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Notes: 1. Short-term (days) hydrodynamic effects of wind-induced mixing on phytoplankton size structure, and C and N uptake characteristics, were studied in the surface mixed layer (epilimnion) of Lake Biwa (North Basin), 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 and particulate P.3. Nutrient concentrations, seston C/N ratios, and uptake rates indicated that phytoplankton biomass and production were limited by P and not N throughout the study. Higher C- and N-based productivity during the typhoon period than before reflected the increased phytoplankton biomass and higher specific uptake rates due to increased nutrient supply.4. Changes in the size-structure of phytoplankton (〈 2 and 〉 2 μm) were associated with variations in the stratification and mixing regime. When vertical stability was high (before the typhoons) concentrations of 〉 2 μm biomass (chlorophyll a, particulate organic C and N) were higher at the bottom of the mixed layer than at the surface whereas, when stability of the mixed layer was low (the typhoon period), the contribution of picoplankton (〈 2 μm) to total Chl a increased at the surface and decreased at the bottom following the first high winds.5. Photoadaptive adjustments of the phytoplankton provided further evidence of hydrodynamic control. The lower intracellular Chl a concentrations and C and N uptake efficiencies in the 〈 2 μm fraction suggest that they experienced, on average, higher irradiance than the larger cells because of their lower sinking rates. During the stability period, picoplankton exhibited higher photosynthetic efficiencies at the bottom of the mixed layer than at the surface. Such differences disappeared during the typhoon period indicating that the mixing rate was then probably higher than the photoacclimation rate in the small size fraction.6. The present results stress the highly transient nature of biological homogeneity in the surface mixed layer of the lake.

Notes: 1. In situ experiments were conducted using specialised incubation devices to grow biofilms under varying light regimes and grazing intensities (by excluding fish and large-sized zooplankton, 〉2 mm) both within and between two sites in Lake Saint-Pierre.2. Biofilms growing under greater in situ UVR and light exposures found in the south water mass were characterised by a greater biomass and nutrient content, but their total fatty acid (FA) contents and ratios of elemental nutrients were not significantly different from the north. There was a relatively greater abundance of chlorophytes and cyanobacteria in the south water mass, along with a greater proportion of low nutritional quality saturated fatty acids (SAFA). Conversely, biofilms growing in the north had a greater relative abundance of diatoms, as well as greater eicosapentaneoic acid (20:5ω3) and docosahexaneoic acid (22:6ω3) concentrations (two FAs implicated in the physiological competency of grazers).3. The prevailing community structures created differences in terms of nutritional status of the biofilms for benthic grazers and their predators at the two sites. The biofilms from the southern site were characterised by greater food quantity at the expense of quality, while biofilms from the northern site contained less food of a better quality. Despite this, the nutritional regime in the south supported a greater productivity at higher trophic levels. The secondary treatments (light and grazing by fish and macro-invertebrates) had lesser effects on food quality.