Legacy effects of Hurricane Katrina influenced marsh shoreline erosion following the Deepwater Horizon oil spill

Disturbance interactions occur when one perturbation influences the severity and perhaps the baseline state of succeeding disturbances. Natural and anthropogenic disturbances are frequent in dynamic coastal ecosystems and can often be linked. We evaluated potential for disturbance interactions associated with the 2010 Deepwater Horizon (DWH) oil spill, which was preceded by disturbance from Hurricane Katrina in 2005, by quantifying marsh shoreline retreat across both events. Our goal was to determine the degree to which Hurricane Katrina altered baseline rates of erosion prior to the DWH spill. We quantified erosion rate and fetch from aerial images of northern Barataria Bay, Louisiana marsh shorelines classified as reference, moderately-oiled, and heavily-oiled over three pre-spill time periods (1998-2004, prior to Hurricane Katrina; 2004-2005, during Katrina; 2005-2010, post-Katrina but pre-oil spill) and a post-spill period from 2010 to 2013. Prior to Hurricane Katrina, marsh shoreline erosion rates were low (from 0.38 to 1.10 m yr^-1). In contrast during Hurricane Katrina (2004-2005), erosion increased by 661% and 756%, respectively, for shorelines that would subsequently become moderately and heavily-oiled; reference shoreline erosion increased by 59%. These high erosion rates were associated with increased fetch and higher wave action due to loss of protective geomorphic features such as small islands and spits and persisted during the post-Katrina/pre-spill period of 2005-2010 (0.62, 1.38, and 2.07 m yr^-1 for reference, moderately, and heavily-oiled shorelines, respectively). Erosion rates increased modestly after the DWH event (reference = 1.13 m yr^-1, moderate oiling = 1.45 m yr^-1; heavy oiling = 2.77 m yr^-1), but not significantly, compared to the post-Katrina period. Consequently, we could not detect a post-spill increase in marsh shoreline erosion. Rather, we concluded that Hurricane Katrina reset the erosion baseline, thereby connecting the two disturbances, and was the major driver of marsh shoreline erosion at our research sites during the study period.

Shoreline oiling effects and recovery of salt marsh macroinvertebrates from the Deepwater Horizon Oil Spill

Salt marshes in northern Barataria Bay, Louisiana, USA were oiled, sometimes heavily, in the aftermath of the Deepwater Horizon oil spill. Previous studies indicate that fiddler crabs (in the genus Uca) and the salt marsh periwinkle (Littoraria irrorata) were negatively impacted in the short term by the spill. Here, we detail longer-term effects and recovery from moderate and heavy oiling over a 3-year span, beginning 30 months after the spill. Although neither fiddler crab burrow density nor diameter differed between oiled and reference sites when combined across all sampling events, these traits differed among some individual sampling periods consistent with a pattern of lingering oiling impacts. Periwinkle density, however, increased in all oiling categories and shell-length groups during our sampling period, and periwinkle densities were consistently highest at moderately oiled sites where Spartina alterniflora aboveground biomass was highest. Periwinkle shell length linearly increased from a mean of 16.5 to 19.2 mm over the study period at reference sites. In contrast, shell lengths at moderately oiled and heavily oiled sites increased through month 48 after the spill, but then decreased. This decrease was associated with a decline in the relative abundance of large adults (shell length 21-26 mm) at oiled sites which was likely caused by chronic hydrocarbon toxicity or oil-induced effects on habitat quality or food resources. Overall, the recovery of S. alterniflora facilitated the recovery of fiddler crabs and periwinkles. However, our long-term record not only indicates that variation in periwinkle mean shell length and length-frequency distributions are sensitive indicators of the health and recovery of the marsh, but agrees with synoptic studies of vegetation and infaunal communities that full recovery of heavily oiled sites will take longer than 66 months.