SIZE-DEPENDENT CHANGES IN PHYTOPLANKTON C AND N UPTAKE IN THE DYNAMICMIXED-LAYER OF LAKE BIWA

1. Short-term (days) hydrodynamic effects of wind-induced mixing on ph ytoplankton size structure, and C and N uptake characteristics, were s tudied in the surface mixed layer (epilimnion) of Lake Biwa (North Bas in),before and during a period of high winds (typhoons). 2. The latter period was characterized by two major typhoon events associated with deepening of the seasonal thermocline, reduced water column stability, decreased underwater irradiance and increased dissolved reactive N an d particulate P. 3. Nutrient concentrations, seston C/N ratios, and up take rates indicated that phytoplankton biomass and production were li mited by P and not N throughout the study. Higher C- and N-based produ ctivity during the typhoon period than before reflected the increased phytoplankton biomass and higher specific uptake rates due to increase d nutrient supply. 4. Changes in the size-structure of phytoplankton ( < 2 and > 2 mu m) were associated with variations in the stratificatio n and mixing regime. When vertical stability was high (before the typh oons) concentrations of > 2 mu m biomass (chlorophyll a, particulate o rganic C and N) were higher at the bottom of the mixed layer than at t he surface whereas, when stability of the mixed layer was low (the typ hoon period), the contribution of picoplankton (< 2 mu m) to total Chl a increased at the surface and decreased at the bottom following the first high winds. 5. Photoadaptive adjustments of the phytoplankton pr ovided further evidence of hydrodynamic control. The lower intracellul ar Chl a concentrations and C and N uptake efficiencies in the < 2 mu m fraction suggest that they experienced, on average, higher irradianc e than the larger cells because of their lower sinking rates. During t he stability period, picoplankton exhibited higher photosynthetic effi ciencies at the bottom of the mixed layer than at the surface. Such di fferences disappeared during the typhoon period indicating that the mi xing rate was then probably higher than the photoacclimation rate in t he small size fraction. 6. The present results stress the highly trans ient nature of biological homogeneity in the surface mixed layer of th e lake.