The effects of projected sea-level rise on Everglades coastal ecosystems: Evaluating the potential for and mechanisms of peat collapse

Background/Question/Methods

Peat soils are critical to the Greater Everglades Ecosystem because peat accretion is a large and important carbon (C) reservoir that contributes to landscape pattern and maintains wetland elevation relative to rising sea level.  With sea level rising at ~3 mm y^-1, coastal freshwater and oligohaline Everglades wetlands are being exposed to increased duration and spatial extent of inundation and salinity from seawater, which can affect soil C balance through soil redox potential, soil respiration, and the intensity of osmotic stress to vegetation.  The term “peat collapse” has been used to describe a relatively dramatic shift in soil C balance, leading to a rapid loss of soil elevation, and culminating in a conversion of vegetated freshwater marsh to open water.  The process has been documented to varying degrees across the U.S. and contributes to instability of coastal marshes that are important for fisheries habitat, shoreline stabilization, and C sequestration.

In the Everglades, significant peat collapse has been locally observed and attributed to of sea level rise and the impacts from canals dug in the 1920s that pierced a marl berm barrier originally protecting the interior freshwater marsh. More recently, evidence of freshwater peat collapse has been observed in lower Shark River Slough suggesting that this process is ongoing and may be affected by a reduction in freshwater head, recent storm surges (e.g., Hurricane Wilma), sea level rise, and possibly fire.

Results/Conclusions

In late 2014, we initiated parallel field and mesocosm experiments to investigate potential mechanisms of peat collapse attributed to increased seawater salinity and inundation in freshwater and oligohaline wetland ecosystems of the southern coastal Everglades. Our previous experiments with mangrove peats showed predicted shifts in soil redox and enhanced C loss from soils exposed to increased salinity.  Results from our present, long-term study illustrate that, despite relatively low salinity dosing, both field and experimental results are beginning to show significant changes in chloride and sulfate concentrations. In the field, based on net ecosystem exchange measurements, marshes continue to be a net C sink despite porewater changes associated with increased salinity. In soil-only mesocosms, we are measuring decreases in phosphatase and increases in cellulase activities in subsurface soils without effects on surface soils or net microbial respiration. Sawgrass may have the capacity to osmoregulate salinity stress or have a higher salinity tolerance than has been demonstrated experimentally, whereas changes in soil microbial responses may be driven by phosphorus-induced subsurface C loss.