An in situ study of the productivity and population dynamics of five freshwater planktonic diatom species

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Abstract
In situ carbon fixation rates were determined through track autoradiography for Asterionella formosa Hassall, Fragilaria crotonensis Kitton, Melosira italica Ehrenberg, Synedra radians Kützing, and Tabellaria fenestrata Kützing during their wax and wane in a small lake. Photoinhibition of primary production was much stronger than predicted from culture studies, while temperature effects were minimal. Despite the severity of photoinhibition, long term cell growth was maximal at or near the surface for all species. The general magnitude of cell growth was determined primarily by the maximum photosynthetic rate which, for at least one species, was in turn strongly correlated with precipitation (as a measure of nutrient loading). Epilimnetic cell division was mildly light limited except for M. itadica and S. radians, each of which had a relatively high saturation intensity. Population fluctuations were correlated quite strongly with estimates of sinking rather than cell growth; sinking was in turn correlated with death. Changes in abundance of these diatom populations were thus largely the result of variables loss rates.