Species-specific responses of benthic primary producers to the increasing CO2 environment with potential ecosystem implications involved in the Baltic Sea

Liina Pajusalu^1*, Gerli Albert¹, Evangeline Fachon², Christopher D. Hepburn^3,4, Jonne Kotta¹, Tiina Paalme¹, Anneliis Peterson¹, Daniel W. Pritchard⁴, Arno Põllumäe¹, Kaire Torn¹, Georg Martin¹

¹ Estonian Marine Institute, University of Tartu, Mäealuse 14, 12618 Tallinn, Estonia

² Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA

³ Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand

⁴ Coastal People Southern Skies, Centre of Research Excellence, Dunedin, New Zealand

Ocean Acidification (OA) research is primarily focused on the negative implications of elevated ocean carbon on calcifying organisms, while the effect on non-calcifying primary producers has received less attention. Benthic macrophytes vary in their ability to utilize carbon in form of HCO₃⁻ and/or CO₂ for photosynthesis. Some functional groups that are solely reliant on CO₂ for photosynthesis could receive competitive advantages from increasing CO₂ compared to groups that have efficient carbon acquisition strategies of HCO₃⁻. The aim of this research was to provide species-specific responses of benthic primary producers representing a broad range of traits to the increasing CO₂ environment. Then an extensive habitat mapping data was used to advise potential implications of the elevated CO₂ environment to the near coastal ecosystem of the NE Baltic Sea. This study focuses on the most common macrophyte species (macroalgae, charophytes, seagrass, and higher plants) found to inhabit the region. Mechanistic assessment of the carbon physiology of macrophytes was used to predict productivity and competitive interactions between different species under future climate. Carbon use strategies in macrophytes were determined by analysing the natural abundances of carbon isotopes (δ¹³C), pH drift experiments and photosynthesis vs. dissolved inorganic carbon (DIC) curves. The increasing CO₂ concentrations in the brackish Baltic Sea are expected to enhance the primary productivity of macrophytes. However, changing seawater carbon chemistry has the potential to influence the macrophyte species composition and community structure, and as a result the reduction of habitat-forming species under future climate.