Physical Conditions during Hypoxic Events in Long Bay, South Carolina: 2006-2014

Hypoxia is commonly found in coastal areas and in enclosed or semi-enclosed basins due to both anthropogenic and natural factors. The effects of hypoxia can adversely influence the local economy and ecosystem by limiting or changing an environment’s biodiversity. Hypoxia occurs most often during the summer months, when increased solar radiation increases vertical stratification, and higher water temperatures reduce saturation concentrations for oxygen. Severe cases of hypoxia have been documented in Long Bay, South Carolina since June 2004. Hypoxia is usually attributed to eutrophication or coastal upwelling, though prior studies in this region suggest that the source of the low oxygen conditions in Long Bay may not be directly due to either of those causes. Water quality and atmospheric data, including water and air temperature, dissolved oxygen concentration, wind speed and direction, and salinity, as well as surface currents, are collected from both nearshore and offshore sensors in Long Bay for periods between 1-8 years depending on the sensor. In this study, hypoxic conditions are categorized as distinct events based on coastal dissolved oxygen thresholds and timing criteria. Corresponding physical parameters such as water temperature, salinity, and wind speed and direction for these events are used to examine relationships between physical signals and hypoxic conditions in the bay. These time series were analyzed primarily using cross correlation functions between the physical parameters and dissolved oxygen concentrations for all events. We find variability in physical conditions present during hypoxic events as well as consistencies between all events. In particular, low wind speed and an increased difference in water temperature between surface and bottom waters were observed consistently during hypoxic events. These conditions are consistent with limited mixing and appear to be necessary conditions for hypoxic events in the open embayment of Long Bay, SC.