The Chandeleur Islands, Louisiana (USA), were among the first coastal locations in the northern Gulf of Mexico (GoM) threatened by exposure to Deepwater Horizon oil. Shoreline oiling data and surface oil trajectories (aerial and satellite imagery) showed oil passing through seagrass beds on the shallow back barrier shelf west of the islands repeatedly between May and early July 2010. Aerial photos in May 2010 revealed a heterogeneous distribution of surface oil crossing the shelf, and MC252 exposure was confirmed in sediments and seagrass tissue during field assessments. We observed 5 seagrasses growing at densities comparable to other northern GoM communities. Ruppia maritima and Halodule wrightii were the most common, followed by Thalassia testudinum. Syringodium filiforme and Halophila engelmannii were rarely encountered. The subtidal and intertidal seascape on the shelf was a mosaic of seagrass patches distributed in varying sizes among unvegetated and sparsely vegetated areas at water depths and in sediment types known to support seagrasses. To quantitatively assess the seagrass response following exposure, sophisticated change detection methodologies were applied to aerial photography acquired in October 2010, 2011, and 2012 in a subsample of 5 locations on the shelf where Deepwater Horizon oil exposure was confirmed. The analysis conservatively estimated a seagrass loss of 104.22 acres (42.18 ha) at these locations. Unexpectedly, the whole back barrier shelf experienced a net gain of 228 acres (92.27 ha) of seagrass between 2010 and 2011, representing a pause in the long-standing trend in seagrass declines in the Chandeleurs and indicating that oil exposure did not result in a shelf-wide catastrophe for seagrasses. Predictions for the impending disappearance of this seagrass resource in the near future may need to be reconsidered.

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