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Non-indigenous seaweed species have been introduced to the coasts of Brittany, France for decades, with an increasing arrival rate since the 1970s due to both the introduction of the Pacific oyster and increased maritime traffic. In this study, seven species of red macroalgae originating from the Pacific Ocean were found in new locations around the coasts of Brittany between 2018 and 2022. The seaweed species belong to four different orders: Ceramiales (Antithamnion hubbsii, Polysiphonia morrowii, and Symphyocladiella dendroidea), Halymeniales (including Pachymeniopsis lanceolata and Polyopes lancifolius), Rhodymeniales (Botryocladia wrightii), and Gigartinales (Solieria sp.). The dispersal mode and putative invasive potential of each species were examined, and the species were described in terms of their macroscopic and microscopic appearance and habitat. These finds result from a combination of citizen science and long-term monitoring. Used together, these two approaches can aid in the detection of these species on other European coasts and in understanding their dispersion.

Intertidal macroalgal habitats are major components of temperate coastal ecosystems. Their distribution was studied using field sampling and hyperspectral remote mapping on a rocky shore of Porspoder (western Brittany, France). Covers of both dominating macroalgae and the sessile fauna were characterized in situ at low tide in 24 sampling spots, according to four bathymetric levels. A zone of ca. 17,000 m2 was characterized using a drone equipped with a hyperspectral camera. Macroalgae were identified by image processing using two classification methods to assess the representativeness of spectral classes. Finally, a comparison of the remote imaging data to the field sampling data was conducted. Seven seaweed classes were distinguished by hyperspectral pictures, including five different species of Fucales. The maximum likelihood (MLC) and spectral angle mapper (SAM) were both trained using image-derived spectra. MLC was more accurate to classify the main dominating species (Overall Accuracy (OA) 95.1%) than SAM (OA 87.9%) at a site scale. However, at sampling points scale, the results depend on the bathymetric level. This study evidenced the efficiency and accuracy of hyperspectral remote sensing to evaluate the distribution of dominating intertidal seaweed species and the potential for a combined field/remote approach to assess the ecological state of macroalgal communities.