Dissolved Organic Matter and Nutrient Dynamics in Forest-Marsh Transition Zones – Implications of Sea Level Rise on Coastal Ecosystems in Southeastern US.

Rising sea level along the relatively flat southeastern US coastal plain significantly changes both vegetation composition and salinity of coastal wetlands, eventually modifying ecosystem functions and biogeochemical processes of these wetlands. We conducted a two-year study to evaluate the dynamics and relationships among aboveground productivity, greenhouse gas emissions, nutrients, and dissolved organic matter (DOM) of a freshwater forested wetland, a salt-impacted and degraded forested wetland, and a salt marsh in Winyah Bay, South Carolina, representing the salinity gradient and the transition from freshwater forested wetland to salt marsh due to sea level rise. The degraded forested wetland had significantly lower above-ground productivity with annual stem growth of 102 g/m^2/yr and litterfall of 392 g/m^2/yr compared to the freshwater forested wetland (230 and 612 g/m^2/yr, respectively). High methane emission [> 50 mmol/m2/day] was only observed in the freshwater-forested wetland but there was a strong smell of sulfide noticed in the salt marsh, suggesting that different redox processes control the decomposition of natural organic matter along the salinity gradient. The highest dissolved organic carbon concentration (28 - 42 mg/L) in monthly water samples was found in the degraded forested wetland, followed by the freshwater forested wetland (19 – 38 mg/L) and salt marsh (9 – 18 mg/L). In addition, a greater annual electrical energy output detected by in-situ microbial fuel cells was observed in the degraded forested wetland, suggesting that greater microbial activity and higher decomposition rates occurred in these saltwater-impacted organic rich soils. Currently, we are examining the microbial community analysis and DOM composition using 16S rRNA:rDNA ratios and Fourier transform ion cyclotron resonance mass spectrometry, respectively. The goal of our study is to identify unique biogeochemical processes in the forest-marsh transition zones that differ from unaltered freshwater forested wetland and salt marsh, and to illustrate the impacts of sea level rise in coastal ecosystems.