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 trophic state of the Taechung Reservoir, Korea. By using conventional criteria, the trophic state of this system ranged from mesotrophic to hypertrophic with spatial and seasonal variations depending 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 a potential trophic deviation. Using Carlson's (1990) two dimensional 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 intensive monsoon indicated that factors other than phosphorus limited algal growth, and that non-algal particles affect light attenuation. These findings were supported by high non-volatile suspended 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 the intensive monsoon was considered to be a result of the short water retention time and reduced light availability through dominance of inorganic suspended solids. Under these circumstances, trophic state assessments based only on nutrient loading may not be a useful indicator for water management.