Using fine-scale variation in ecosystem state parameters to evaluate alternate stable states within a salinizing peat marsh in the Florida Coastal Everglades

Abstract:

One of the most important threats to coastal wetland communities is sea level rise. Within the Florida Coastal Everglades, a rising ocean is pushing saltwater further inland, altering ecosystem functioning in pre-historically freshwater wetlands, and contributing to a process known as “peat collapse.” Peat collapse has been observed in coastal Everglades sawgrass peat marshes, resulting in rapid declines of soil surface elevation and coastal carbon storage capacity, and has the potential to hinder inland transgression of mangrove forests. While our mechanistic understanding of peat collapse is strong though field and outdoor lab mesocosms experiments, we lack robust characterizations of peat collapse at the landscape level. Additionally, the concept of alternate stable states and resilience may be useful in characterizing this process in a manner relevant to land and water managers. Here, we use a fine-scale survey of hydro-geomorphic parameters across dominant ecosystem states within a salinizing peat marsh that is observably collapsing as evidence for alternate stable states. We conducted transect surveys of soil surface elevation, soil depth, porewater salinity, and water depth across three vegetation classes: vegetated marsh, submerged aquatic vegetation, and unvegetated open-water.

Correlation coefficients revealed strong relationships among all parameters measured while MANOVA and Tukey post-hoc tests reveal significant differences among ecosystem states. Probability density estimates revealed bi-modality with non-overlapping peaks associated with vegetated marsh and open-water. Principle component and cluster analysis revealed strong clustering in the reduced dataset with regard to vegetation class. These results provide spatially explicit, empirical evidence for the existence of alternate stable states within a coastal Everglades peat marsh, occurring across abrupt spatial transitions. Coupled with information from experimental peat collapse research, it is likely that peat collapse is facilitating a regime shift from healthy, vegetated marsh, to open-water/bare soil.