The effect of summer monsoon on the ecosystem of Lake Soyang, Korea

Bomchul Kim, Kwang-Soon Choi, U-Hee Lee, Yuni Kim, Woo-Myung Heo*

Kangwon National University, *Samchok National University, Korea

In Korea one half of 1200mm annual precipitation is concentrated in several heavy showers in July and August, which are sometimes greater than 100 mm/day. The ratio of highest/lowest flow rates in the river is about 300. The change of hydraulic residence time has the greatest impact on the aquatic ecosystems. The concentrations of nutrients in the stream increase by up to one order of magnitude. Because there are few natural lakes in Korea, reservoirs and rivers are the major large freshwater ecosystem. Plankton is washed away in the flood season in the reservoirs of short residence time and shallow depth, whereas turbid flood runoff is stored in the intermediate layer of the deep reservoirs with longer residence time, such as Lake Soyang. Nutrients loading is concentrated in storm runoff and most of them are discharged from the dam before they are dispersed to the epilimnion. Because nutrients in the intermediate layer are dispersed slowly to the epilimnion, cyanobacterial blooms follow heavy storm runoff. Most of phosphorus export via storm runoff comes from fertilizer and cattle manure. Phosphorus is accumulated in the topsoil of agricultural fields which is eroded in heavy showers. Therefore phosphorus concentration is highest in late summer after storms and it depends on the storm runoff in the preceding summer monsoon. In the years of large precipitation we have denser algal blooms than dry years in Lake Soyang. Usually chlorophyll a concentration increase from 2 mg/m3 before monsoon to >6 mg/m3 after monsoon. DOC from watershed that has very low decomposition rate does not seem to contribute much to the oxygen budget because it is washed out of reservoirs before oxidized. And therefore though allochthonous organic loading is as large as autochthonous generation, autochthonous organic matter is thought to be the major energy source in the ecosystem of Lake Soyang.