Coastal Riverine Wetland Biogeochemistry in the Anthropocene: Relationships with Vegetation Transition and Saltwater Intrusion

Subtropical coastal wetlands are at the forefront of sea-level rise induced vegetation transition. As sea level rises, increased tidal range brings mangrove propagules further inland, promoting the transition of herbaceous wetlands to woody swamps. Freshwater wetland soils are highly reduced and are major sources of non-anthropogenic methane emissions. Sulfate, the third most prevalent ion in seawater, provides an alternative anaerobic respiration pathway to freshwater wetlands, potentially decreasing methane emissions. However, underground biogeochemical conditions are difficult and time-consuming to detect from a land manager\u27s perspective. Chapter 2 assessed the soil biogeochemistry spanning marsh-mangrove ecotones along three rivers on the west coast of Florida to determine if patterns were consistent across the landscape and if visible shifts in vegetation corresponded to belowground processes. Furthermore, how biogeochemical properties respond to salinity was addressed in an intact core set-up from two marsh-mangrove transition in Chapter 3. Results indicated landscape-scale patterns in soil biogeochemistry differed significantly by river and were most strongly correlated with soil organic matter content, regardless of vegetation community or salinity regime. Methane production was observed in moderate- (S = 12) and high- (S = 34) salinity mangrove communities. The vegetation ecotone experienced seasonally variable salinity and did not serve as a true biogeochemical intermediate between the marsh and mangrove communities. Overall, soil origin was determined to be the most important predictor of the biogeochemical response to elevated salinity in chapter 3. Bacterial abundance showed a moderate response to increased salinity 10 days into the manipulations. These studies indicate how biogeochemistry along the marsh-to-mangrove ecotone will not respond in the same ways to sea-level rise at the landscape level