Benefits and challenges of UAV imagery for eelgrass (Zostera marina) mapping in small estuaries of the Salish Sea

Abstract

Seagrasses are a fundamental component of nearshore marine habitats and as such, concerted effort has been put into developing remote sensing methods for mapping and monitoring these important habitats. However, in the small coastal bays of the Salish Sea, traditional aerial or satellite remote sensing can be cost-prohibitive or lack sufficient spatial resolution to detect the small, fringing, and often patchy eelgrass (Zostera marina) meadows. Bridging the gap between remotely sensed data and ground-based mapping techniques, aerial imagery collected by Unmanned Aerial Vehicle (UAV) is revolutionizing the study of fine-scale ecological phenomena. This paper presents a method for collection and processing of UAV imagery to map eelgrass (Zostera marina) at three small coastal estuaries in the Salish Sea of British Columbia. A quad-copter style XAircraft X650 UAV equipped with a rectilinear GoPro Hero 3+ was used to acquire images with a ground resolution of 2 cm. Pix4D Pro software was used to orthorectify, georeference, and mosaic UAV imagery into continuous orthomosaics. To delineate eelgrass, a manual classification approach of segmented image objects was used on an eelgrass presence or absence basis using ground reference data collected in the form of underwater videography collected by kayak. Mapping accuracies of 95.3%, 88.9%, and 90.1% were achieved for Village Bay, Horton Bay, and Lyall Harbour respectively. The benefits (flexible acquisition, high resolution) and challenges (inconsistent radiometry, small footprint) of UAV mapping of seagrasses are compared to the use of conventional remote sensing technologies