Author : Rustadi

Summary : The research in Sermo dam of Kulonprogo regency, Yogyakarta Special Province aims : to find source and load of nitrogen (N) and phosphorus (P) over space and time, to evaluate eutrophication level, suitability of water quality for fish life, to predict carrying capacity for fish culture and to control N and P concentrations. The research was conducted by direct observation and measurement on water samples taken from inflow and outflow rivers, upstream, middle and downstream areas of reservoir. The samples in reservoir were taken at water depth: 0, 2, 4 m and the bottom. Observation was done monthly from June 2006 to May 2007. The variables are ammonia, ammonium, nitrite, nitrate, and phosphate; N-total and P-total contents in the water; water temperature, transparency, turbidity, total dissolved and suspended solids, dissolved O2, pH, free CO2, and alkalinity, plankton and benthos. The research conclude that N and P nutrients come from river inflow and fish culture waste; its concentration is spatially flat and temporarily higher during rainy season compare to dry season; there were residue of N and P; eutrophication level varies from eutrophic to hipereutrophic; water quality was suitable for fish culture; light pollution was found in fish culture area and when the highest concentration of P; carrying capacity of water environment for fish culture 93.858 ton/year or 3.76-17.354 ton/month; controlling of N and P by fish culture using double nets, fish stocking and harvesting outside net cage may eliminate residue and decrease loading of N and P in the water.