Changes in ecosystem carbon responses to saltwater exposure: Implications of sea level rise in the Florida coastal Everglades
Coastal wetlands, which have immense potential to store carbon (C) in vegetation and sediments, are a vital part of the global C cycle. How C storage in coastal wetlands will be affected by accelerated sea level rise as a result of a warming climate, however, is uncertain. In oligotrophic wetlands such as the Everglades in the southeastern USA, saltwater intrusion will bring ions (Cl-, SO42-) and phosphorus (P), a limiting nutrient for ecosystem productivity. It is hypothesized that shifts in stressors and subsidies can shift the soil carbon balance from a net C sink to a C source, stimulating peat collapse, which will, in turn, accelerate the effects of sea level rise. Previous research on mangrove peat soils found an increase in CO2 efflux with elevated salinity, suggesting sea level rise may make soils susceptible to peat collapse. The objective of this study is to investigate how simulated saltwater intrusion into freshwater and oligohaline wetlands will change net ecosystem productivity and affect the soil C balance. Using coupled field and mesocosm experiments, we are examining how plant gross primary production, plant respiration, ecosystem respiration, microbial C processing, and net ecosystem exchange in freshwater and oligohaline wetlands will change when exposed to saltwater and an increase in P loading.
Results from a short-term mesocosm experiment revealed that elevated salinity increased soil CO2 efflux in exposed soils, yet elevated salinity had no effect on inundated soils. CH4 efflux increased in response to saltwater addition at the freshwater site but not at the oligohaline site, yet this response diminished during the onset of winter. We predict that a saltwater load will increase ecosystem respiration while decreasing ecosystem productivity, possibly shifting the C balance of these marshes from a net sink to a source. In contrast, increased P loading has been shown to increase ecosystem productivity in oligotrophic wetlands. Therefore, it is still unknown how the interaction of an increased P subsidy coupled with saltwater intrusion will affect overall net ecosystem productivity and the C balance. Results from this study will reveal how the soil C balance in freshwater and oligohaline wetlands changes with saltwater intrusion due to sea level rise.