Three sequential hurricanes, Dennis, Floyd, and Irene, affected coastal North Carolina in September and October 1999. These hurricanes inundated the region with up to 1 m of rainfall, causing 50-to 500-year flooding in the watershed of the Pamlico Sound, the largest lagoonal estuary in the United States and a key West Atlantic fisheries nursery. We investigated the ecosystem-level impacts on and responses of the Sound to the floodwater discharge. Floodwaters displaced three-fourths of the volume of the Sound, depressed salinity by a similar amount, and delivered at least half of the typical annual nitrogen load to this nitrogen-sensitive ecosystem. Organic carbon concentrations in floodwaters entering Pamlico Sound via a major tributary (the Neuse River Estuary) were at least 2-fold higher than concentrations under prefloodwater conditions. A cascading set of physical, chemical, and ecological impacts followed, including strong vertical stratification, bottom water hypoxia, a sustained increase in algal biomass, displacement of many marine organisms, and a rise in fish disease. Because of the Sound's long residence time (Ϸ1 year), we hypothesize that the effects of the short-term nutrient enrichment could prove to be multiannual. A predicted increase in the frequency of hurricane activity over the next few decades may cause longer-term biogeochemical and trophic changes in this and other estuarine and coastal habitats.

Using shipboard bioassays, w e examined the roles rainfall, individual and combined nutrients play in accelerating primary production in coastal, Gulf Stream and pelagic (Sargasso Sea) locations in the North Atlantic Ocean off North Caroli.na, USA, from 1993 to 1995. Photosynthetic CO, fixation and net chlorophyll a (chl a) production were measured In replicated bioassays to assess individual and combined impacts of d~fferent constituents of atmospheric deposition, including natural rainfall, a synthetic rain mix, dissolved inorganic nitrogen (DIN; NH,', NO3-), dssolved organlc nitrogen (DON; urea), phosphorus (POA3-) and iron (as EDTA-chelated and unchelated FeC13). Natural rainfall and DIN additions most often stimulated CO2 fixation and chl a production, but frequencies and magnitudes of biostimulation, relative to controls, varied between these indicators. Spatial differences in the types and magnitudes of stimulation were also observed. When added in equimolar amounts, NH.,' was, at times, more stimulatory than NO<. The NOT stimulation was significantly enhanced by Fe-EDTA. Urea was marginally stimulatory at the coastal location. Pod3-was never stimulatory. Fe-EDTA and EDTA by themselves stimulated production only at the offshore locations, suggesting increased Fe limitation with increasing distance from land. Synthetic rain, which contained both sources of DIN, but not Fe, generally proved less stimulatory per unit N than natural rainfall. Results lndlcate a broad sensitivity of these waters to N additions, which in the case of NOT are enhanced by Fe-EDTA. At all locations, the h~g h level of stimulation of primary production attributable to natural rain may be due to the supply of both DIN and CO-limiting nutnents (e.g. Fe), contributing to the eutroph~cation potential of waters downwind of urban, industrial and agricultural emissions.

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