Long‐term temporal and spatial trends in phytoplankton biomass and class‐level taxonomic composition in the hydrologically variable Neuse‐Pamlico estuarine continuum, North Carolina, U.S.A.

Phytoplankton diagnostic photopigments in near‐surface waters (≈0.5 m) were identified and quantified by high‐performance liquid chromatography beginning in April 1994 in the Neuse River Estuary and in October 1999 in the Pamlico Sound, North Carolina. Photopigment concentrations were analyzed using ChemTax to determine the class‐specific biomass of the dominant phytoplankton groups. Long‐term annual and seasonal trends in phytoplankton biomass and composition were characterized along the river‐estuarine continuum and compared to river flow rates, which were variable because of droughts, uncharacteristic seasonal rainfall patterns, and, since 1996, an increase in the frequency of tropical storms and hurricanes. We tested the hypothesis that temporal and spatial patterns of phytoplankton biomass and composition were largely controlled by changes in river flow rate through associated changes in salinity and residence time or through physical transport and advection of phytoplankton classes with river flow along the estuary. Significant interannual, seasonal, and spatial variability in phytoplankton biomass and composition was observed and coincided with variability in river flow rates. The five dominant phytoplankton classes (Chlorophyceae, Cryptophyceae, Cyanobacteria, Bacillariophyceae, and Dinophyceae) were physically displaced downstream during periods of elevated river flow. Dinoflagellates were reduced in abundance during high flow conditions, especially in the upper Neuse River Estuary. The abundance of cyanobacteria was also reduced throughout the system during elevated river flow conditions, although chlorophytes were more abundant. Changes in hydrology can be a useful indicator of seasonal phytoplankton distribution and higher level compositional changes along hydrologic gradients in these and similar systems.