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Drainage enhancement effects on a waterlogged Rhode Island (USA) salt marsh
Citation:
Raposa, K., R. Weber, W. Ferguson, J. Hollister, R. Rozsa, N. Maher, AND A. Gettman. Drainage enhancement effects on a waterlogged Rhode Island (USA) salt marsh. Estuarine Coastal and Shelf Science. Elsevier Science Ltd, New York, NY, 231:106435, (2019). https://doi.org/10.1016/j.ecss.2019.106435
Impact/Purpose:
Salt marshes are prone to waterlogging for a variety of reasons. This may become more prevalent as sea levels rise. To better manage salt marshes and increase their resilience, managers need proven methods for reducing waterlogged conditions on marshes. Traditional approaches (e.g., ditch digging) have been used to reduce waterlogging, but little information exists evaluating the effectiveness of this approach. We provide this information with an experimental approach on a waterlogged marsh in Rhode Island and dug several ditches on the marsh and monitored the response over several years. The height of the marsh surface dropped 3 years after the ditches were added and previously bare portions of the marsh had revegetated. The impact of this work is that it appears that enhancing drainage on a marsh may help improve biological condition of the marsh through revegetation, but will likely not provide significant protection against future sea level rise.
Description:
Drainage enhancement (e.g., ditch digging, open-marsh water management, runnelling) has long been used to reduce tidal marsh soil waterlogging and surface ponding to promote salt hay production and mosquito control. Now it is also being used as a tool to enhance marsh resilience to sea-level rise despite a lack of studies that evaluate its effectiveness as an intervention approach. We therefore conducted a controlled field experiment to evaluate short-term responses to drainage enhancement of a waterlogged Rhode Island (USA) salt marsh. Drainage enhancement elicited rapid physical changes including declines in water levels and marsh elevation, but biological communities were largely unaffected. In some locations of the marsh, surface inundation duration declined from > 75% to 3-10% and low water levels dropped by 20 cm. Elevations across the marsh increased 5 mm one year after drainage enhancement but dropped to 11 mm below initial conditions after three years. The decline in elevation varied among habitats, with the greatest decline (18 mm) found in areas dominated by Spartina alterniflora and/or bare ground. Vegetation communities were unchanged, but areas that were initially bare had fully revegetated after three years. Drainage enhancement also had no effects on bird communities or marsh sparrow (Ammodramus spp.) density. Our study provides evidence that drainage enhancement can relieve marsh waterlogging and help bare areas quickly revegetate, without any apparent adverse effects to existing biological communities. At the same time, it can induce a loss of marsh platform elevation that has the potential to offset declining water levels and inhibit high marsh enhancement. Finally, unanticipated findings from our study provide evidence that drainage enhancement can increase marsh crab abundance and impacts, and that the effects of larger-scale drivers, such as sea-level rise, may predominate over localized responses to drainage enhancement itself.
