Factors regulating bluegreen dominance in a reservoir directly influenced by the Asian monsoon

Current hypotheses, based on physical and chemical theory, that account for bluegreen-blooms in lakes were evaluated in Taechung Reservoir, Korea, during May 1993-November 1994. Seasonal patterns of chlorophyll (Chl) were similar in 1993 and 1994, but the taxonomic composition and size structure of the phytoplankton communities differed between years. During August-September 1994, bluegreens comprised > 80% of total cell numbers and net-Chl (fraction of Chl > 35 mu m) was 63% of total Chl, whereas in 1993, diatoms (Melosira) were the major taxa, and ultra-Chl (< 11 mu m) dominated. The major factor influencing bluegreen dominance in 1994 was a weak monsoon which was directly linked to strong water column stability (> 25 kg mu m(-)2), high water temperature (> 28 degrees C), and reduced silica input. Low N/or N:P ratios and the concentration of monovalent cations (Na+ and K+) were not determining factors in this system, but light and pH seemed to act as secondary factors. Our study suggests that the magnitude and frequency of bluegreen blooms in Asian waterbodies may be reduced during an intense monsoon, but increased during a weak monsoon.