Short-term dynamics of a Melosira population in the plankton of an impoundment in central Chile

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Abstract
Relationships among the chlorophyll-specific photosynthetic behaviour of a Melosira -dominated phytoplankton, net variations in the quantity of Melosira in suspension, the rates of sinking loss of Melosira cells and did fluctuations in water-column stability were investigated in Embalse Rapel. central Chile. The short study was carried out during the autumnal equinoxial period (March. 1984) of maximal Melosira abundance to determine the extent to which these various components might contribute to the distinctly bimodal seasonal distribution of phytoplankton abundance in this reservoir. Diurnal photosynthetic production within the illuminated layer and supposed column respiration were found to be of a similar order, suggesting that net increase through cellular growth was already substantially limited by self-shading. Thus observed net changes in the standing population may have depended more on the relative magnitude of sinking loss rates and recruitment through resuspension, which processes are profoundly influenced by alternations in wind-induced mixing and intense, near-surface, thermal stratification. As the relative frequency and duration of these episodes vary during the year, so does the capacity of the Melosira to maintain or increase its standing biomass. High cellular photosynthetic efficiency coupled with rapid sinking rate appear to be of selective advantage when the cycle of alternation is approximately diel. The application of these findings to the seasonal distribution of M. granulata in the Rapel reservoir and to the ecology of planktonic Melosira populations elsewhere is briefly discussed.

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