MEPS 461:15-30 (2012)  -  DOI: https://doi.org/10.3354/meps09796

ABSTRACT: Hypoxic episodes in the shallow nearshore waters of Long Bay, South Carolina, USA, an open coast embayment on the South Atlantic Bight, have been observed on a number of occasions during the summer months from 2004 to 2009, with the most pronounced ones occurring in July of 2004 and August and September of 2009. We present a synthesis of data that included time-series oxygen measurements, synoptic surveys of water quality and oxygen consumption rates, and circulation numerical modeling aimed at explaining these hypoxic events. Periods of hypoxia coincide with coastal upwelling conditions (i.e. winds from the southwest and colder bottom waters). Field flow data and numerical simulations confirmed that these conditions create onshore-directed bottom flows. The latter also showed that this onshore water intrusion can penetrate into very shallow waters (5 to 7 m depth) and is separated from the very shallow waters by a mixing front. Nutrient and organic matter concentrations, especially the particulate forms, were higher in the nearshore (~0.3 km from the coastline) compared to offshore (2 to 3 km from the coastline) waters, while dissolved oxygen concentrations tended to increase with distance from shore. Oxygen consumption rates via water-column respiration were highly variable (5.6 to 73.6 µg O₂ l⁻¹ h⁻¹), but showed significant correlations with spatial variability in particulate nutrient and organic matter concentrations. These observational and modeling results suggest that occurrences of low dissolved oxygen in Long Bay result from regional oceanographic processes that constrain water masses to the nearshore and thus prevent the wider dispersion of local terrestrial inputs. The resulting enhanced material concentrations stimulate heterotrophic oxygen demand, leading to hypoxia in the immediate nearshore waters.

KEY WORDS: Nearshore hypoxia · Upwelling · Respiration · Eutrophication · South Atlantic Bight