Freshwater-marine gradients along coastal ecosystems are rapidly compressing with sea-level rise and tidal expansion into coastal ecosystems. Saltwater intrusion is transforming coastal landscapes, and a comparison of ecosystem-level responses is needed to understand commonalities of spatio-temporal changes. We tested how saltwater intrusion alters above and belowground carbon stocks and fluxes among diverse coastal wetlands with distinct climate, hydrology, nutrient biogeochemistry, and coastal geomorphology. We synthesized data from salinity dosing experiments coupled with coastal gradient observational studies that have specifically measured changes in plant and soil wetland carbon stocks and/or fluxes with changes in surface/porewater salinity. We build on two recent conceptual frameworks of (i) saltwater intrusion and (ii) ecosystem development trajectories to illustrate how ecosystem responses to saltwater intrusion can yield gains, losses, or no effects on carbon storage based on a suite of environmental conditions.
Results/Conclusions
Experimental manipulations of elevated salinity paired with long-term observations in freshwater and brackish wetlands have revealed similar effects on wetland ecosystems across diverse coastal regions. These studies have found that elevated salinity alters wetland porewater chemistry through changes in ionic exchange and oxidation-reduction, which facilitates the release of inorganic nutrients and carbon. Although salinity tolerance varies among species and prior exposure, saltwater intrusion leads to net carbon loss among coastal ecosystems. The mechanisms of carbon loss vary among ecosystems ranging from reduction in plant inputs of organic carbon to soils in salt-sensitive species, enhanced soil microbial respiration, and increases in the processing and export of dissolved organic carbon. Increases in ionic strength, sulfidation, and alkalinization with saltwater intrusion may have stronger negative impacts on wetland carbon storage in lower-productivity wetlands with lower tidal influence and those ecosystems that do not undergo rapid species or community replacement. Drought and nutrient legacies interact to increase wetland carbon losses but may also reduce carbon export from tidal freshwater ecosystems. Saltwater intrusion leads to carbon loss from coastal wetland ecosystems, but the relative rate and magnitude of changes is a function of environmental conditions, including ecosystem age, nutrient limitation, productivity, multiple stressor interactions, elevation increase relative to sea-level rise, and geomorphology.