Response of reservoir water quality to nutrient inputs from streams and in-lake fishfarms

This study demonstrates how land-use characteristics influence stream water chemistry in a Korean watershed and how stream conditions and in-lake point sources determine trophic state, algal biomass and phytoplankton composition. Measurements of nutrient loads showed that 68% of total phosphorus (TP) loads was originated from the watershed and the remaining 32% came directly from in-lake fish farms. Based on mass balance models, annual areal phosphorus (L_p) and water loading (q_s) were 1.032 g m^-2 and 25 m³ m^-2 yr^-1, respectively, and the total annual P-input exceeded dangerous loading levels according to Vollenweider's classification (1976). In situ Nutrient Stimulation Bioassays (NSB), Trophic State Index (TSI), and mass N:P ratios showed that phosphorus was the primary element regulating algal chlorophyll in the system, and non-algal turbidity and zooplankton effects on algal growth were minor. A high regression coefficient in the empirical relationship between chlorophyll (Chl) and TP supported the evidence of strong P-limitation, and model comparisons suggested that our observed Chl values were largely underestimated in models developed for world-wide lakes. Direct P-input to the trophogenic zone from the in-lake fishfarms and streams with intense human-use resulted in massive bluegreen algal blooms, indicating that the highly available input fraction may explain the deviation. Under the circumstances, reduction in P-loading is an essential measure for long-term eutrophication control in this system.