Benthic Geologic Habitats

Mapping Benthic Geologic Habitats

Selected maps available for download here.

 

Benthic geologic habitats are spatially recognizable areas of the seafloor, with geologic characteristics different than adjacent areas (Oakley et al., 2012).  Identification of these habitats is done using side-scan records in conjunction with ground-truth data and existing or estimated tidal current, wind, and wave information.  Side-scan sonar is a method of imaging the seafloor using sound waves, which are reflected off the substrate.  The returning side scan signal is manually interpreted and digitized in a Geographic Information System (ESRI ArcMap) based on the texture and intensity of the returning acoustic energy.  The spatially recognizable areas with different backscatter in the side-scan record patterns represent side-scan sonar facies.

Selected side-scan images of geologic features. A. Facies boundary between bayfloor sand sheet and low-energy basin organic silt in southwest Greenwich Bay. B. Wave-formed bars in southeast Greenwich Bay. C. Isolated boulders cropping out on the depositional platform, Greenwich Bay. D. Tidal bedforms on the depositional platform sand sheet, north of Sandy Point, Greenwich Bay. Crest to crest spacing averages 8 m. The distinct furrow in the center of the image was produced by a hydraulic dredge used for collecting shellfish.

Anthropogenic impacts mapped in Greenwich Bay. A. Trails formed by rakes dragged across the bottom during the harvesting of shellfish (Mercenaria mercenaria, locally called Quahogs). Scale across each rake trail is ~0.6 m. Habitat is low-energy basin silt. Box shows the extent of figure 7B. B. Inset of figure A showing no portion of the bayfloor in this area untouched by a rake. C. Mooring drag in southeast Greenwich Bay. The circular feature in the lower right of the image is caused by the ground chain dragging along the bayfloor as the boat pivots around mooring anchor. The vertical mooring chain and keel of the boat are also visible in the image. D. Shipwreck on the depositional platform south of Oakland Beach in Greenwich Bay.

Facies, are bodies of sediment or rock, with certain readily identifiable characteristics such as color, particle size, sorting, structure, biologic content, plus others, discernable in either the field or laboratory (Walker, 1990).  Side-scan sonar facies are the geologic facies interpreted using the strength and texture of the returning sonar signal; in general, the harder (or denser) the bottom, the stronger the return signal  and the darker the side-scan sonar record (using an inverse color scheme) (Goff et al., 2000).  This relationship is complicated by bed roughness, vegetation and bioturbation (Nitsche et al., 2004), and surface sediment samples and underwater video imagery will be collected to aid interpretation.

Oakley et al., (2012) proposed a definition for benthic geologic habitats, and a new naming convention for shallow estuarine environments.  This naming scheme allows for the characterization of the benthic geologic habitats utilizing a flexible naming convention that combines information about geologic processes, morphologic form, particle size, biota, and anthropogenic impacts.   The key to this naming convention is the flexibility, and new depositional environments, descriptors, and additional sediment textures can be added as needed in new study areas.

Much of my previous work in mapping shallow estuarine environments was undertaken as part of the MapCoast partnership (www.mapcoast.org), which was a NOAA funded project that involved academic and federal partners, most notably, soil scientists with the Rhode Island office of the NRCS and the University of Rhode Island.  I have also applied these techniques offshore of Fire Island, NY, and in Pleasant Bay, MA, working with Mark Borrelli of the Provincetown Center for Coastal Studies.

 

I will be expanding this work into Long Island Sound in 2014 and 2015, partnered with the Tolland, CT Office of the USDA-NRCS.

 

Benthic Geologic Habitats of Greenwich Bay, RI (Oakley et al., 2012)

Benthic Geologic Habitats (Oakley et al., 2012)

Benthic Geologic Habitat Maps

References

 

Goff, J.A., Olson, H.C., and Duncan, C.S., 2000, Correlation of side-scan backscatter intensity with grain-size distribution of shelf sediment, New Jersey margin: Geo-Marine Letters, v. 20, p. 43-49.

Nitsche, F.O., Bell, R., Carbotte, S.M., Ryan, W.B.F., and Flood, R., 2004, Process-related classification of acoustic data from the Hudson River Estuary: Marine Geology, v. 209, p. 131-145.

Oakley, B.A., Alvarez, J.D., and Boothroyd, J.C., 2012, Benthic geologic habitats of shallow estuarine environments: Greenwich Bay and Wickford Harbor, Narragansett Bay, Rhode Island, USA: Journal of Coastal Research, v. 28, p. 760-773.

Walker, R.G., 1990, Facies modeling and sequence stratigraphy: Journal of Sedimentary Petrology, v. 60, p. 777-786.

Comments Off

Comments are closed.