Identification of Coastal Hypoxia Mechanisms in Inner Shelf Waters of Long Bay, South Carolina
Eric Koepfler1a, Susan Libes1b, Joseph Bennett1b, Erik Smith2, Alan Lewitus3, Brent Lewis1a
1aCoastal Carolina University, Dept. of Marine Science
1bCoastal Carolina University, Burroughs and Chapin Center for Marine and Wetlands Studies
2University of South Carolina, Baruch Institute for Marine and Coastal Sciences
3Algal Ecology Laboratory
The factors which may be important in the development of coastal hypoxia in Long Bay can be described in a conceptual model where terrestrial inputs and offshore oceanic forcing are key drivers of nearshore nutrient and organic carbon dynamics that promote hypoxic conditions in the immediate nearshore waters of Long Bay. In this model, oxygen concentrations in the bottom waters are controlled by the balance between downward physical flux of oxygen across the pycnocline and the biological oxygen demand associated with heterotrophic respiration of the organic carbon pool. The organic carbon concentrations in the bottom water are a function of inputs associated with the deposition of organic matter produced in situ through primary production, which is itself a function of nutrient loading at both the local and regional scale, and inputs/deposition associated with regional river discharge and local-scale stormwater discharge including surficial groundwater flows. The downward physical flux of oxygen is a function of the oxygen concentration in the surface waters, which is itself a function of the production- respiration balance in the surface waters plus the magnitude of air-sea exchange, and the strength of pycnocline, which is a function of thermal and saline stratification resulting from freshwater input and regional scale oceanic conditions. Heterotrophic oxygen demand is located within the lower layer of the water column, relegating the benthic community to a largely negligible role in controlling bottom water oxygen concentrations. This model represents a consensus view developed during a September 2004 meeting convened following the July event, and as part of the original/initial research effort on the Long Bay hypoxia episode funded through SECOORA.
This research effort is structured around testing the relative roles of the various physical inputs and forcings of this conceptual model in the development of nearshore hypoxia in Long Bay. Given the documented hypoxia occurrence in 2004 and indications of prior events, and understanding the great negative consequences of continued and perhaps more extreme events, the project will study the mechanisms (natural and anthropogenic) that may contribute to nearshore hypoxia. Due to the lack of baseline information on the water quality and hydrography in Long Bay, the research plan takes a very focused and multi-staged approach to addressing questions concerning hypoxia in coastal South Carolina. Top priority will be given to characterizing seasonally average water quality and hydrographic conditions in the nearshore, onshore and terrestrial end members. Some process information may be gleaned from these data by correlating results to spatial features, such as proximity to freshwater sources, to hydrographic events, such as seasonal shifts in density stratification, and other phenomena, such as the seasonal shifts in temperature, light availability and terrestrial water discharges. These results will be used to design and perform laboratory experiments that simulate some of the likely phenomena influencing rates of dissolved oxygen supply and demand to the nearshore. The goal of the study is to ascertain the conditions and specific mechanisms that promote the development of hypoxia in Long Bay.