Physical Conditions of Coastal Hypoxia in the Open Embayment of Long Bay, South Carolina: 2006–2014

Abstract

Episodic cases of low dissolved oxygen have been documented in Long Bay, SC, since June 2004. Hypoxia is commonly attributed to eutrophication or coastal upwelling, though prior studies in this urbanized region are inconclusive as to whether the source of the low oxygen conditions in Long Bay may be attributed to either of those causes. Since 2006, water property and atmospheric data are collected from both nearshore and offshore sensors in Long Bay for periods between 1 and 8 years depending on the sensor. In this study, hypoxic conditions are categorized as distinct events based on coastal dissolved oxygen concentration thresholds and timing criteria. These events are then further categorized based on event duration. Corresponding physical conditions 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 data are analyzed primarily using cross correlation functions between the physical parameters and the dissolved oxygen concentrations for low-oxygen events of various durations as well as irrespective of duration. We find variability in physical conditions present during hypoxic events as well as a few consistencies between all events. In particular, significant correlations between decreasing oxygen and decreasing wind speeds and increasing stable stratification occur regardless of event duration, but these relationships are often stronger for long-duration events (>24 h). These results suggest limited mixing co-occurs with low-oxygen events in the open embayment of Long Bay, SC.