Seagrasses in coastal wetlands of the Algarve region (southern Portugal): Past and present distribution and extent

The global biodiversity crisis urges to update the distribution maps of natural habitats, especially those that are in rapid decline and play a key role in ecosystem functioning, such as seagrasses. In Portugal, seagrass meadows have declined substantially since the 1980s and are considered one of the most endangered marine habitats. Here we aimed to 1) compile records on the distribution and area extent of seagrass meadows in the four wetlands of the Algarve region (southern Portugal) with historical occurrence of seagrasses, based on a systematic review, and 2) update the distribution maps and area extent of seagrass meadows in this region using a combination of manual and machine learning mapping techniques. The systematic review showed a general lack of data on the distribution of seagrass meadows in the Algarve, with patchy information at different times and using incomparable mapping techniques. Based on the updated mapping from 2019 to 2023, well-developed seagrass meadows were only found in the Ria Formosa lagoon and the Guadiana estuary. In the Ria de Alvor, some sparce scattered patches of seagrass were observed in the intertidal zone, while no seagrass was found in the Arade estuary. The total extent of seagrass in the Algarve region was estimated at 1231 ha, with nearly all (99 %) occurring in the Ria Formosa, where 85 % was intertidal. Although assessing temporal trends remains challenging due to methodological differences and data limitations, the estimated 20 % decline and ongoing pressures on seagrass meadows in Algarve wetlands underscore the urgent need for continuous, standardized monitoring of their distribution, extent, and condition. • A combination of approaches was used to update seagrass mapping in the Algarve • Seagrass area in the Algarve was estimated at 1231 ha • 99 % of total seagrass meadows are in the Ria Formosa and 85 % are intertidal