Annual cycles of phytoplankton community-structure and bloom dynamics in the Neuse River Estuary, North Carolina

Abstract

The spatiotemporal distributions of major phytoplankton taxa were quantified to estimate the relative contribution of different microalgal groups to biomass and bloom dynamics in the eutrophic Neuse River Estuary, North Carolina, USA. Biweekly water samples and ambient physical and chemical data were examined at sites along a salinity gradient from January 1994 through December 1996. Chemosystematic photopigments (chlorophylls and carotenoids) were identified and quantified using high-performance liquid chromatography (HPLC). A recently-developed factor-analysis procedure (CHEMTAX) was used to partition the algal group-specific chlorophyll a (chl a) concentrations based on photopigment concentrations. Results were spatially and temporally integrated to determine the ecosystem-level dynamics of phytoplankton community-constituents. Seasonal patterns of phytoplankton community-composition changes were observed over the 3 yr. Dinoflagellates reached maximum abundance in the late winter to early spring (January to March), followed by a spring diatom bloom (May to July). Cyanobacteria were more prevalent during summer months and made a large contribution to phytoplankton biomass, possibly in response to nutrient-enriched freshwater discharge. Cryptomonad blooms were not associated with a particular season, and varied from year to year. Chlorophyte abundance was low, but occasional blooms occurred during spring and summer. Over the 3 yr period, the total contribution of each algal group, in terms of chl a, was evenly balanced, with each contributing nearly 20% of the total chl a. Cryptomonad, chlorophyte, and cyanobacterial dynamics did not exhibit regular seasonal bloom patterns. High dissolved inorganic-nitrogen loading during the summer months promoted major blooms of cryptomonads, chlorophytes, and cyanobacteria.