Responses of phytoplankton and Pfiesteria-like dinoflagellate zoospores tonutrient enrichment in the Neuse River Estuary, North Carolina, USA

The recently described toxic dinoflagellate Pfiesteria piscicida and morpho logically similar Pfiesteria-like dinoflagellates have become a major water quality issue with possible fish mortality and reported human health impli cations. The linkages between accelerated nutrient loading, eutrophication, and the proliferation of this group of dinoflagellates, however, are not w ell established for natural systems. Phytoplankton primary production may p rovide a key link between nutrient inputs and potential outbreaks of Pfiest eria-like biflagellated zoospores in the Neuse River Estuary, North Carolin a. The impacts of nutrient (NO3--nitrogen and PO43--phosphorus) supply rate s, sediment-water column exchange, water column mixing, and phytoplankton p rey on the abundance of Pfiesteria-like non-toxic biflagellated zoospores w ere examined seasonally over 18 mo in a region of the Neuse River Estuary w here fish-kills attributed to P, piscicida have been reported. Phytoplankto n community responses to the manipulated variables indicated that biomass a nd productivity were consistently N Limited. Dominant phytoplankton taxa, i ncluding chlorophytes, diatoms, and cyanobacteria, exhibited significant bi omass increases in response to N (as NO3-) additions. Phosphate (as PO43-) enrichments did not additionally influence the relative growth and abundanc e of individual algal groups. Pfiesteria-like zoospores did not exhibit sig nificant increases in abundance in response to mixing, sediment, or nutrien t-addition treatments. Seasonally, the number of Pfiesteria-like zoospores was positively correlated with phytoplankton biomass and productivity. The abundance of Pfiesteria-like zoospores followed general trends in phytoplan kton biomass and production in the estuary, suggesting that the source of o rganic nutrition supporting growth is likely phytoplankton based. Lowering of phytoplankton growth and bloom potentials through proposed nutrient-inpu t reduction strategies should translate into broad-based water quality impr ovement, including declines in the frequency and magnitudes of nuisance alg al blooms, O-2 depletion, and associated fish and shellfish mortality in th e Neuse River Estuary.