Monitoring intertidal seagrass meadows with Sentinel-2: Insights for coastal management

Intertidal seagrass meadows face increasing threats from sea-level rise, water quality degradation, and coastal modification, despite providing critical ecosystem services, including climate regulation through carbon sequestration. Accessible and effective monitoring is therefore essential for conservation and restoration. This study assessed spatio-temporal trends in monospecific Nanozostera noltei intertidal meadows at four sites in the Republic of Ireland (i.e. Fenit and Spa on the SW coast; Sligo on the NW coast; and Dungarvan on the SE coast) using a series of 320 cloud-free, low-tide Sentinel-2 images (2018-2025) processed in Google Earth Engine and validated through in-situ observations. Seagrass percentage cover (SPC), meadow extent, and rates of change were estimated from the Normalized Difference Vegetation Index (NDVI > 0.179). Clear seasonal patterns were identified at all four sites, with peak SPC in August-September and maximum extent in October, whereas interannual trends differed among meadows, the most pronounced being a progressive decline in Sligo since 2020. The highest positive SPC change rates occurred in June-July, indicating the period of most active seagrass growth, which is directly relevant for timing conservation and restoration action. A site-specific decoupling between meadow extent and density was also observed. Furthermore, a satellite-derived Ecological Quality Ratio based on seagrass metrics was explored as an indicator of the ecological status of these transitional waters; however, it revealed limitations that mean such estimates must be interpreted with caution.