Wetland-Water Column Exchanges of Carbon, Nitrogen, and Phosphorus in a Southern Everglades Dwarf Mangrove

We used enclosures to quantify wetland-water column nutrient exchanges in a dwarf red mangrove (Rhizophora mangle L.) system near Taylor River, an important hydraulic linkage between the southern Everglades and eastern Florida Bay, Florida, USA. Circular enclosures were constructed around small (2.5-4 m diam) mangrove islands (n = 3) and sampled quarterly from August 1996 to May 1998 to quantify net exchanges of carbon, nitrogen, and phosphorus. The dwarf mangrove wetland was a net nitrifying environment, with consistent uptake of ammonium (6.6-31.4 μ mol m-2 h-1) and release of nitrite + nitrate (7.1-139.5 μ mol m-2 h-1) to the water column. Significant flux of soluble reactive phosphorus was rarely detected in this nutrient-poor, P-limited environment. We did observe recurrent uptake of total phosphorus and nitrogen (2.1-8.3 and 98-502 μ mol m-2 h-1, respectively), as well as dissolved organic carbon (1.8-6.9 μ mol m-2 h-1) from the water column. Total organic carbon flux shifted unexplainably from uptake, during Year 1, to export, during Year 2. The use of unvegetated (control) enclosures during the second year allowed us to distinguish the influence of mangrove vegetation from soil-water column processes on these fluxes. Nutrient fluxes in control chambers typically paralleled the direction (uptake or release) of mangrove enclosure fluxes, but not the magnitude. In several instances, nutrient fluxes were more than twofold greater in the absence of mangroves, suggesting an influence of the vegetation on wetland-water column processes. Our findings characterize wetland nutrient exchanges in a mangrove forest type that has received such little attention in the past, and serve as baseline data for a system undergoing hydrologic restoration.