Subtidal resource inventories at Assateague Island: resolving Hurricane Sandy's impact on benthic assemblages and habitats
Date
2016
Authors
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Publisher
University of Delaware
Abstract
As part of a benthic habitat mapping project for the National Park Service, I aim to characterize the subtidal habitats and invertebrate communities of the nearshore zone of Assateague Island National Seashore (ASIS), providing an updated assessment to inform management decisions at ASIS. I will also outline any changes in benthic assemblages between previous and current surveys, to determine if I can resolve any impacts of Hurricane Sandy. In October 2014 and 2015, I sampled the benthic zone with modified Young grabs, taking subsamples to determine grain size distributions and total organic carbon of sediments. Both pre-storm and post-storm, benthos assemblages significantly differ between sediment classes (ANOSIM R=0.677, 0.263 respectively, p<0.01). Assemblages are unchanged at class level, but the opportunistic Mediomastus ambiseta increasingly dominates across all sediments from 2011 to 2015 (SIMPER). Post-storm samples are more interspersed (nMDS) and show less similarity within sediment class groupings (SIMPER) than pre-storm samples. Shannon and Simpson diversity consistently decrease across the shared sediment classes. Distance-based RDA ordinations (DistLM) relating environmental variables to assemblage patterns show that sediment classification explains the greatest proportion of assemblage variation both pre-storm (17.8%, p<0.01) and post-storm (39%, p<0.01). Dominant sediment classes at the grab level show a considerable shift towards finer grain sizes from 2011 to 2015. Assemblage composition is significantly different between post-storm sampling years (ANOSIM R=0.093, p<0.01), largely driven by increased dominance of M. ambiseta in 2015. Post-Sandy, assemblages are characterized by the same or similar species but have lower diversity, richness, and total abundance, and show less similarity within-class. I am unable to isolate the impacts of Hurricane Sandy on benthic assemblages from other storms in the study period due to their timing around sampling. Sandy is not unique in its ability to shape benthic assemblages, as storms with comparable wave heights recur approximately every 2.2 years in the Delmarva region. It is more realistic to characterize the shifts in community composition as the result of continuous changes over a multi-year scale due to many storm events of varying intensities, rather than from direct impacts of Hurricane Sandy.