DISTRIBUTION AND CHARACTER OF SCOUR DEPRESSIONS IN EASTERN RHODE ISLAND SOUND: AN IMPORTANT CONTROL ON BENTHIC DIVERSITY

Multibeam bathymetry, collected for charting purposes during NOAA hydrographic survey H11922, is coupled with USGS data from sampling and photographic stations to map seabed morphology and composition, provide information on sediment transport and benthic habitat, and extend a series of studies providing a fundamental framework for research and management activities along the New England inner continental shelf. Patchworks of scour depressions control much of the benthic diversity on seaward-facing slopes and adjacent bathymetric highs in eastern Rhode Island Sound. These depressions average about 0.5 m deep, have relatively steep well-defined sides and gravel floors, and occur in a variety of shapes, sizes, and configurations. Some are elongate and narrow; others are broad and rounded. In places, individual scour depressions have expanded to combine with adjacent depressions, forming larger eroded areas that commonly contain outliers of the original sandy sea-floor sediments. Cobbles and scattered boulders on the depression floors indicate that the finer grained Holocene sediments have been removed, exposing winnowed relict Pleistocene deposits. Relatively weak tidal currents and the lack of asymmetrical scour features suggest that storm-generated currents are more important than bi-directional tidal currents in forming these features, and the presence and morphology of the depressions in the study area suggest they are in equilibrium with the present hydrodynamic regime. These depressions are apparently formed, expanded, and maintained under high-energy shelf conditions owing to hydraulic forces imposed by storm-driven waves and down-welling currents. Because sessile fauna (e.g. hydrozoans, anemones) dominate on gravel floors of the depressions and infauna (e.g. amphipods, polychaetes) are prevalent in the sandy Holocene deposits, we conclude that the resultant close juxtaposition of sand- and gravel-dependent communities promotes regional faunal complexity.