The occurrence and depth penetration of macroalgae along environmental gradients in the northern Baltic Sea

Eutrophication is known to affect the community structure of macroalgae by e.g. decreasing the depth penetration of species and by shifting dominance from perennial to annual species. However, there is substantial lack of knowledge in the Baltic Sea regarding the distribution of many of the macroalgal species, how natural environmental factors affect their occurrence and how they respond to eutrophication. As macroalgae are used as indicators of the quality of the sea areas in the EU legislation (Water Framework Directive, Marine Strategy Framework Directive), this kind of knowledge is essential. The aim of this study was to determine which variables were related to variation in species occurrence and their lower limit of occurrence in the Finnish marine area. The study was carried out on data from five study areas along the Finnish coastline and included about 30 taxa. Our results showed that both the macroalgal communities and the occurrence (presence/absence) of most of the species differed between the study areas and that the differences were mainly related to salinity and exposure, although also eutrophication related factors played a role. Of the perennial species, eutrophied conditions seemed to favour only the occurrence of Sphacelaria arctica and Polysiphonia fucoides. Secchi depth was important in determining the lower limit of occurrence of brown and red algal species. However, Secchi depth was rarely the only factor causing variation in the lower limit of occurrence as also exposure, salinity and slope of the shore affected it. We conclude that in the northern Baltic Sea, the taxonomic composition of the macroalgal communities is not a very useful indicator of eutrophication as perennial species seem to tolerate rather eutrophied conditions, when suitable substrate is available. The lower limit of occurrence of many of the brown and red algal species is a good indicator of eutrophication but due to lack of suitable substrate in more eutrophied areas, especially in the depths where light becomes limiting, it is only applicable in the middle and outer archipelago areas. Furthermore, when planning monitoring programmes or setting thresholds for evaluating the ecological status of the sea, the natural variation in the lower limit of occurrence of macroalgae across sea areas is problematic and should carefully be taken into account.

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► The occurrence of many macroalgae mainly related to salinity and exposure.
► Secchi depth important in determining the lower limit of occurrence (LLO) of algae.
► Also exposure, salinity and slope of the shore play a role in the LLO of species.
► LLO a useful indicator of eutrophication but environmental gradients must be considered.