MAPPING BENTHIC GEOLOGIC HABITAT IN SHALLOW ESTUARINE AND LAGOON ENVIRONMENTS: CASE STUDIES, EXPERIENCE AND RESULTS FROM SOUTHERN NEW ENGLAND

An integrated mapping approach using high-resolution side-scan sonar, surface sediment grab samples, digital aerial and orthophotography as well as underwater video imagery has been applied to map the Holocene sediment cover and Late Wisconsinan glacial outcrop in numerous shallow estuarine and coastal lagoon environments in southern New England, (>50 km²) over the last decade. Water depths in these areas range from <1 to >10 m, often with an average depth of only a couple of meters, making it challenging to map using larger research vessels. Our research has been conducted using a variety of research vessels, including a 5.5 m pontoon boat and various outboard powered vessels ranging from 4 to 7.3 m, allowing collection of full coverage sonar data in water > 1 m deep. The variety of vessels available was the result a collaborative effort known as the MapCoast project, a consortium of geologists, soil scientists, benthic ecologists and managers from state and federal agencies, non-governmental organizations and academic institutions.

While the use of side-scan sonar to characterize the seafloor has become common in a variety of different marine environments, challenges remain in classifying the map units into a naming convention that is useful to both scientists and managers, particularly in shallow areas with complex facies distributions. Benthic geologic habitats, defined as spatially recognizable areas of the seafloor with geologic characteristics different from adjacent areas, are described utilizing a flexible naming convention that combines the depositional environment (which includes information about the geologic processes and morphologic form), particle size, biota, and anthropogenic impacts. The high resolution of the data, and large mapping scale (typically < 1:5,000) allows for a minimum mapped polygon size < 300 m². Low-energy basins of varying depths have been the areally largest depositional environment mapped in the project areas. These basins are often only slightly deeper than surrounding habitats, however they represent a significant sink for fine-grained, often organic rich sediment, and are likely a sink for carbon. How much carbon sequestration occurs in these areas has not been analyzed in this work and could represent a future research application.