Under natural conditions, biogeochemical processes in sediments attain a steady state, with recycling of important nutrient elements matching the needs of the wetland plant community. Perturbation of this natural balance with an influx of contaminants may cause dramatic changes in the wetland ecosystem structure. In south Florida wetlands, contamination from P, S, and other elements and changes in the hydrology of the system have unbalanced the natural system in many areas. Some well known changes include the replacement of oligotrophic sawgrass by eutrophic cattails, a significant reduction in hydrologic period, and mercury contamination of fish. However, a detailed analysis of the ecosystem's response to this perturbation and the potential effects of proposed remediation are lacking. P is considered to be the principal nutrient element limiting plant growth in south Florida wetland environments, and the introduction of excess P into the ecosystem from canals draining the Everglades Agricultural Area (EAA) is presumed to be a principal cause of changes in plant growth patterns. The cycling of S in the south Florida ecosystem is of interest because of the central role sulfate reduction plays in the methylation of mercury, and the bioaccumulation of methyl mercury (a potent neurotoxin) in fish.

The principal objectives of this project are to examine the sources, sinks, and recycling of the biologically important elements P, S, C, and N in wetland sediments of south Florida. A secondary goal is to develop a geochemical history of the ecosystem from chemical studies of dated sediment cores. Our work on the geochemical history of the ecosystem is closely linked to other USGS paleoecology and sediment dating projects. Two aspects of this project component that we are currently focused on are: 1) historical salinity changes in the lower Taylor Slough area, and 2) the use of organic marker compounds to trace seagrass history in Florida Bay sediments.

The sources of P, S, C, and N to the ecosystem are being investigated principally through the use of isotopic methods. Uranium concentration and isotopic composition in sediment and water samples are being used as a proxy to trace fertilizer-derived P from canals draining the EAA to marshes of the Water Conservation Areas (WCAs) (see abstract by Zielinski and others in this volume). Similarly, stable isotopes of S are being used to trace the sources of sulfate to freshwater marshes of the WCAs, and the transformation of S between reduced and oxidized forms in wetland sediments and waters (see abstract by Bates and others in this volume).

Geochemical modeling of data obtained by chemical analyses of pore water and sediments is being used to establish the major biogeochemical processes in the sediments, to estimate rates of nutrient recycling and biodegradation, to estimate fluxes of chemical species between the sediment and the surface water, and to determine major sinks for P, S, C, and N in the sediments. Our results suggest that background concentrations of total P in surface sediments from freshwater wetland areas of south Florida range from 300 to 500 5 mg/g, which is similar to ranges from other freshwater wetlands of the eastern United States. (Okefenokee Swamp and Great Dismal Swamp). In contrast, highly contaminated areas of the WCAs near the canals draining the agricultural areas have total P concentration in surface sediments that exceed 5,000 5 mg/g, or more than 10 times greater than pristine areas. Accumulation rates of P at contaminated sites are 20-50 times higher than those in pristine areas (200 gm cm-2 yr-1 and 4 to 9 gm cm-2 yr-1, respectively). Geochemical modeling of pore water data suggests that recycling of sedimentary P and fluxes back to the surface water are also higher at contaminated sites. The higher recycling rates of P, as well as C, N, and S, in P-contaminated areas may be due to the stimulation of microbial biodegradation processes by the much greater input of "fresh" organic matter from the quick-growing, eutrophic macrophytes (cattails)

Current work is focused on the fate of the large "plug" of excess P stored in the sediments of contaminated areas. We wish to determine if this excess P will ultimately be buried at the sites, or if it will be recycled to surface waters for transport elsewhere. In large part, this question is linked to the relative biodegradation rates of organic detritus from cattails and sawgrass. We are currently using organic geochemical methods such as 13C nuclear magnetic resonance spectroscopy and lignin phenol analysis to examine the biodegradation rates of cattail and sawgrass peat in carefully collected cores at sites with known accumulation rates. Results also indicate that the mangrove fringe area of Taylor Slough is an important zone for concentration of P. However, the mechanism of this concentration effect is unclear.

Sulfur accumulation rates are up to 5 times higher at P-contaminated sites in WCA 2A compared to pristine areas of the freshwater Everglades (5,000 gm cm-2 yr-1 and 1,000 gm cm-2 yr-1, respectively). The excess S in the sediments of contaminated areas results from the reaction of microbially produced H2S with organic matter to form organic sulfur compounds. High sulfate concentrations are common for freshwater marsh areas in the WCA 2A near the Hillsboro Canal. Pore water profiles of sulfate and sulfide may be useful predictors of methyl mercury production. In P-contaminated areas, high rates of sulfate reduction produce high levels of pore water sulfide. High sulfide content may poison the bacteria involved in methyl mercury production and also immobilize Hg in the sediments as insoluble HgS. In pristine freshwater marsh areas, low levels of sulfate would limit sulfate reduction and thus methyl mercury production. Freshwater marsh areas with moderate levels of sulfate and intermediate sulfide content may represent zones of maximum methyl mercury production. This issue is discussed further in the abstract by Bates and others (this volume).

Back to Project Homepage