Reproducibility, Precision, and Accuracy of a Hydroacoustic Method to Estimate Seagrass Canopy Height and Percent Cover in Massachusetts

Abstract

Measured decreases in the spatial extent of seagrasses in Massachusetts are attributed to declining water quality, disease, or direct and indirect impact from dredging or construction. Routine aerial monitoring of seagrasses along the 2400-km coastline is costly and therefore limited both temporally and spatially. Diver surveys are used at two sites for in situ assessments of seagrass meadow structure. In order to examine spatial and temporal trends in seagrass extent and structure, a survey methodology that is less expensive and better at capturing structural characteristics than aerial photography and more efficient than divers is needed. Hydroacoustic methods are relatively inexpensive and rapidly deployed and can measure characteristics including canopy height and percent cover of eelgrass meadows. We tested the capabilities and limitations of a BioSonics DT-X echosounder using the EcoSAV algorithm to measure presence/absence, canopy height, and percent cover of eelgrass. Measurements were found to be reproducible on duplicate transects but had low precision: 36 cm for canopy height and 34% for percent cover. To assess accuracy, we compared EcoSAV estimates of canopy height and percent cover with diver measurements. The EcoSAV estimates of canopy height and percent cover were significantly higher than diver measurements by 60 cm and 46%. We conclude that the BioSonics with EcoSAV processing is an efficient tool to map eelgrass meadows and assess relative change over time, but because of differences in the way the echosounder measurements are made compared with diver measurements, echosounder data should not be used to directly compare with diver data or for surveys that require high accuracy and precision.