I compared the production ecology of phytoplankton from two basins of an oligotrophic lake which are characterized by the presence and absence of summer stratification. The objective of this study was to assess the impact of mixing and stratification of the water column on phytoplankton photosynthesis and the fate of primary production.

My results firstly show that estimates of photosynthetic capacity (Pmax) and efficiency (a), calculated using photosynthesis vs irradiance curves, vary depending on the model used. Estimates of Pmax using the exponential and hyperbolic tangent models are in good agreement (4% difference). The same comparaison for a shows poor agreement (24% difference between the two models). The lack of agreement increases when an intercept parameter is included in the models.

The relationship between the size-distribution of phytoplankton biomass and production was used to characterize the fate of primary production on daily and seasonal time scales. The phytoplankton community was dominated by small cells (pico- and nanoplankton), which reflects the oligotrophic status of the lake where competition for nutrients was likely to favour smaller-sized phytoplankton. The nanoplankton fraction accounted for most of the seasonal change in total biomass and production, whereas the contribution of picoplankton to both biomass and production remained relatively constant throughout the season. The observed seasonal variations in the size structure of phytoplankton do not agree with the usual paradigm of dominance of small organisms during the summer stratified period, and of larger algae during the mixing events of the spring and autumn. The relative constancy of picoplankton biomass throughout the season as well as the close relationship between potential growth and loss rates suggest an equilibrium controlled by herbivore grazing. In such environments where allochthonous nutrient inputs are limited, the principal source of nutrients would be autochthonous (recycling within the mixed layer) and mixing and stratification processes would not have a major impact on productivity.

The size-fractionated photosynthesis was used to characterize the influence of environmental factors on the phytoplankton community, on daily and seasonal time scales. Seasonal variations of Pmax showed size- related differences, with maximum values in July for the picoplankton and in November for the nanoplankton. Results also indicate that picoplankton require less light to saturate photosynthesis, and can sustain a wide variation in ambient irradiance, which suggests a euryphotic capacity of adaptation.

Diel periodicities of Pmax and a were observed for both the pico- and the nanoplankton, each size fraction exhibiting distinct patterns of variability in the timing and amplitude of daily oscillations. Maximum photosynthetic capacity was generally observed for the two size fractions around noon and in the morning, but without synchronicity. The picoplankton exhibit larger amplitudes in the oscillations of Pmax and a than the nanoplankton, which suggests differences in the photonutritive history of each size fraction. Hydrodynamic differences in the two basins account for the adaptation to high irradiances of picoplankton in Basin 1. The vertical and seasonal changes in photosynthetic parameters show that both fractions are photoadapted to their environment and that the stratification and mixing regime modifies the photosynthetic response depending on the physiological characteristics associated with size.