Changes in Marsh Vegetation, Stability and Dissolved Organic Carbon in Barataria Bay Marshes Following the Deepwater Horizon Oil Spill

Coastal wetlands in Southern Louisiana were contaminated by the release of record volumes of oil between April and July 2010. Barataria Bay was extensively impacted, resulting in the oiling and dieback of marsh grasses along a discontinuous margin up to 30m into the marshes. Shoreline stability and biogeochemistry have been monitored over three site visits between between July 10 and August 25, 2010. Initial observations in early July were that grasses, dominantly shape Spartina alterniflora and shape Juncus roemerianus, were bent over under the weight of a thick oil coating. The bent-over grasses were broken off along some reaches, leaving a stubbled shoreline more susceptible to erosion. Repeated site visits in mid and late August found visible erosion along some of the effected shorelines. Water samples were collected from the shoreline marsh remnants and from visibly unaffected marshes to characterize dissolved organic carbon (DOC) and total dissolved nitrogen (TDN). In spite of visible oil sheens in unfiltered water from contaminated shorelines and no visible sign of impact on vegetation in the “control” sites with no visible oil on vegetation, DOC concentrations were similar in impacted and visibly unimpacted sites in Barataria Bay. Subsequent samples in mid- and late-August had increased DOC concentrations relative to previous sample events regardless of whether the site was visibly impacted. There was a general increase in specific UV absorbance (SUVA), an index of aromaticity, with increasing DOC concentrations, either due to seasonal effects or continued dissolution of petroleum compounds. Further chemical characterization using fluorescence and gas chromatography will be used to confirm the presence of petroleum compounds. The ratio of DOC to TDN also increased over time, which may have implications for vegetation regrowth and plant community structure, including the shift of grass species dominating effected marshes. These initial findings suggest that while some shorelines have overtly visible impacts from the oil slick that reached Barataria Bay, there are also less visible indications that there may be a more comprehensive biogeochemical effect from the oil incursion throughout Barataria Bay.