Does High Silicate Supply Control Phytoplankton Composition and
Particulate Organic Matter Formation in Two Eutrophic Reservoirs in the Brantas
River Catchment, Java, Indonesia?
Abstract: 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 nonbiomineralizing 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.
Keywords: Eutrophication, artificial lakes, suspended organic matter, nutrients, phytoplankton, dam
