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 an oligotrophic lake (Quebec, Canada), which are characterized by the absence (shallow Basin 1) and presence (deeper Basin 2) of seasonal thermal stratification. Size-fractionated photosynthesis was used to characterize changes in phytoplankton characteristics during periods of seasonal mixing and stratification. Seasonal variations of P-max showed size-related differences; with maximum values in July for the picoplankton and in November for the nanoplankton. Similar patterns of variability in alpha and l(k) were observed for the two size fractions throughout the season. However, size-related differences in the magnitudes of l(k) indicate that the picoplankton have lower light requirements than the nanoplankton to saturate photosynthesis and can sustain large variations of irradiance in the surface layer during their growth season. Tight inverse correlations between in situ irradiance and alpha, at the seasonal scale, indicate that the two size fractions respond to lower irradiance by increasing their photosynthetic efficiencies. P-max and alpha are strongly correlated for the two size fractions, which suggests that biochemical processes controlling these two parameters are closely coupled. Hydrodynamics may influence photosynthesis differently for pico- and nanoplankton. Picoplankton in Basin 1 are adapted to higher irradiance than those in the euphotic zone of Basin 2, as shown 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 photosynthetic parameters in Basin 2 show that both pico- and nanoplankton were photoadapted as a function of depth. The size-related pattern of residence in the water column coupled to the turbulence regime in the epilimnion may be responsible for size-related differences in photoadaptation.