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Mapping and measuring eelgrass beds with an MB1 sonar

eel-grass

Eelgrass plays many important roles in temperate coastal ecosystems, including as habitat for many species, and as a bio-indicator for water quality in many areas.

This presentation was given at the Teledyne Marine Technology Workshop in San Diego 2015 by Ashley Norton from Center for Coastal and Ocean Mapping

The deepest edges of eelgrass beds are considered more vulnerable to water quality issues because of the pre-existing light limitation with increasing depth due to natural light attenuation. However, the deep edges of beds are also often the most difficult to delineate with satellite and aerial imagery often used for large-scale seagrass mapping programs. We are developing a methodology to characterize the depth limit (‘deep edge’), percent cover and canopy height of eelgrass beds at high resolution (~1 m) using water column acoustic backscatter data from an MB1 multi-beam echo-sounder.

An automated data processing workflow is being developed that will use a combination of digital signal and image-processing techniques, including techniques originally developed for medical ultrasound imagery. These data can provide georeferenced acoustic imagery and depth information needed to document the location, structure, and spatial heterogeneity of eelgrass beds, with more spatial coverage than existing acoustic tools that mostly utilize single-beam echosounders.

Water column data were collected over beds at 3 locations in the estuary in the summer of 2014, and preliminary data analysis shows that eelgrass patches as small as 1m2 and as short as 20 cm are detectable. Data was also collected concurrently in the summer of 2015, and they include1) ground-truth data from drop camera imagery and field surveys; 2) aerial surveys; and 3) acoustic backscatter data. The ability to process multi-beam water column data for eelgrass characterization may provide a new data source and tool for ecologists and managers interested in eelgrass distribution and characterization, as well as bathymetric information used for charting depths.