LINKAGES BETWEEN SURFACE AND SUBSURFACE HYDROLOGY AND ECOLOGICAL FUNCTIONING OF MANGROVE ECOSYSTEMS IN FT. PIERCE, FLORIDA

Recent studies indicate that hydrological processes control ecosystem structure and function in mangroves systems. To date, however, few ecological processes have been directly related to variations in hydrological conditions in mangrove systems. Mangrove systems comprise ~2,500 km² of south Florida. Many of these mangrove systems are located in the Indian River Lagoon, a series of connected estuaries that extends ~250 km along the east coast of Florida. Most of the mangrove systems in the Indian River Lagoon have been ditched and impounded for mosquito control but are still hydrologically linked by surface water and groundwater pathways to the Indian River Lagoon.

Our study site is a mosquito impoundment located on a carbonate barrier island near Ft. Pierce, Florida. The common conceptual model of carbonate barrier island hydrogeology is that a freshwater lens rests on top of saline groundwater and that this lens expands and contracts seasonally due to changes in precipitation and subsequent freshwater runoff and recharge. Our results suggest that this conceptual model is incorrect. Salinities vary spatially, with surface water and groundwater salinities ranging from ~10 ppt in the upland groundwater, to ~30 ppt in the regularly-flushed mangrove surface water and groundwater, to ~75 ppt in the irregularly-flushed mangrove surface water and groundwater. However, salinities do not significantly differ seasonally, and cation and anion concentrations do not vary spatially or seasonally.

These results indicate that salinity variations are largely due to mixing between precipitation and lagoon water and the subsequent evapoconcentration in the irregularly-flushed areas rather than to precipitation and subsequent freshwater runoff and recharge. The spatial variations in salinities correlate with spatial variations in species composition and abundance, with maritime hammock, red mangrove, dense black mangrove, sparse black mangrove, and salt pan being arranged on a gradient of increasing salinities. In turn, spatial variations in nutrient cycling correlate with spatial variations in species composition and abundance, with N, P, and K patterns and rates of nitrogen mineralization and denitrification differing significantly between community types.