PICOPHYTOPLANKTON BIOMASS IN RELATION TO LAKE TROPHIC STATE AND THE TN/TP RATIO OF LAKE WATER IN JAPAN

Picophytoplankton biomass and its contribution to total phytoplankton biomass were investigated in relation to the nutrient concentration and total N : total P ratio of the epilimnetic waters of 42 Japanese lakes during the warm season in 1991 (April-October). Picophytoplankton biomass (as chlorophyll a) in meso-, eu-, and hypertrophic lakes was significantly higher than those observed in oligotrophic lakes. However, picophytoplankton biomass increased significantly with increased total P concentrations in all systems excluding hypertrophic lakes. Picophytoplankton contribution to total chlorophyll a content was significantly higher in oligo- and mesotrophic lakes than in eu- and hypertrophic lakes and was inversely correlated with total P concentrations in lake water. Picophytoplankton contribution to the total phytoplankton biomass was positively (r = 0.54, n = 42, P = 0.0003) correlated with the total N : total P ratio of lake waters. Each lake trophic type, with the exception of hypertrophic lakes, showed this trend, although the correlation was not significant. We suggest that picophytoplankton contribution is influenced by the total N : total P ratio rather than lake trophic state; however, picophytoplankton were of little importance in hypertrophic lakes.