Seasonal variations of size-fractionated phytoplankton along the salinity gradient in the York River estuary, Virginia (USA)

The dynamics of phytoplankton size structure were investigated in the freshwater, transitional and estuarine zones of the York River over an annual cycle. The contribution of large cells (microplankton, \u3e20 mu m) to total concentrations of chlorophyll a increased downstream during winter, whereas that of small cells (nanoplankton, 3-20 mu m; picoplankton,m) increased downstream during summer. In the freshwater region, the contribution of micro phytoplankton to total concentrations of chlorophyll a was significant during warm seasons (spring and summer) but not during colder seasons (winter), whereas the contribution of small-sized cells (especially picoplankton) increased during cold seasons. Temperature, light and high flushing rate appear to control phytoplankton community structure in the freshwater region. In the transitional region, nano-sized cells dominated the phytoplankton population throughout all seasons except during the spring bloom (April) when the chlorophyll a concentration of micro playtoplankton increased. Size structure in the transitional region is most likely regulated by light availability. In the mesohaline region, nano- and pico-sized cells dominated the phytoplankton population during the summer bloom, whereas micro-sized cells dominated during the winter bloom. Factors controlling phytoplankton community Size structure in the mesohaline zone may be riverine nitrogen input, temperature and/or advective transport from up-river. Based on these results, the spatial and seasonal variations in size structure of phytoplankton observed on the estuarine scale may be determined both by the different preferences for nutrients and by different light requirements of micro-, nano- and picoplankton. The results suggest that analyses of phytoplankton size structure are necessary to better understand controls on phytoplankton dynamics and to better manage water quality in river-dominated, estuarine systems