Instream swamps and their effect on dissolved oxygen dynamics within blackwater streams of the Georgia Coastal Plain

Abstract

Blackwater streams on the Georgian Coastal Plain are distinguished by violation of the state’s dissolved oxygen (DO) standard leading to TMDL development as required by the Clean Water Act. These streams are characterized by low slopes, high summertime temperatures, and extensive inundation of surrounding floodplains. Typically lasting from winter to early spring, the long inundation period creates a multitude of instream floodplain swamps that play a vital role in overall water quality. One of the hypothesized reasons for the relatively low DO levels in these systems is the slow movement or water and extended contact with underlying sediments. This dissertation investigated the role of these instream swamps on a watershed scale and focused specifically on the influences of sediment oxygen demand (SOD), the distribution of organic sediments and the role of hydrology on water column DO concentrations. Results support the idea of extended travel times through these instream swamps due to tortuous flow pathways and extensive transient storage. SOD values were measured well above previously published values and were correlated with increasing organic carbon content. Results show that organic sediments are widespread and become more prevalent in higher order streams. While DO dynamics are a complicated mix of natural and anthropogenic factors, instream swamps play a critical role in overall watershed oxygen dynamics and support the hypothesis that these systems are naturally low in DO. Further, these studies suggest that biota may be particularly vulnerable to any additional anthropogenically induced oxygen demand because of these already naturally lowered DO conditions.