Hurricanes’ full havoc yet to be felt

Sediment-rich floodwaters gushing from southern Pamlico Sound after Hurricane Floyd are swept northeast (arrow) by the Gulf Stream in this Sept. 23, 1999, image. ©1999 Orbital Imaging Corporation

Forget about a mere one-two punch. When Hurricanes Dennis, Floyd, and Irene pummeled North Carolina in the fall of 1999, they delivered a three-punch combination that for years to come may disturb coastal ecosystems there and disrupt fishing in the Atlantic Ocean.

The three hurricanes struck the eastern United States within a 6-week period. Along their paths, they dumped a total of 1 meter of rainfall upon areas that drain into North Carolina’s Pamlico Sound. That body of water—a lagoon protected by the barrier islands rimming the state’s shore—is a major fish and shellfish nursery for the entire Atlantic coast, says Hans W. Paerl, a marine ecologist at the University of North Carolina’s Institute of Marine Sciences in Morehead City. These spawning grounds ultimately are the source of more than 90 percent of the fish caught by North Carolina’s commercial fishers, he notes.

As sediment- and nutrient-rich floodwaters spilled from the North Carolina piedmont in the wake of the hurricanes, they radically altered the environment in Pamlico Sound, Paerl notes. More than 75 percent of the sound’s brackish water was flushed into the Atlantic Ocean as fresh water from Piedmont rivers filled the sound. The resulting spike of dissolved nutrients in the sound fed algae blooms. In the months following the storms, the concentration of algae rose to four to six times typical values and remained greater than normal more than a year later, Paerl says.

After the first bloom of algae died and sank, its decomposition sopped up much of the oxygen along the bottom of the sound for a month or so, significantly affecting all local life.

“The slower [the animals] were, the worse their predicament was,” says Paerl. At some sites, most oysters and clams died and blue crab populations dropped by 90 percent. Paerl’s team reports its findings in the May 8 Proceedings of the National Academy of Sciences.

There were few fish kills in the sound because the initial pulse of fresh water from Hurricane Dennis drove the fish into the Atlantic Ocean—right into the nets of the waiting fishermen. This fish stampede led to some of the best catches in years, but these commercial successes don’t indicate the hurricanes’ effects on upcoming generations of fish, notes coauthor Larry W. Ausley, a marine biologist at the North Carolina Department of Environment and Natural Resources in Raleigh.

Although the ecosystems in Pamlico Sound are resilient, it may take years for them to fully recover, and it may be a decade or more before the full effects on fish populations are seen, notes Ausley.

Other scientists in the region had been concerned that petroleum products, pesticides, and agricultural chemicals flushed into Pamlico Sound would end up trapped in the sediments there. Damian Shea, a toxicologist at North Carolina State University in Raleigh, says that the surge of floodwaters did, in fact, boost the concentrations of many such chemicals in the sound’s water for a couple of weeks. However, a program that monitored the sediments in the sound during the 18 months following the hurricanes showed that those increases didn’t translate into additional long-term contamination.

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