Center for Coastal Resources Management

Current Research Project: 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

Final Report

Objectives
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.  Benthic habitat will be mapped and quantified within the nearshore of Mobjack Bay, including the Severn, Ware, North and East Rivers, and compared with described aquatic habitat distribution (e.g. SAV, tidal marsh) from other sources (CCRM, wetlands program and VIMS SAV program), to characterize the extent and distribution of habitats.

Click on map for larger image and video links

Data for the Mobjack Bay include submerged aquatic vegetation (SAV) distributions derived from aerial photography, benthic characterization (SAV, oyster reef and broad sediment class distributions) 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), and data derived from verification activities including sediment samples and video imagery. These data sets are provided in shapefile formats and delineated by river: Severn River, East River, North River, and Ware River.

Acoustic Surveys
Testing and calibration of equipment was completed prior to survey work to ensure the accuracy of data collected.  Survey tracks were plotted based on the results of pilot surveys, which helped establish the most appropriate settings and protocols for the shallow-water habitat mapping of Mobjack Bay (Figure 1).

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).
 
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.  The breakdown for each river is as follows:

Figure 1. Acoustic survey tracklines completed in the Mobjack Bay: Severn, Ware, North and East Rivers.

Post-processing of Acoustic Data
Survey data were analyzed with Questar Tangent software (July-August 2007) for the entire area.  The echo-sounder single beam data were processed with QTC Impact, and the side-scan sonar data with QTC Sideview. The 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 (Figure 2). 

Figure 2.  Benthic characterization of the Mobjack Bay based on side-scan sonar classification by QTC Sideview.  Each color represents a unique acoustic class, which may be representative of unique surficial benthic habitat.  In this image, similar colors have similar acoustic signals and thus may have similar benthic habitat characters.

Validation of Acoustic Classification
Acoustic classes were stratified by river and three regions were randomly selected for each class in each stratum (river) for ground-truthing.  Field evaluation is currently underway and consists of two major elements:  underwater video imagery and sediment type assessments.

Benthic imagery was obtained with a modified benthic sled outfitted with a forward and downward-facing video camera (Aqua-vu), that is flown along transects at each site of a given acoustic class (Figure 3).  Sediment-probes are conducted along the same transect with a handheld PVC rod with an adaptive clear piece at the end for sampling the top (~10-17 centimeters) of sediment depending on the sediment type/penetration. For each sediment probe, images are recorded of the sample (Figure 4), estimated depth of sediment layers within the probe noted, and descriptions of sediment type by percent and biogenic materials (e.g. shell, root matter) in the top and bottom layers of the sediment plug recorded independently.  Information on demarcations in sediment layers is critical to relate to dual-frequency echo-sounder data which describes sediment at differing depths of penetration (i.e. 28 kHz generally penetrates deeper into the sediment than 200 kHz which describes more surficial conditions). Sites beyond the reach of the sediment probe (in excess of 3.5m), or with impenetrable conditions are assessed by physically sampling surficial sediments when necessary.

	Figure 3.  Video imagery of submerged aquatic vegetation benthic habitat in the Mobjack Bay
	Figure 4.  Sediment probe to ascertain sediment type, presence of biogenic materials and depth of individual sediment layers.

Next Steps
We will complete field ground-truthing of acoustic classes, and further validation with auxiliary datasets, such as submerged aquatic vegetation surveys.  We will then examine and summarize underwater imagery in relation to the ground-truth data compiled, and will associate acoustic classes with the appropriate benthic characterization. Lastly, we will estimate the amount of each benthic class and create geo-referenced inventories and maps for use in management and restoration activities.