Unique Interactions Cause Long Bay “Dead Zones”
After twice in six years finding sizable “dead zones” in Long Bay, scientists now believe that an unusual interaction of forces caused these low-oxygen—or hypoxic—events. Waters that reach hypoxic levels are no longer able to sustain most animal life.
Long Bay (the coastal ocean from Cape Fear, N.C. to Winyah Bay, S.C.) experienced the first observed hypoxic event in the summer of 2004, and two additional events were observed in the summer of 2009.
Low-oxygen levels in 2004 caused a flounder jubilee in the Myrtle Beach area. The flatfish were plentiful and slow moving because of a zone of low-oxygen water that drove suffocating flounder toward the shoreline. In response to the 2004 event, a Long Bay Working Group (LBWG) of state and federal agency and academic partners was formed to study the phenomena and identify causes of low-oxygen problems.
“We believe that Long Bay’s hypoxic events are caused by an interaction of physical and biological processes that’s unique to the region,” said Denise Sanger, LBWG facilitator and assistant director for research and planning with the S.C. Sea Grant Consortium.
Physical oceanographic processes in Long Bay are naturally occurring but play a crucial role. In the summers of 2004 and 2009, strong southwesterly winds pushed cold, deep water from far offshore toward the Long Bay beachfront in a process called upwelling. These southwesterly winds and resulting upwelling were persistent, constraining a mass of colder water in the nearshore zone just seaward of the surf zone.
At the same time, hot summer weather days added to temperature stratification in the water column. That is, warm surface waters could not mix with cold waters at the bottom.
On the landward side, nutrients and organic matter draining off highly developed uplands is thought to be another important contributor to low-oxygen events. Tidal creeks (locally known as swashes), stormwater-discharge pipes, and groundwater all carry nutrients and organic matter into the coastal ocean.
Under typical conditions, this material is widely dispersed. But when constrained within Long Bay’s nearshore zone, the material stimulates bacterial activity. The bacterial communities consume oxygen at a faster rate than it can be replenished, resulting in hypoxia.
After the 2004 event, the LBWG established research and monitoring efforts. The LBWG continues to collect data critical to increasing our understanding of conditions in Long Bay. The scientific studies are funded and conducted by the S.C. Sea Grant Consortium, S.C. Department of Natural Resources, S.C. Department of Health and Environmental Control–Office of Ocean and Coastal Resource Management, University of South Carolina, and Coastal Carolina University, among others.
The Coastal Conservation Association, a recreational fishing group, and the Apache Pier Campground have also helped fund continuous monitoring sensors for oxygen levels at Apache Pier, which are critical to identifying and tracking hypoxic events. As a result, the researchers in 2009 could identify a relatively narrow zone affected by hypoxia, beginning just beyond the surf zone to about one mile offshore from N. Myrtle Beach to Surfside Beach.