Seasonal variation in the regulation of phytoplankton by nitrogen and grazing in a salt‐smarsh estuary

Abstract

In North Inlet, a tidally dominated salt-marsh estuary near Georgetown, South Carolina, the summer chlorophyll maximum correlates with an annual peak in ambient NH,+ concentration. This relationship suggests that phytoplankton population growth during the summer bloom is limited by factors other than nutrient supply, because NH,+ is the major inorganic nitrogen source available to phytoplankton in North Inlet, and phosphorus should not be limiting (N : P is generally -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 pica-, 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 NH,' addition, but was greatly stimulated by dilution that reduced microzooplankton grazing pressure. During the winter, when diatoms dominated the phytoplankton, the response to dilution was relatively minor, while NH,' 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.

Use of Metabolic Inhibitors to Characterize Ecological Interactions in an Estuarine Microbial Food Web

“…The experiment was conducted three times. Substrates were initially deployed on 14 May, 13 July and 15 October 1996, which represented periods during the upslope, peak, and downslope, respectively; of the annual phytoplankton bloom in North Inlet [25]. For example, at Oyster Landing in 1996, mean chlorophyll-a concentrations measured bi-hourly over two tidal cycles varied from 7.8 ± 3.3 in mid-May to 17.9 ± 9.7 in mid-July to 8.8 ± 4.3 and 7.9 ± 2.5 in early and late October, respectively (data from the NOAA North Inlet/Winyah Bay National Estuarine Research Reserve monitoring program).…”

“…However, when attention is paid to time-course patterns, inhibitor effects can lend valuable qualitative information to interpreting trophic interactions as, for example, used here for comparing seasonal differences. The inhibitor concentrations used in this study were chosen based on previous demonstrations of effectiveness and specificity, including microscopic verification using North Inlet estuarine communities [24][25][26]42]. …”

“…More recent studies have also shown that bacteria and microzooplankton (ciliates, heterotrophic nanoflagellates) abundances during the summer in North Inlet were at the high end of those reported in the literature [26,45]. Because the major proportion of annual planktonic primary production in North Inlet occurs during the summer [25,44], knowledge of the trophodynamics of microbial communities during this period is important to understanding overall ecosystem function in this estuary.…”

“…Lewitus et al [25] stressed the importance of microbial loop dynamics to material and energy flux in North Inlet (Georgetown, South Carolina), a high salinity salt marsh estuary characterized by the predominant influence of tidal exchange in driving nutrient flow. Freshwater input to North Inlet through surface-or groundwater is restricted (3% of tidal volume; [30]), and ca.…”

“…The frequent tidal flushing of this shallow estuary (2.2 m maximal tidal amplitude) results in a highly variable nutrient pool, regulated primarily by the magnitude of flux through the mouth, exchange between the water column and the intertidal sediments, and trophic interactions inside the estuary [30]. Lewitus et al [25] characterized the annual phytoplankton bloom (peak in July-August) in North Inlet as a microbial loop, based on the predominance of phototrophic pico-and nanoflagellates, regenerated nitrogen sources (e.g., NH 4 ), and microzooplanktonic grazing control of phytoplankton population growth. More recent studies have also shown that bacteria and microzooplankton (ciliates, heterotrophic nanoflagellates) abundances during the summer in North Inlet were at the high end of those reported in the literature [26,45].…”