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The recently described toxic dinoflagellate Pfiestena piscicida and morphologically similar Pfiesteria-like dinoflagellates have become a major water quality issue with possible fish mortality and reported human health implications. The linkages between accelerated nutrient loading, eutrophication, and the proliferation of this group of dinoflagellates, however, are not well established for natural Systems. Phytoplankton pnmary production may provide a key link between nutrient inputs and potential outbreaks of Pfiesteria-like biflagellated zoospores in the Neuse River Estuary, North Carolina. The impacts of nutrient (NO3- -nitrogen and PO43- -phosphorus) supply rates, sediment-water column exchange, water column mixing, and phytoplankton prey on the abundance of Pfiesteria-like non-toxic biflagellated zoospores were examined seasonally over 18 mo in a region of the Neuse River Estuary where fish-kills attnbuted to P piscicida have been reported. Phytoplankton community responses to the manipulated vanables indicated that biomass and productivity were consistently N limited. Dominant phytoplankton taxa, including chlorophytes, diatoms, and cyanobacteria, exhibited significant biomass increases in response to N (as NO3-) additions. Phosphate (as PO43- ) enrichments did not additionally influence the relative growth and abundance of individual algal groups. Pfiesterialike zoospores did not exhibit significant increases in abundance in response to mixing, sediment, or nutnent-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 phytoplankton biomass and production in the estuary, suggesting that the source of organic nutrition supporting growth is likely phytoplankton based. Lowering of phytoplankton growth and bloom potentials through proposed nutrient-input reduction strategies should translate into broad-based water quality improvement, including declines in the frequency and magnitudes of nuisance algal blooms, 02, depletion, and associated fish and shellfish mortality in the Neuse River Estuary.

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