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.