Physical Conditions Associated with Hypoxia in Long Bay, SC

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 the environment’s biodiversity. Frequent cases of low dissolved oxygen have been documented in Long Bay, South Carolina since June 2004. Hypoxia is commonly attributed to eutrophication or coastal upwelling, though prior studies in this region are inconclusive as to whether the source of the low oxygen conditions in Long Bay may be attributed to either of those causes.

Water quality and atmospheric data have been 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 environmental properties and low oxygen conditions in the bay. These time series were analyzed primarily using cross correlation functions between the physical parameters and the dissolved oxygen concentrations for all observed 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 gradient in water temperature between surface and bottom waters were most consistently observed  during hypoxic events. These conditions suggest limited mixing and appear to be  co-occurring with low oxygen events in the open embayment of Long Bay, SC.