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Blackwater rivers are common throughout the Atlantic coastal plain and water quality is heavily influenced by the flat topography, sandy soils and floodplain swamp forests. In the southern coastal plain of Georgia, streams regularly violate dissolved oxygen (DO) standards established by the Georgia Department of Natural Resources. Total Maximum Daily Load (TMDL) management plans must be developed for watersheds that are drained by DO-impaired streams but previous studies suggest DO may be naturally low. At nine sites throughout the region, eighteen passive nutrient diffusion periphytometers were deployed to determine if algal growth was nutrient and/or light limited. Periphyton biomass for treatments in the sun, measured as chlorophyll a, was significantly (p < 0.05) greater than corresponding treatments in the shade and algal growth was nutrient-limited at several sites where DO concentrations were well below regulatory standards. Factors other than algae may be responsible for low DO concentrations during summer.