W&M > VIMS > CCRM > Research > Mapping & Surveying > Benthic & Shallow Water Habitat > Mobjack Bay

Surveying and summarizing the spatial arrangement of benthic habitat types within the nearshore of Mobjack Bay, Virginia

Principle Investigators:  Donna Marie Bilkovic and Carl H. Hershner, Funding Agency:  NOAA/NCBO

Objectives
To survey, map and quantify benthic habitat within the nearshore of Mobjack Bay, including the Severn, Ware, North and East Rivers using remote-sensing technologies.  Final output includes digital geospatial characterization of the extent and distribution of prevalent nearshore habitats, such as submerged aquatic vegetation and oyster shell.

Background
Estuarine habitat research and restoration efforts historically targeted a single habitat type, such as seagrass beds or saltmarshes.  As such, little is known about how intertidal marshes and adjacent subtidal vegetated and unvegetated habitats interact and function together (Rozas and Odum 1987; Hettler 1989).  However, there is a growing body of work that indicates the spatial arrangement and heterogeneity of habitats may have significant influence on biotic community interactions, such as foraging behavior, predation, competition as well as recruitment (Coen et al. 1981; Mittelbach 1986; Werner and Hall 1988; Danielson 1991; Irlandi and Crawford 1997; Micheli and Peterson 1999). In the Chesapeake Bay, there is currently no comprehensive assessment of aquatic habitat heterogeneity or understanding of the effects of multiple stressors on the viability of these habitats.

Mobjack Bay and its associated tributaries historically contained a diverse array of critical habitat types including oyster reefs, seagrass beds and tidal wetlands.  Currently, multiple restoration efforts are underway throughout this watershed to mitigate losses from disease, and habitat destruction and modification. We collected detailed information on the quantity and distribution of nearshore subtidal habitat within Mobjack Bay Watershed. The result was the delineation of important Chesapeake Bay habitats for tributaries containing a variety of habitat restoration and monitoring efforts, such as oyster reef placement and SAV plantings. 

mobjack bay video locator map

Benthic habitat characterization was completed with multiple acoustic technologies and data derived from validation activities including sediment samples and video imagery. Click for more.

Methods - Acoustic Surveys
All benthic habitats between 1 and 4 meter depths were surveyed in four tributaries of the Mobjack Bay (Severn, Ware, North, East rivers) during May through July 2007 with multiple acoustic technologies.  Benthic characterization was completed with side-scan sonar technology (Sea Scan Marine Sonics, 600 kHz) and an echo-sounder (Knudsen 320 BP; Kel 28/200 kHz dual-frequency transducer).

severn river camera sled

Side-scan sonar surveys covered a distance of 50 m on either side of the nadir for a total width of 100 m.  Where there was an extensive broad reaches of shallow waters, multiple passes were completed to ensure all benthic habitats between 1 and 4 meters were scanned.  Our track line encompassed 158.3 km, with a total swath area of 12.69 km². 

table and map

Survey data were analyzed with Questar Tangent software for the entire area.  The echo-sounder single beam data were processed with QTC Impact, and the side-scan sonar data with QTC Sideview. Complementary acoustic datasets and associated post-processing output were used in conjunction with field observations to select five primary acoustic classes to represent benthic habitat types (see maplink). 

Habitat Maps and Tables
Complementary acoustic datasets and associated post-processing output were used in conjunction with field observations to select five primary acoustic classes to represent benthic habitat types. Auxiliary datasets, such as VIMS aerial submerged aquatic vegetation monitoring surveys, were overlaid with benthic characterization for validation from a secondary source in overlapping areas.  Preliminary aerial survey data from  2007, spatially displayed with our acoustic surveys, indicated a consistent overlap in regions with identified SAV.  The SAV aerial survey can delineate SAV within the shallows (<1 m depth) which the side-scan sonar survey could not cover effectively due to harsh backscatter overwhelming imagery.  In reaches where SAV was present in deeper waters (> 1m) and often non-identifiable in aerial survey images, SAV was accurately delineated with acoustic imaging at small spatial resolutions (~20m²).  Aerial survey estimates in concert with ground-truth information validated the benthic categorization output. Likewise, known oyster reef locations were accurately categorized with the side-scan sonar imagery. Additional oyster shell habitat located by benthic mapping may be useful for targeting of future restoration efforts.

Summary
Acoustic benthic habitat charaterization is a valuable tool in dynamic estuary systems in which low water column visibility is common and visual survey methodology impratical.  Reaches of subtidal habitat ill-defined by aerial imagery were successfully identified with acoustic systems.  We were able to survey, map and quantify benthic habitat within the nearshore of Mobjack Bay using remote-sensing technologies and classification software.  These technologies are especially useful to produce digital geospatial characterization of the extent and distribution of prevalent nearshore habitats, including submerged aquatic vegetation and oyster shell habitat. Benthic habitat data may be used to assess biotic interrelationships among habitats and biota, as well as the significance of specific spatial arrangements of habitat, and target/evaluate restoration or conservation sites.

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