Study: Hurricanes Left Lasting Effects On Coastal Ecosystems
The hurricanes that hit North Carolina over the last 10 years left behind ecological effects that lasted long after flood waters receded, according to a study by marine scientists from the University of North Carolina at Chapel Hill (UNC) and other institutions.
Effects such as enhanced growth conditions for potentially harmful algae, low oxygen levels in the bottom waters, and declines in some 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 (http://www.marine.unc.edu/Paerllab.
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 said.
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.
The study, announced on May 26, 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 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 said. 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-induced stress," Paerl said.
The study also found a surprising trend: since 1996, the start of the elevated hurricane period, the annual amount of nitrogen released into the Neuse River Estuary has actually decreased, even though North Carolina didn't mandate a reduction in nitrogen releases for this system until the late 1990s. Nitrogen inputs control the growth of algae, and excessive nitrogen has been blamed for excessive algal growth or "blooms," a major cause of water quality degradation.
"The elevated hurricane activity may be affecting the manner in which the Neuse River Estuary's watershed is processing its nitrogen load," Paerl said. "We're not sure how, but Mother Nature may be helping us during the hurricane years in terms of reducing the load. The loads may go back up once we get into a quieter storm period."
"Overall, hurricane cycles appear to be accompanied by large-scale, long-term changes in the manner by which nutrients are processed on impacted land and in receiving water," Paerl said. "Nutrient management plans and controls need to incorporate the impacts of these cycles."
"What this is telling us is that we need to stay the course with long-term monitoring of the efforts and benefits of reducing nitrogen and other nutrient inputs," Paerl said. "This study points out that it is easy to be fooled into thinking that relatively short-term declines in nutrient input trends are entirely due to human nutrient input reduction."
The study is based on two long-term monitoring programs supported by North Carolina's Department of Environment and Natural Resources: ModMon, in which scientists have modeled and monitored the Neuse River Estuary since 1994, and FerryMon, an automated water quality monitoring system aboard the North Carolina Dept. of Transportation ferries crossing the Neuse Estuary and Pamlico Sound since 2000.
"The results point to the importance of these long-term monitoring programs," Paerl said. "They're the annual and longer-term checkup that we have for these important systems."