Effects of climatic variability on phytoplankton community structure and bloom development in the eutrophic, microtidal, New River Estuary, North Carolina, USA

The roles of climatically driven freshwater flow, nutrient loading, and temperature on phytoplankton community biomass and composition were examined along a downstream transect within the New River Estuary, North Carolina from 2007 to 2010. This microtidal system has a history of eutrophication symptoms, including harmful algal blooms. Riverine discharge strongly controlled nutrient loading to the estuary. Except during very high flow conditions, loads were generally assimilated near the head of the estuary and low concentrations of dissolved inorganic nitrogen and phosphate, <2 and 0.4 μmol L−1, respectively, were found throughout most of the estuary. Total phytoplankton biomass (chlorophyll a) was maximal under moderate discharge conditions when flushing time was approximately four days and salinity was in the range of 3–18 psu. The observed non-monotonic relationship between phytoplankton biomass and flushing time reflects a balance between nutrient stimulation of phytoplankton biomass and advective losses associated with river flow. Pigment indicators of major phytoplankton taxa also revealed non-monotonic responses to flushing time with some significant inter-taxa differences in slopes and peak location. These differences suggest that flushing time or environmental covariates of flushing time are important determinants of community composition. We suggest these group-specific differences are due to advantages gained by phytoflagellates through vertical migration, ability of pico-cyanobacteria to exploit low residual nutrient concentrations, and subsidies to the diatom population via sediment resuspension events. In addition to effects of flow, pico-cyanobacteria and harmful raphidophyte taxa demonstrated positive relationships with temperature. Overall, results demonstrate a strong linkage between the phytoplankton community and two important climatic drivers, flow and temperature. The observed sensitivity of phytoplankton biomass to nutrient loads suggests that load reductions (or increases) are likely to decrease (or increase) total phytoplankton biomass in this system. However, phytoplankton community composition appears to be largely determined by environmental factors other than nutrient loading, particularly temperature, stratification and sediment resuspension.