Coastal subsidence as a function of salinity intrusion and peat decomposition in a karst environment

Background/Question/Methods: Large scale, restoration of the Everglades hydrology is mostly on hold as the State of Florida designs, builds and manages StormWater Treatment Areas (STAs) to remove TP from upstream basins. In the mean-time, sea level is rising (2-3 mm per year) and saltwater intrusion into the groundwater has begun. Therefore, we ask two key questions: How long will the resilience of the Florida coastal ecosystems to climate change defer the impacts of saltwater intrusion, increased tidal inundation, and hypersalinity? And, how is the phenomenon of peat collapse associated with sea level rise? To answer these questions, we examined 19 years of Surface Elevation Tables (SETs) in the coastal mangroves of Florida Bay and the first year of a large, in situ salinity dosing experiment from an area in Shark River Slough on the brink of peat collapse and another area far upstream, many years removed from saltwater intrusion.

Results/Conclusions: The porewater chemistry in the brackish sawgrass marsh was very surprising. As expected, Treatment chambers had higher concentrations of salt, chlorides and sulfates, than the Controls. However, they had lower concentrations of total dissolved phosphorus, nitrogen and organic carbon. Concentrations of TDP and TDN averaged around 90 ppb and 5 mg/l respectively, in the Treatment chambers, and 150 ppb and 8 mg/l, respectively, in the Control chambers. The nutrient concentrations in this brackish site were also significantly higher than the freshwater site where, TDP and TDN averaged around 12 ppb and 1.4 mg/l respectively, in the Treatment chambers, and 9 ppb and 0.9 mg/l, respectively, in the Control chambers. The high nutrients in the brackish site suggest high decomposition rates, which agrees with the SET relationships found with hydrology. In continuously flooded mangroves, the annual average elevation change (0.15 cm) was lower than the average annual accretion rate (0.21 cm), indicating that subsidence is so high that these sites cannot keep pace with current sea level rise. On the other hand, frequently flooded mangroves, ones that occasionally have low water and low salinity, had an elevation change of 3.1 mm/yr, but interestingly, only an accretion of 0.11 cm/yr. Since these sites are accreting very little, their ability to keep pace with sea level rise is likely the result of belowground processes; processes that our dosing chambers are illuminating.