Influence of Seasonal Monsoon on the Trophic State Deviation in an Asian Reservoir

Trophic parameters including total nitrogen (TN), total phosphorus (TP), Secchi depth (SD), and chlorophyll-a (CHL) were analyzed to evaluate how the Asian monsoon influences the trophicstate of the Taechung Reservoir, Korea. By using conventional criteria, the trophic state of this system ranged from mesotrophic to hypertrophic with spatial and seasonal variationsdepending on landuse pattern and flow regime. Based on TP, CHL, and transparency, the overall trophic state declined from the headwaters to the dam. However, based on TN, the entire reservoir was hypereutrophic thoughout the year. Comparing nutrients and CHL as trophic parameters, large disparities of the trophic state have been observed in the headwaters during intensive monsoon. The trophic state was much greater when assessments were based on nutrients instead of CHL, indicating apotential trophic deviation. Using Carlson's (1990) twodimensional approach, deviations of the Trophic State Index(TSI), based on the relation between TSI (CHL) minus TSI(TP), and TSI (CHL) minus TSI (SD) during the intensivemonsoon indicated that factors other than phosphorus limitedalgal growth, and that non-algal particles affect lightattenuation. These findings were supported by high non-volatilesuspended solids (NVSS), short water residence time, and low CHL:TP ratios during monsoon. Such deviations, however, did not occur during weak monsoon. The trophic disparity during theintensive monsoon was considered to be a result of the shortwater retention time and reduced light availability throughdominance of inorganic suspended solids. Under these circumstances, trophic state assessments based only on nutrientloading may not be a useful indicator for water management.