 than in May 2001. Phytoplankton responded to the amount and composition of nutrients in such a way that diatoms dominated when silicate concentrations and N/P ratios were high in the Sutami reservoir in May 2001. The mass occurrence of the water hyacinth Eichhornia crassipes in the Selorejo reservoir was probably responsible for a high uptake of dissolved nutrients resulting in an N/P ratio <8. This favoured the growth of cyanobacteria which can fix atmospheric nitrogen. Excessive phytoplankton growth in the Sutami reservoir in June 2002 led to a drastic silicate reduction (165 µM in May 2001, 95 µM in June 2002). This and an N/P ratio <4 consequently resulted in a much lower abundance of diatoms and a much higher abundance of cyanobacteria than in May 2001. The biogenic extraction of silicate by diatoms in reservoirs has often been observed in high latitude regions where it, in combination with high anthropogenic additions of nitrogen and phosphorus and a lack of silicate replenishment downstream of reservoirs, leads to eutrophication of coastal waters and a shift from biomineralizing to non-biomineralizing phytoplankton. Because of the high weathering rates in the tropical Brantas River catchment, silicate concentrations in the downstream portion of the river were as high as in the headwaters and coastal phytoplankton was dominated by diatoms despite nutrient extraction in reservoirs. It indicates that human activities in river catchments in the humid tropics affect coastal ecosystems in a different way than in high latitude regions.