Production of submersed aquatic vegetation (SAV) is promoted for waterfowl forage through hydrological management in brackish tidal impoundments along the south Atlantic coast, USA. This management also promotes production of aquatic invertebrates as food resources for many bird species. We conducted a field experiment to compare effects of traditional complete drawdown to fissure substrates versus a novel partial drawdown (i.e., mudflat to 10 cm depth) on aquatic invertebrate biomass in impounded and non-impounded tidal wetlands in the Ashepoo, Combahee, and Ed- isto Rivers (ACE) Basin, South Carolina. We sampled 20 randomly selected impoundments (complete drawdown, n = 8; partial drawdown, n = 12) and adjacent non-impounded tidal marsh across three properties in August 2016, November 2016, January 2017, and April 2017. Partially drawn-down impoundments contained 4–13 times greater benthic invertebrate biomasses than complete drawdown impoundments in August 2016, November 2016, and April 2017. Benthic invertebrate biomass in complete and partial drawdown impoundments was five times greater than in non-impounded tidal marshes in January 2017. Total invertebrate biomass was 3–15 times greater in partial than complete drawdown impoundments in August 2016, November 2016, and April 2017. We also detected a significant positive association of total invertebrate biomass with SAV biomass across all sampling periods. Dabbling duck energetic use-days (EUDs), based on combined SAV and total invertebrate biomasses, were three times greater in partial than complete drawdown impoundments across sampling periods. We suggest annual partial drawdowns to increase invertebrate and SAV biomasses in brackish impoundments for ducks and other waterbirds but acknowledge need for periodic complete drawdowns to consolidate substrates.
