Impacts of mixing and stratification on the fate of primary production were studied in an oligotrophic lake by comparing the size-distributions of phytoplankton standing stock and production in two basins, only one of which experiences seasonal thermal stratification. In both basins, the phytoplankton was dominated by small cells (pico- and nanoplankton). The contribution of picoplankton to both biomass and production remained relatively constant throughout the season in both basins. Seasonal variations in the size structure of phytoplankton communities do not agree with the paradigm of dominance by small cells during summer stratification and dominance of larger cells during spring and fall mixing events. Nutrient control of productivity through mixing and stratification is unlikely to affect the structure of phytoplankton communities when nutrients (allochthonous) derived from the catchment basin or sediments are in short supply. In such environments, nutrients (autochthonous) are largely derived in the lake through heterotrophic food web processes such as grazing, excretion and decomposition. Maximum rates of production and losses in July and August in both basins are consistent with increased regeneration and may represent a response of larger-sized cells to higher nutrient availability resulting from enhanced grazing on picoplankton. The high correlation between the rates of loss and of potential growth for the phytoplankton community during all sampling periods, and the relative constancy of the picoplankton biomass, leads us to propose a long-term, steady-state equilibrium in the phytoplankton community under the control of grazing by herbivores and/or other loss processes.
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Frenette et al. (1994) studied this question.