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Albemarle-Pamlico NAWQA

Salinity Variation in an Estuary Used for Oyster Cultivation in Southeastern North Carolina During the Passover of the Eye of Hurricane Bertha

Douglas A. Harned, Douglas J. Newcomb, Edward T. Hudson, and Jay F. Levine

person in boat near a series of rectangular floatsOyster garden, Alligator Bay, NC.


In 1995 a salinity monitor was established to help demonstrate the feasibility of oyster cultivation using floating trays and bags in Alligator Bay behind the barrier island of Topsail Island N. C. This plot is monitored on an hourly basis for specific conductance and temperature. The 2.6-square-kilometer area of Alligator Bay is bisected by the intracoastal waterway, the primary source of saline water for the Bay, and is fed by Mill Creek, a 15.4-square-kilometer freshwater drainage.

Satellite image of Hurricane Bertha with North Carolina and Eastern US Coast outlines superimposed in greenHurricane Bertha off of North Carolina Coast (NOAA image)

The monitor was in operation during the passover of Hurricane Bertha on July 12, 1996. The eye of Hurricane Bertha passed over Alligator Bay at approximately 5:30 p.m. with maximum sustained winds of 168 kilometers per hour. A storm surge of 2 to 3 meters caused overwash of the barrier island and rainfall of 12 to 20 cm caused extensive flooding. High-water marks on the barrier island, with land-surface elevations near 2 meters, were recorded near overwash areas at 2.5 meters.

The influx of fresh and saltwater drove rapid changes in salinity in Alligator Bay. Surficial salinity at the monitor, which had a mean concentration of 28 parts per thousand (ppt) during the previous 3 weeks, dropped within a few hours to about 10 ppt. Daily variability also showed an increased range from 2 ppt salinity before the storm to around 7 ppt salinity.

On September 5, 1996 the eye of Hurricane Fran made landfall 60 kilometers south of the monitoring site with maximum sustained winds of 200 kilometers per hour (125 mph). On the barrier island for Alligator Bay 80 percent of the structures were damaged or destroyed. A storm surge of 4 to 5 meters caused extensive overwash of saltwater over the island and raised the water level of Alligator Bay approximately 2 meters. The oyster cultivation racks were broken up during the hurricane and the salinity monitor sank to the bottom of the Bay.

Doug Harned, in waders, holding a batterned salinity monitor out of the waterSalinity monitor that survived both hurricanes.

Although the influx of saltwater from overwash of the barrier island was extensive, the pattern in salinity variation was similar to that observed for Hurricane Bertha. The drop in salinity from 32 ppt to 5 ppt was more precipitous than that for Bertha. Daily variability was also greater, showing an initial range of 10 ppt for the first few days, probably due to the mixing of saltwater from overwash and continued inputs of fresh water from rainfall. Salinity concentrations in Alligator Bay remained relatively unstable for a full week after the storm, due in large part to continued rainfall in the days after the passing of the hurricane.

Water quality effects of Hurricane Bertha and Fran at an estuarine station near New Bern were reported by Bales and Childress (1996). In this estuarine station the salinity initially increased after the hurricane passed and then sharply decreased with the influx of fresh water from the drainage basin upstream. The hurricanes also caused a loss of stratification of surface and bottom waters resulting in an initial short-term increase and then a long-term decrease in dissolved oxygen concentrations.

The larger volume and rapid input of fresh rainwater had a greater influence on salinity than saltwater overwash during both hurricanes. Oysters require salinities ranging from 10 to 32 ppt to survive. An optimal range for oyster growth is about 16 to 26 ppt. The salinities recorded during Hurricane Bertha and Fran are not optimal for oyster growth or survival.

About 15 people in a boatEd Hudson's Aquaculture class at Dixon High School.

The monitoring of salinity and temperature at Alligator Bay is supported by the U. S. Geological Survey National Water Quality Assessment Program (NAWQA). NAWQA is designed to integrate chemical, physical, and biological data to assess the status and trends in the Nation's water quality at local, regional, and national levels. The monitoring is done in cooperation with Ed Hudson of Dixon High School, Sneads Ferry N.C. and Jay Levine of the North Carolina State University School of Veterinary Medicine.


Bales, J.D. and Childress, C.J.O., 1996, Aftermath of Hurricane Fran in North Carolina—Preliminary data on flooding and water quality: U.S. Geological Survey Open-File Report 96-499, 6 p.

Harned, D.A., Newcomb, D.J., Hudson, E.T., and Levine, J.F., 1996, Salinity variation in an estuary used for oyster cultivation in Southeastern North Carolina during the passover of the eye of Hurricane Bertha [abs.]: Transactions of the American Geophysical Union 1996 Fall Meeting, December 1996, San Francisco, California, EOS, v. 77, no. 46. logo  Take Pride in America button

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