Effects of simulated drought on physiological activity of Everglades short-hydroperiod marsh

Background/Question/Methods

During the last 130 years, hydrologic regimes in the Everglades have been interrupted by 2500km of spillways, levees, and canals that were designed for flood protection and to provide water to urban areas in south Florida. This has led to changes in hydrology that have transformed the characteristics of the Everglades freshwater marsh ecosystems. Moreover, IPCC-projected climate change for the Everglades indicates there will be an increased occurrence of drought due to changes in precipitation patterns in the southeastern U.S.  Sawgrass (Cladium jamaicense) communities dominate the Everglades; altered hydroperiods and drought stress in sawgrass communities can have significant impacts on carbon cycling for the entire Everglades. The goal of this study was to determine how freshwater marl marsh ecosystems in the Everglades respond to changes in hydroperiod and drought. To determine the effects of hydroperiod and drought on carbon dynamics, we varied hydroperiods using 3 simulated drought scenarios (immediate, intermediate, and a gradual transition into drought), on 18 freshwater marl marsh monoliths collected from the greater Everglades.  Simulated drought scenarios were repeated twice over a 22-week period.  Components of CO₂ fluxes (by static chamber) and spectral reflectance properties were measured weekly to track ecosystem responses to treatment.

Results/Conclusions

Variations in hydrology resulted in significant changes in carbon dynamics. Simulated drought reduced net ecosystem carbon storage (NEE) by increasing ecosystem respiration (Reco) and decreasing carbon uptake (GEE) (p<0.001). The second drought reduced fluxes (NEE and Reco) and increased the amount of carbon released to the atmosphere (p<0.05) relative to carbon uptake. This difference may have been due to changes in leaf area (p=0.110), which can impact maximum midday NEE. Changes in community composition were also observed. Following the final simulated drought there was a decrease in sawgrass dominance from >80% to 47% and a 35% increase in the dominance of Panicum repens, an invasive species, indicating an additive effect of drought stress on the recovery of sawgrass. Fluctuations in reflective indices, which serve as proxies for light use efficiency (PRI), water content (WI), energy absorption and chlorophyll content (NDVI), changed in response to simulated drought. WI increased with the onset of drought, while NDVI and PRI values decreased, indicating a change in physiological potential in response to drought stress. These results suggest that future changes in precipitation patterns and drought occurrence/duration can change the carbon storage capacity of freshwater marshes from sinks to sources of carbon to the atmosphere.