Examining the relative contribution of ‘blue carbon’ to coastal shelf environments via optical properties of dissolved and base-extracted particulate organic matter

Coastal wetlands represent a key interface between terrestrial and marine ecosystems in the global landscape. ‘Blue carbon’ is carbon accumulated in vegetated coastal wetlands through primary production. Although the contribution of terrestrial organic carbon (OC) to the coastal shelf is well documented, the focus has been on the role of rivers, and less is known about the export of ‘blue carbon’ from coastal wetlands. We investigated the chemical characteristics of the organic matter (OM) in estuarine complexes of the Gulf of Mexico with the goal of shedding light on the relative contribution from riverine upland and wetland sources to the coastal ocean. We conducted synoptic sampling campaigns in the Apalachicola and Barataria Bays to determine the concentration and isotopic composition of the dissolved OC (DOC), and the absorbance and fluorescence of the dissolved OM (DOM) and base-extracted particulate OM (BEPOM). Preliminary results show clear differences in the biogeochemical properties of the OM within and between each bay. Apalachicola Bay samples from March 2015 suggest the presence of two sources of terrestrial DOM that we initially attribute to the Carabelle River and marsh production in the East Bay. Compared to Aplalachicola Bay, we found a lower humification index, higher slope ratio (indicative of lower molecular weight OM), and higher amino-acid fluorescence in the DOM from Barataria Bay, sampled in summer 2015. This contrast suggests key differences in the freshness and molecular weight of the DOM between bays. Ongoing work in these systems will help elucidate patterns associated with temporal drivers, while mixing models based on stable isotopes and PARAFAC analysis of the fluorescent OM will allow us to identify the potential OC sources to the coastal pool. The cumulative results of these field measurements, together with remote sensing efforts and other geochemical analyses conducted by collaborators in the project will help constrain carbon cycling dynamics across coastal regimes.