Climate change can cause complex responses in Baltic Sea macroalgae: A systematic review

- Declining salinity and elevated temperatures promote growth of filamentous green algae.
- Climate change is expected to intensify ongoing eutrophication of coastal ecosystems.
- Declining salinities will have negative impacts on foundation species such as Fucus.
- Results from ocean acidification experiments are mixed, and merit more research effort.

Estuarine macroalgae are important primary producers in aquatic ecosystems, and often foundation species providing structurally complex habitat. Climate change alters many abiotic factors that affect their long-term persistence and distribution. Here, we review the existing scientific literature on the tolerance of key macroalgal species in the Baltic Sea, the world's largest brackish water body. Elevated temperature is expected to intensify coastal eutrophication, further promoting growth of opportunistic, filamentous species, especially green algae, which are often species associated with intensive filamentous algal blooms. Declining salinities will push the distributions of marine species towards south, which may alter the Baltic Sea community compositions towards a more limnic state. Together with increasing eutrophication trends this may cause losses in marine-originating foundation species such as Fucus, causing severe biodiversity impacts. Experimental results on ocean acidification effects on macroalgae are mixed, with only few studies conducted in the Baltic Sea. We conclude that climate change can alter the structure and functioning of macroalgal ecosystems especially in the northern Baltic coastal areas, and can potentially act synergistically with eutrophication. We briefly discuss potential adaptation measures.