The northern Portuguese coast is a biogeographic transition zone where many macroalgal species have their distribution limits; it is thus a particularly interesting region for investigating species distribution shifts. An updated and complete list of species for this region is not available in spite of its baseline importance for comparative studies with past and present data. Based on new records, literature references, and herbarium data, we provide an updated checklist of the benthic marine algae of the northern Portuguese coast. This checklist includes 346 species: 26 Cyanobacteria, 200 Rhodophyta, 70 Ochrophyta, and 50 Chlorophyta. From these, 21 species are new records for the Portuguese coast (Lyngbya aestuarii, Lyngbya semiplena, Microcoleus acutirostris, Myxosarcina gloeocapsoides, Aiolocolax pulchella, Antithamnion densum, Antithamnion villosum, Antithamnionella spirographidis, Dasya sessilis, Furcellaria lumbricalis, Neosiphonia harveyi, Porphyrostromium boryanum, Chorda filum, Dictyopteris ambigua, Sphacelaria rigidula, Undaria pinnatifida, Vaucheria coronata, Vaucheria velutina, Ulothrix implexa, Ulva scandinavica, and Umbraulva olivascens) and 33 were recorded for the first time in the north of Portugal. Alien species have increased in number and extended their distribution range over the last 10 years in the study area. Distribution shifts of northern cold water species with southern distribution limit in the north of Portugal were not consistent among species.

established by the Italian law were not rare and they were found close to the mainland and the main river outflows. The comparison with previous data to update the knowledge on the trophic conditions of the lagoon showed that the overall trophic load is reducing. Only nitrate concentrations did not change.The seasonal samples allowed to confirm the above observations highlighting the behaviour of nutrients in relation to the weather conditions and to the primary producer fluctuations.