Hydrological Management for Submersed Aquatic Vegetation in South Carolina Coastal Impoundments

We conducted a field experiment to test effects of complete drawdown to dried substrates versus partial (shallow water, 0–10 cm) drawdown on widgeongrass ( Ruppia maritima ) and other native submersed aquatic vegetation (SAV) biomass (dry) in managed brackish tidal impoundments (hereafter, impoundments) in the Ashepoo, Combahee, and Edisto Rivers Basin, South Carolina, USA. We sampled SAV in 20 impoundments (complete drawdown, n = 8; partial drawdown, n = 12) during August 2016, November 2016, January 2017, and April 2017. We detected a drawdown effect on SAV biomass for August 2016 samples before Hurricane Matthew in October 2016, which subsequently confounded drawdown treatment effects. Mean SAV biomass in August 2016 was nearly 4 times greater in partially drawndown ( = 35.98 g/m 2 ± 6.05 [SE]) than completely drawndown impoundments ( = 9.52 g/m 2 ± 7.41). We suggest partial drawdowns to promote SAV biomass in coastal South Carolina and other south Atlantic brackish wetlands. We also encourage managers to conduct complete drawdowns periodically to reduce flocculent soils and organic matter and promote rooting by SAV. Our results may aid the Atlantic Coast Joint Venture of the North American Waterfowl Management Plan to estimate potential energetic carrying capacity of brackish impoundments in South Carolina and elsewhere as applicable for wintering ducks and American coots ( Fulica americana ), and to illustrate the effect of a hurricane that decreases SAV biomass in impounded wetlands.