SIZE-RELATED PHOTOSYNTHETIC CHARACTERISTICS OF PHYTOPLANKTON DURING PERIODS OF SEASONAL MIXING AND STRATIFICATION IN AN OLIGOTROPHIC MULTIBASIN LAKE SYSTEM

Phytoplankton photosynthetic responses were studied in two basins of a n oligotrophic lake (Quebec, Canada), which are characterized by the a bsence (shallow Basin 1) and presence (deeper Basin 2) of seasonal the rmal stratification. Size-fractionated photosynthesis was used to char acterize changes in phytoplankton characteristics during periods of se asonal mixing and stratification. Seasonal variations of P-max showed size-related differences; with maximum values in July for the picoplan kton and in November for the nanoplankton. Similar patterns of variabi lity in alpha and l(k) were observed for the two size fractions throug hout the season. However, size-related differences in the magnitudes o f l(k) indicate that the picoplankton have lower light requirements th an the nanoplankton to saturate photosynthesis and can sustain large v ariations of irradiance in the surface layer during their growth seaso n. Tight inverse correlations between in situ irradiance and alpha, at the seasonal scale, indicate that the two size fractions respond to l ower irradiance by increasing their photosynthetic efficiencies. P-max and alpha are strongly correlated for the two size fractions, which s uggests that biochemical processes controlling these two parameters ar e closely coupled. Hydrodynamics may influence photosynthesis differen tly for pico- and nanoplankton. Picoplankton in Basin 1 are adapted to higher irradiance than those in the euphotic zone of Basin 2, as show n by their lower alpha and higher l(k) during most of the study period , which may be explained by more frequent exposure to high irradiance through mixing in the shallowest basin. Vertically changing photosynth etic parameters in Basin 2 show that both pico- and nanoplankton were photoadapted as a function of depth. The size-related pattern of resid ence in the water column coupled to the turbulence regime in the epili mnion may be responsible for size-related differences in photoadaptati on.