Biogeochemistry of light hydrocarbons in south Florida wetlands

Light hydrocarbons are ubiquitous in south Florida wetlands. Enrichment or depletion of dissolved light hydrocarbons is effected by natural phenomenon and anthropogenic activities. Natural processes include: air/water exchange, seepage and diffusion across the sediment/water interface, advective transport, microbial consumption during migration, and in situ biological and photochemical production. Anthropogenic inputs result from offshore and onshore oil and natural gas production, manufacturing, and transportation operations.; The diffusive flux of light hydrocarbons were estimated from measured surface water concentrations using an empirically derived gas exchange model. The mean diffusive flux from 5 freshwater lakes was 12.4 g {dollar}rm CHsb4/msp2/yr.{dollar} The mean diffusive flux for an urbanized, sub-tropical estuary (Tampa Bay) was 0.96 g {dollar}rm CHsb4/msp2/yr.{dollar} The diffusive methane flux from the Everglades sawgrass marsh system varied widely, ranging from 2.89 g {dollar}rm CHsb4/msp2/yr{dollar} for densely vegetated regions to 32.3 g {dollar}rm CHsb4/msp2/yr{dollar} for sparsely vegetated, carbonate-mud areas. A strong methane flux (30.0 g {dollar}rm CHsb4/msp2/yr){dollar} was estimated for an organic-rich brackish pond, near Florida Bay. The diffusive flux accounted for 15 to 35% of the total methane flux from these environments.; The biogenic flux of ethane from several south Florida wetlands is reported. The average diffusive flux varied from 4.6 mg {dollar}rm Csb2Hsb6/msp2/yr{dollar} from densely vegetated sawgrass marshes to 110 mg {dollar}rm Csb2Hsb6/msp2/yr{dollar} from Tampa Bay. Ethane flux from estuarine environments was significantly larger than from freshwater sites. The annual biogenic ethane flux was estimated at 0.02 Tg (Tg = 10{dollar}sp{lcub}12{rcub}{dollar}g). The contribution of natural wetlands to the global budget (16 Tg/yr) is relatively insignificant.; The isotopic composition of methane released to the atmosphere by ebullition is reflective of the methane in the sedimentary gas phase; however, methane diffusing across the sediment/water interface and eventually into the atmosphere will have a higher probability of undergoing fractionation effects associated with microbial oxidation. Stable isotope data and the {dollar}rm CHsb4/Csb2Hsb6{dollar} ratio were used to evaluate the importance of methane oxidation in south Florida wetlands. Microbial oxidation reduces the diffusive methane flux from the Florida Everglades greater than 95%.