Ecosystem Responses of a Tidal Freshwater Marsh Experiencing Saltwater Intrusion and Altered Hydrology
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Tidal freshwater marshes exist in a dynamic environment where plant productivity, subsurface biogeochemical processes, and soil elevation respond to hydrological fluctuations over tidal to multi-decadal time scales. The objective of this study was to determine ecosystem responses to elevated salinity and increased water inputs, which are likely as sea level rise accelerates and saltwater intrudes into freshwater habitats. Since June 2008, in situ manipulations in a Zizaniopsis miliacea (giant cutgrass)-dominated tidal freshwater marsh in South Carolina have raised porewater salinities from freshwater to oligohaline levels and/or subtly increased the amount of water flowing through the system. Ecosystem-level fluxes of CO2 and CH4 have been measured to quantify rates of production and respiration. During the first 20 months of the experiment, the major impact of elevated salinity was a depression of plant productivity, whereas increasing freshwater inputs had a greater effect on rates of ecosystem CO2 emissions, primarily due to changes in soil processes. Net ecosystem production, the balance between gross ecosystem production and ecosystem respiration, decreased by 55% due to elevated salinity, increased by 75% when freshwater inputs were increased, and did not change when salinity and hydrology were both manipulated. These changes in net ecosystem production may impact the ability of marshes to keep up with rising sea levels since the accumulation of organic matter is critical in allowing tidal freshwater marshes to build soil volume. Thus, it is necessary to have regional-scale predictions of saltwater intrusion and water level changes relative to the marsh surface in order to accurately forecast the long-term sustainability of tidal freshwater marshes to future environmental change.
KeywordsClimate change Primary production Ecosystem respiration Carbon dioxide Methane Waccamaw River, South Carolina
I thank Liana Nichols, Paul Kenny, Tom Marshall, Shan Deeter, Christina Mitchell, Leigh McCallister, Lindsey Koren, and Troy Washum for invaluable assistance in the field; Stephen Forehand for help constructing the flux chambers and collars; and Michelle, Isabella, and Ryleigh Neubauer and Dorothy Silvernail for help constructing the porewater samplers. Erik Smith and Amy Willman helped access weather data. I especially thank Bob Jewell, Mike Ammons, and the staff at Brookgreen Gardens for allowing and facilitating access to the study site. This research was supported by a grant from the University of South Carolina, Office of Research and Health Sciences Research Funding Program to S.C.N. The weather data were collected by staff at the North Inlet—Winyah Bay National Estuarine Research Reserve under an award from the Estuarine Reserves Division, Office of Ocean and Coastal Resource Management, National Ocean Service, National Oceanic and Atmospheric Administration. This is contribution number 1633 from the University of South Carolina’s Belle W. Baruch Institute for Marine and Coastal Sciences.
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