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Long Bay Hypoxia Working Group

In 2004, anglers were startled by unusually large catches of flounder in the waters off the Myrtle Beach area, which oceanographers call Long Bay, in the northern coastal ocean of South Carolina. The problem with this flounder “jubilee” was that low-oxygen levels in the water had created a “dead zone” that drove fish toward the shoreline. What caused this low-oxygen—or hypoxic—event in the first place?

“The 2004 hypoxia event was a surprise,” said Denise Sanger, then assistant director for research and planning with the S.C. Sea Grant Consortium. “We had not seen this kind of oceanographic dead zone in South Carolina before.”

About the Project

South Carolina resource managers and scientists, including Sanger, assembled a Long Bay Working Group to collaborate on research efforts and understand the causes of the hypoxic event.

In a series of research projects sponsored by the S.C. Sea Grant Consortium, S.C. Department of Health and Environmental Control-Office of Ocean and Coastal Resource Management , and S.C. Department of Natural Resources , scientists studied physical, biological, chemical, and geological coastal-ocean processes in Long Bay.

It seemed that a series of oceanographic conditions in Long Bay (oscillatory wind stress, upwelling, and hot summer weather) in 2004 stratified the water column near the coast, causing the flounder jubilee. Cold water along the ocean bottom was not mixing with warmer surface water, reducing oxygen levels. These conditions, plus the contribution of nutrients in stormwater runoff and groundwater discharge, apparently led to low-oxygen events in Long Bay.

“We have learned a great deal about the Long Bay system,” said Sanger. “In recent years, we have not observed another long-term hypoxic event like the one in 2004. But periodically we are seeing additional low dissolved-oxygen conditions in this environment, and that’s unusual. The levels observed are similar to low dissolved-oxygen that we observe in the naturally stressful, small estuarine headwater creeks. Now we’re finding these conditions in Long Bay just beyond the surf zone, despite strong currents and waves.”

Goals of the Research

The goal of the S.C. Sea Grant Consortium research effort was to develop tools for future use in forecasting hypoxic events in Long Bay. The research results will be of interest to coastal and fishery managers and local communities.

Consortium researcher George Voulgaris of the University of South Carolina (USC) developed a high-resolution numerical circulation model for Long Bay. The model is being used to identify the physical scenarios under which low-oxygen events can occur.

In a related study, Eric Koepfler and his colleagues at Coastal Carolina University (CCU) and USC evaluated the potential and relative roles of marine and terrestrial factors that can affect oxygen levels in Long Bay.

The two studies helped provide insights into threshold conditions of future hypoxic events. And they made it possible to identify relative significance of oceanic conditions and human-made sources of nutrients. This information was been identified as critically important by the state’s coastal-zone management program, which funded the first years of these studies.

Richard Viso of CCU and a student pump up groundwater from a beachfront area, measuring parameters such as dissolved oxygen and nutrients. Photo: S.C. Sea Grant Consortium.

The REMUS AUV was deployed in near-shore waters to collect dissolved oxygen (DO), current speed and direction, temperature, and salinity. Photo: S.C. Sea Grant Consortium.

Additional Studies

Richard Viso of CCU conducted research to identify groundwater seep locations along the shelf waters of Long Bay. The information will enable researchers to conduct fieldwork to quantify submarine groundwater discharge, and the contribution of that discharge to water-quality issues, a valuable component to understanding the nutrient fluxes and pollutant transport.

These research projects fostered additional efforts, including research by Viso to evaluate the contribution of groundwater discharge to water-quality issues and two pilot studies using autonomous underwater vehicles (AUVs) to validate and expand the findings of Voulgaris and Koepfler.

In addition, the S.C. Department of Natural Resources funded a study by Susan Libes of CCU to monitor the surface and bottom dissolved-oxygen levels, among other parameters, at Apache Pier on the Grand Strand. This information is providing continuous measurements of the conditions in the area, which researchers have not had in the past.