Trophic State, Seasonal Patterns and Empirical Models in South Korean Reservoirs

ABSTRACT Data from 59 reservoirs in South Korea, sampled monthly during 1993–2000, showed that about one-third were mesotrophic, nearly one-half were eutrophic and the remainder were hypereutrophic based on conventional criteria for total phosphorus (TP), chlorophyll (Chi) and Secchi depth. Most reservoirs 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 trophicstate metrics. During die 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 mat 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, die 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 fail bloom. About half of the time maximum Chl values were measured during the monsoon or post-monsoon (July-September). Maximum Chl was ~3 times the annual mean and during summer maximum Chl was ~ double the mean. The log relation between Chl and Secchi depth matched mat found in North American lakes and the seasonal phenology for Secchi depth was die opposite of Chl and suspended solids. The analysis confirms that die monsoon is a major source of variation within and among Korean reservoirs.

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