Distribution and species composition of red algal communities in the northern Baltic Sea

Highlights • Prevalence and depth distribution of several red algal species were studied. • Distribution patterns were highly variable across environmental gradients. • Reduced depth penetration and occurrence were found in comparison to older data. • Most representative areas for red algae were revealed with GIS analyses. • The results advance management and conservation of these threatened habitats.

Red algae are an important component of the Baltic Sea rocky shores. Within the Finnish marine area, in the northern Baltic Sea, red algal communities (on habitat level) are currently listed as endangered, due to eutrophication effects and potential future decrease in salinity. At species level, Rhodomela confervoides and Ceramium virgatum are red-listed. Despite their common use in different management contexts within the Baltic Sea, e.g. their depth distribution used as an indicator of the status of the sea areas, we know little about the commonness and more detailed distribution patterns of the red algal species. This limits the reliability of algae-based assessments. The aim of this study was to describe the general occurrence patterns, abundance and prevalence of erect perennial and annual red algae, based on extensive scuba-diving inventories within the Finnish marine area. Furthermore, the most representative areas for the red algal communities were identified. The results show high variation in depth distribution and prevalence across environmental gradients for many red algal species, e.g. for the relatively common perennial species Furcellaria lumbricalis and Vertebrata fucoides. In comparison to older data, reductions in depth penetration and occurrence were identified especially for deep-occurring species such as Rhodomela confervoides. With improved knowledge on the occurrence patterns of red algal species in relation to environmental variation, our results enable more reliable use of red algae as indicators of the status of the sea areas. By identifying areas where red algal communities are currently most representative, the results also allow better targeting of management efforts aiming to improve their status, such as areal protection measures or reducing nutrient input from point sources.