Coastal wetlands store immense amounts of carbon (C) in vegetation and sediments, making them a vital part of the global C cycle. Wetland C storage depends on an interplay of factors such as water availability, nutrient availability, and various stressors such as saltwater intrusion. Given current global climate change, accelerated sea level rise is predicted to increase saltwater intrusion into historically freshwater wetlands. The largest wetland system in the U.S., the coastal Everglades, is characterized as an “upside-down” estuary, meaning that the limiting nutrient in the ecosystem, phosphorus (P), comes from the downstream source (the Gulf of Mexico) rather than the upstream source, as is the case in most river-driven estuarine systems. Therefore, during periods of saltwater intrusion, the coastal Everglades will be exposed to both a stress (elevated salinity) and a subsidy (P enrichment). The objective of this study is to test the subsidy-stress response of a freshwater Cladium jamaicense marsh experiencing both P enrichment and elevated salinity. Using a mesocosm experimental setup, we elevated salinity from freshwater to brackish (~10 ppt) salinity and subjected one treatment to a P load (0.49 g P m-2 yr-1). We quantified changes in gross ecosystem production (GEP), ecosystem respiration (ER), soil C efflux, and net ecosystem production (NEP) in freshwater wetland peat soil monoliths.
After two years, elevated salinity reduced soil CO2 efflux by 41%, while elevated P increased soil CO2 efflux by 24%. Unsurprisingly, soil CH4 efflux decreased by 98% when exposed to elevated salinity. When exposed to an increased P load, however, soil CH4 efflux increased by 277% within the freshwater monoliths. In terms of ecosystem CO2 flux, elevated P increased NEP by 98%, while, surprisingly, elevated salinity had no significant effect on NEP. The increase in NEP with elevated P was highly correlated to increases in GEP (107%) and ER (103%), suggesting that physiological responses of plants was the main driver to changes in the overall C flux balance. Based on our results, freshwater Cladium marshes within the Everglades may be able to withstand small to moderate increases in porewater salinity and perhaps even thrive if this response is coupled with low-level P enrichment. However, more measurements of soil processes, such as root inputs and soil decomposition, are needed to fully understand how freshwater marshes respond to a subsidy-stress event.