Mapping eelgrass and assessing anthropogenic stressors in Placentia Bay and Trinity Bay, Newfoundland and Labrador

Abstract

Seagrasses are declining globally at an accelerating rate. A lack of information on seagrass condition and spatial extent is hindering seagrass monitoring, conservation, and management. This study provides a baseline of eelgrass’ (Zostera marina) extent at several sites in Placentia Bay and Trinity Bay, Newfoundland and Labrador, Canada, providing a reference point for future eelgrass monitoring. Eelgrass was mapped using high-resolution UAV imagery and underwater video. The presence of physical disturbance and eutrophication was assessed using visual analysis of both imagery sources. Additionally, changes in eelgrass landscape structure were assessed in areas with and without the invasive European green crab (Carcinus maenas). The total area of eelgrass mapped was approximately 1km2 across seven sites in Placentia Bay and three sites in Trinity Bay. There were few indications of physical disturbance and eutrophication across the study sites. The site with the largest change in eelgrass (a loss of 98.9% of eelgrass area between 2014 and 2020) in Placentia Bay also had the greatest abundance of beach-washed European green crab carapaces. Observations agree with previous research that has shown that eelgrass in Placentia Bay is predominantly threatened by the invasive European green crab.