Development of a new acoustic mapping method for eelgrass and macroalgae using a multi-beam echo-sounder

Abstract

Eelgrass and various macroalgae play important roles in temperate coastal ecosystems, including as habitat for many species, and as a bio-indicator for water quality. However, in turbid or deeper waters, the optical remote sensing methods commonly used for mapping eelgrass do not provide the necessary range for analysis. We are developing a methodology for detecting and characterizing eelgrass and macroalgae beds using water column backscatter data from multi-beam echosounder systems. We are specifically developing methods to map the maximum depth limit, percent cover, functional type (i.e., macroalgae or eelgrass) and canopy height of the beds, because these are difficult to characterize using existing optical and acoustic methods. Water column data was collected using an Odom MB1 sonar in 2014 and 2015 over a variety of vegetated sites selected to represent a range of conditions: dense/sparse eelgrass, long/short eelgrass, mixed macroalgae and eelgrass, eelgrass on muddy or hard substrates, etc. In addition to sonar data, drop camera data was collected, and data from a regional aerial mapping program also exist for comparison. Initial data analysis shows good agreement between drop camera and sonar detections, and patches as small as 1m2 and as short as 20 cm are detectable.