Study examines hurricanes and coastal ecosystems

CHAPEL HILL, NC —   The spate of hurricanes that hit North Carolina over the last ten years left behind ecological effects that lasted long after flood waters receded, according to an upcoming study by marine scientists from East Carolina University and colleagues.

Effects such as increased growth of potentially harmful algae, low oxygen levels in the water, and declines in fisheries lasted as long as two to three years after a storm such as 1999’s Hurricane Floyd, said Dr. Hans Paerl, Kenan Professor of Marine and Environmental Sciences at UNC’s Institute of Marine Sciences in Morehead City.

The period of elevated hurricane activity that began in 1995 added yet another stressor to waterways already affected by man-made nutrient over-enrichment and degradation of fisheries habitat, Paerl said.

“With another above-normal hurricane season predicted for the north Atlantic this year, the study points to the need for adaptive ecosystem management approaches to accommodate these large-scale events over long time spans,” he added.

Paerl and colleagues detail these and other findings in a study to be published this fall in a special issue of Estuaries and Coasts, the journal of the Estuarine Research Foundation. Other authors from UNC’s Institute of Marine Sciences are postdoctoral researcher Dr. Lexia M. Valdes, research technician Alan R. Joyner, doctoral student Benjamin L. Peierls, assistant professor Dr. Michael F. Piehler, and professor and director Dr. Richard A. Luettich.

Additional authors are Dr. Stanley R. Riggs and Dr. Robert R. Christian from East Carolina University; Dr. Lisa A. Eby of the University of Montana; Dr. Larry B. Crowder and Dr. Joseph S. Ramus of the Duke Center for Marine Conservation and Duke University Marine Laboratory; Dr. Erica J. Clesceri of the Darling Marine Center at the University of Maine; and Dr. Christopher. P. Buzzelli of the National Oceanic and Atmospheric Administration.

The study shows that the highest-category storms don’t necessarily cause the most serious long-term ecological change. Instead, waters were most profoundly impacted by storms that brought heavy rains and inland flooding – like 1999’s Floyd and 1996’s Fran.

Such flooding contributes to incidences of increased vertical “stratification,” in which a layer of low-oxygen water is trapped on the bottom.

“When flooding delivers large amounts of freshwater on top of the heavier salt water of the estuaries, it causes the system to resist mixing so that the bottom water can’t breathe and exchange with the atmosphere,” Paerl said. “When that happens, the bottom water can rapidly run out of oxygen.” This low-oxygen water stresses and can even kill finfish and shellfish.

The flood-heavy hurricanes also increased algae growth. “In the case of Floyd in particular, which was a very large event, we had a period of at least a year of elevated algae growth in the Pamlico Sound and in the lower estuarine systems like the Neuse,” Paerl said. “It took about a year for that to come back to what we think is normal.”

Fran and Floyd also depressed fisheries. Blue crab catches, for example, were below average for at least three years following Hurricane Floyd.

“We also observed higher incidences of fish disease such as sores and lesions on fish, well into a year or longer after a hurricane hit,” Paerl says. Both low oxygen and low salinity caused by floodwaters can hurt fish health, the study notes.

“The good news is that, in general, fish health bounced back after this period of stress, but it is also important to recognize that stocks of numerous finfish and shellfish species are more vulnerable to additional stressors such as habitat disturbance and overexploitation during the period of post-hurricane.