SEASONAL-VARIATION IN THE REGULATION OF PHYTOPLANKTON BY NITROGEN ANDGRAZING 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 an nual peak in ambient NH4+ concentration. This relationship suggests th at phytoplankton population growth during the summer bloom is Limited by factors other than nutrient supply, because NH4+ is the major inorg anic nitrogen source available to phytoplankton in North Inlet, and ph osphorus should not be Limiting (N:P is generally similar to 7). We te sted the hypothesis that phytoplankton population growth during the bl oom was controlled by grazing. Natural samples were incubated in treat ments designed to differentiate between nutrient and grazing effects, and time-course changes in total phytoplankton biomass and phototrophi c community composition were followed. Marked seasonal differences wer e observed in the relative contribution of pico-, nano-, or microplank ton to phytoplankton community biomass, as well as the mechanisms cont rolling phytoplankton population growth. During the summer bloom, phot otrophic picoplankton (mostly Synechococcus spp.) and nanoplankton (mo stly flagellates) were relatively abundant, and phytoplankton populati on 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 dil ution was relatively minor, while NH4+ addition significantly stimulat ed the growth of various phytoplankton groups and total chlorophyll. T he results indicate a seasonal transition in microbial food-web trophi c structure and regulation in North Inlet estuary. During the summer, microzooplankton grazing is an important factor regulating phytoplankt on population growth during the nanoflagellate-prevalent bloom, wherea s in the winter, a diatom-dominated community is limited by nutrient s upply.