May 8, 2001 CHAPEL HILL - In as few as six weeks in late summer and early autumn 1999, three major hurricanes -- Dennis, Floyd and Irene -- walloped and partially drowned great swaths of eastern North Carolina under more than three feet of rain. Floodwaters at some sites rose more than 20 feet.
Besides their human toll, the storms powerfully altered the nation's largest lagoonal estuary, Pamlico Sound, new studies show. They flushed unprecedented amounts of nutrients and sediments into this key Mid-Atlantic fisheries nursery, sickening and killing countless thousands of fish and shellfish and greatly diluting the shallow waterway's natural saltiness.
The studies, published as a single paper in the May 8 issue of the Proceedings of the National Academy of Sciences, showed floodwaters displaced three-fourths of the sound's volume. Salinity also declined by two-thirds, and the sound's annual intake of nitrogen jumped by at least 50 percent in the aftermath. "Within six weeks, the entire water content of Pamlico Sound was replaced by the flood," said Dr. Hans W. Paerl, Kenan professor at the University of North Carolina at Chapel Hill's Institute of Marine Sciences and lead author of the paper. "Normally, that would take a year because the sound has only four small inlets restricting exchange with the sea."
Freshwater algae and higher organisms that normally live far up in the freshwater tributaries feeding the Pamlico Sound were driven far down into the sound, Paerl said. This was accompanied by a period of low-oxygen conditions in the sound's bottom waters, restricting habitat for shellfish and finfish." Resident crab, shrimp and fish communities either scurried out of the system, took up residence near the inlets or, in some cases, just died.
"While extensive low-oxygen conditions were documented during fall 1999, we didn't see equivalent low oxygen during the summer of 2000 because that spring and summer were very windy, keeping the waters well-mixed," the scientist said. "Those windy periods were kind of a blessing."
But because much of the load of nitrogen and other nutrients washed into the sound remains, insufficient oxygen could become a problem this summer if winds are minimal and stagnation occurs, he said. Over-abundant algae and other microorganisms that thrive in nitrogen-rich waters can produce low-oxygen conditions that suffocate finfish and shellfish.
Co-authors of the new paper are Dr. Jerad D. Bales of the U.S. Geological Survey in Raleigh; Dr. Larry W. Ausley of the N.C. Department of Environment and Natural Resources; Dr. Christopher P. Buzzelli, a postdoctoral fellow at UNC; Dr. Larry B. Crowder, professor of marine sciences at Duke University; Lisa A. Eby, a graduate student of Crowder's; UNC graduate students John M. Fear and Benjamin L. Peierls; Dr. Tammi L. Richardson, formerly of UNC and now at Texas A&M University, and Dr. Joseph S. Ramus, professor of marine biology at Duke.
"This was a team of hard-working, multi-talented investigators who just jumped in and got important early data on storm effects on the system," Paerl said. "They include hydrologists, marine biologists and water quality specialists."
One surprise of the pooled research was how sensitive the Pamlico Sound system was to nutrient fertilization, he said. Another surprise was that the researchers found no devastating negative impact on fish in the system, just significant localized impacts.
Floodwaters affected the state's blue crab fishery most strongly, researchers found. Neuse River fishermen reported reduced catches beginning last May. Sampling last summer in the Neuse River estuary showed blue crabs down by at least 90 percent from the same periods in 1997, 1998 and 1999. Fishermen also reported smaller oyster and clam catches in affected areas.
Catches of many species, such as croaker, spot, bay anchovy and shrimp, dropped by 50 percent or more in the estuary over the previous year, Paerl said. In the western part of the sound, however, catches of some species tripled or more since the more mobile animals moved there from out of the estuaries in response to the fresh water influx.
Also, Dr. Edward Noga, a professor at N.C. State University's College of Veterinary Medicine, found that by late October, 1999 between 10 percent and 20 percent of pinfish, spot and croaker suffered lesions, sores or sloughing skin, and between 50 percent and 70 percent showed signs of systemic bacterial infections. A year earlier, the incidence of external sores in the Neuse River estuary was 0.18 percent in sport and 0.14 percent in croaker.
Overall, Pamlico Sound appears to be rebounding well from the major storms, Paerl said. "The real question is -- if we're going to have 20 or more years of more frequent hurricanes as our meteorologist friends predict -- how many times can the system bounce back in the face of high fishing pressure?" he said.
Support for the studies came from the National Oceanic and Atmospheric Administration, the N.C. Sea Grant Program, the UNC Water Resources Research Institute, the U.S. Department of Agriculture, the U.S. Environmental Protection Agency, the N.C. Department of Environment and Natural Resources and the U.S. Geological Survey.
Both the UNC Institute of Marine Sciences and the Duke University Marine Lab provided ship time to the scientists.
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