Trophic state, seasonal patterns and empirical models in South Korean reservoirs

Data from 59 reservoirs in South Korea, sampled monthly during 1993-2000, showed that about onk-third were mesotrophic, nearly one-half were eutrophic and the remainder were hypereutrophic based on conventional criteria for total phosphorus (TP), chlorophyll (Chl) and Secchi depth. Most reservoir; had >1 mg.L-1 total nitrogen (TN) resulting in high mass ratios of TN:TP (range 23 to 243, median 76) relative to many temperate lakes. To compensate, conventional TN criteria were provisionally adjusted upward by about 2.5-times to classify Korean reservoirs uniformly across all trophic state metrics. During the summer monsoon, TP and TN typically peaked in mesotrophic reservoirs and declined in the hypereutrophic group. The inference is that monsoon inflow produces these patterns by increasing non-point external inputs that dominate the nutrient budgets of mesotrophic reservoirs while diluting point-source inputs important in hypereutrophic impoundments. Eutrophic reservoirs showed both response patterns, so that taken in aggregate a seasonal response was not apparent. The log relation between Chl and TP was linear and showed an average yield of Chl per unit of TP on par with other temperate lakes. Seasonally, the Chl TP relation was strongest during summer and weaker during fall-winter which is consistent with increased light-limitation during mixis in these monomictic impoundments. Seasonal development of Chl did not show strong evidence of a spring or fall bloom. About half of the time maximum Chl values were measured during the monsoon or post-monsoon (July-September). Maximum Chl was similar to3 times the annual mean and during summer maximum Chl was similar to double the mean. The log relation between Chl and Secchi depth matched that found in North American lakes and the seasonal phenology for Secchi depth was the opposite of Chl and suspended solids. The analysis confirms that the monsoon is a major source of variation within and among Korean reservoirs.