Seasonal variation in the regulation of phytoplankton by nitrogen and grazing in a salt-marsh estuary

In North Inlet, a tidally dominated salt-marsh estuary near Georgetown, South Carolina, the summer chlorophyll maximum correlates with an annual peak in ambient NH4+ concentration. This relationship suggests that phytoplankton population growth during the summer bloom is Limited by factors other than nutrient supply, because NH4+ is the major inorganic nitrogen source available to phytoplankton in North Inlet, and phosphorus should not be Limiting (N:P is generally similar to 7). We tested the hypothesis that phytoplankton population growth during the bloom was controlled by grazing. Natural samples were incubated in treatments designed to differentiate between nutrient and grazing effects, and time-course changes in total phytoplankton biomass and phototrophic community composition were followed. Marked seasonal differences were observed in the relative contribution of pico-, nano-, or microplankton to phytoplankton community biomass, as well as the mechanisms controlling phytoplankton population growth. During the summer bloom, phototrophic picoplankton (mostly Synechococcus spp.) and nanoplankton (mostly flagellates) were relatively abundant, and phytoplankton population growth was unaffected by NH4+ addition, but was greatly stimulated by dilution that reduced microzooplankton grazing pressure. During the winter, when diatoms dominated the phytoplanktan, the response to dilution was relatively minor, while NH4+ addition significantly stimulated the growth of various phytoplankton groups and total chlorophyll. The results indicate a seasonal transition in microbial food-web trophic structure and regulation in North Inlet estuary. During the summer, microzooplankton grazing is an important factor regulating phytoplankton population growth during the nanoflagellate-prevalent bloom, whereas in the winter, a diatom-dominated community is limited by nutrient supply.