Submerged marine habitat mapping at Assateague Island National Seashore: A post-Hurricane Sandy study

Trembanis, Arthur; Miller, Douglas; Ebula, Elimujiang; Rusch, Hannah; Rothermel, Ella

We have surveyed the nearshore zone of Assateague Island National Seashore (ASIS) to determine what changes in bottom sediments and benthic fauna may be attributable to hurricane Sandy. Specifically, we (1) mapped the full survey area with side-scan sonar and multibeam bathymetry at a resolution comparable to that of the pre-Sandy (2011) survey, (2) resampled a subset of the benthic stations that represented all sediment strata previously identified, (3) compared newly obtained data to that from the pre-Sandy (2011) survey and determined changes that could be attributed to hurricane Sandy, and (4) synthesized geological and biological findings in a habitat map. This survey met the aims of the National Park Service including documenting storm-related changes in sediments and marine habitats on scales from meters to kilometers, demonstrating the use of multiple survey methodologies, and applying consistent analysis approaches and interpretation to both pre-Sandy (2011) and post-storm data sets. We have successfully addressed each of these objectives, outlining changes in surficial sediment texture, bathymetry, and benthic assemblage composition through dominant species patterns and decreased abundance, diversity, and species richness. Although storms have varied impacts on benthic environments, the benthos at ASIS have experienced decreases in abundance, diversity, and richness that are consistent with literature reports and published studies. The current study shows that while nearshore benthic invertebrate assemblages in the post-storm period are still significantly ordered by sediment class and other environmental variables, these factors explain less assemblage variation than in 2011. Assemblages differ by sediment classes, but they show more dispersion within them in the post-storm survey, making the resident assemblages of the same sedimentary habitats less similar to one another. Total abundance of individuals, species richness, and two indices of diversity have generally decreased across sediment classes. Characteristic species in different sedimentary habitats have changed slightly, although the same or similar taxa are still present. We present a CMECS classification based on the dominant species identified by total abundance and macrobenthic assemblage analysis. Biotopes were matched to acoustic sediment classes, and this synthesis of acoustic and benthic efforts produced a benthic habitat map for the nearshore zone. In the most notable faunal change, the opportunistic polychaete Mediomastus ambiseta substantially increased dominance across all sediment classes from 2011 to 2015, likely taking advantage of the disturbed benthic environment and decreased competition from the less-resilient generalists. The decreased diversity, proliferation of opportunistic polychaetes, and the shift towards finer grain sizes throughout much of the study area suggest habitat homogenization in the post-Sandy period. While the interest and opportunity for comparing these benthic surveys revolves around the impacts of hurricane Sandy, it is likely not the sole agent of change observed in this study among benthic assemblages at Assateague Island National Seashore. Further, we cannot exclude the possibility of impacts from other storms throughout the study period. This is largely due to the study timeline and constraints of sampling logistics around unexpected storm events such as hurricane Joaquin in October 2015, as well as the life history of Mediomastus. It is more realistic to characterize the sum of assemblage differences we see between the two surveys as integrated changes over a multi-year scale due to several storm events of varying intensities. Despite this inherent uncertainty in the study and results, the benthic survey conducted here is a valuable resource to guide and inform management of publicly-valued and historically under-studied habitats. The completion of an updated assessment of sediment morphology and benthic assemb

Submerged marine habitat mapping at Assateague Island National Seashore: A post-Hurricane Sandy study

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