Monitoring intertidal golden tides dominated by Ectocarpus siliculosus using Sentinel-2 imagery

Abstract

Golden seaweed tides are a global environmental and social problem, that have been occurring along Ireland's eastern coastline since the 1990s. This study focused on analysing the spatiotemporal dynamics of golden seaweed (Ectocarpus siliculosus) coverage at Dollymount Strand in Dublin Bay (Ireland), between 2016 and 2022, and its relationship with meteorological conditions. Hyperspectral measurements in the field and Sentinel-2 imagery were utilized to monitor macroalgal blooms with a spatiotemporal resolution of 10 m and minimum costs. The normalized difference vegetation index (NDVI) values filtered between 0.1 and 1 were used to identify the coverage of golden seaweeds. The results showed that the golden seaweed coverage extended over 99% of the study area (54 ha) in June 2020 with an average NDVI of 0.25. A seasonal pattern of golden seaweed abundance was observed and modelled from May to October using a Generalized Additive Model. Approximately 28% of the coverage was correlated with daily average global radiance, and 38% of the mean NDVI was associated with daily average maximum air temperatures and wind direction by means of the Generalized Linear Models. A greater biomass of Ectocarpus spp. was accumulated on the beach when the wind direction was from the north-east and south-east. The results also suggested that freshwater nutrient inputs in winter from nearby estuaries may be contributing to golden tides on the shoreline. These findings are useful in developing strategies aimed at controlling and managing golden tides globally, as well as using them as bioindicators of ecological status in coastal environments.