Abstract
Macrophytes vary in their ability to utilize carbon in the form of HCO3- and/or CO2 for photosynthesis. Some functional groups that solely use CO2 for photosynthesis could receive competitive advantages from the predicted increase in CO2 compared to groups with efficient carbon acquisition strategies of HCO3-. The aim of this study was to identify carbon use strategies in the common macrophytes (macroalgae, charophytes, seagrass, and other angiosperms) that represent a broad range of functional traits to CO2 concentrations in the northeastern Baltic Sea. Mechanistic assessment of the carbon physiology of macrophytes was used to predict productivity and competitive interactions between different functional groups under future climate. Carbon use strategies in macrophytes were determined by analysing the carbon isotopes (& delta;C-13), pH drift experiments, and photosynthesis versus dissolved inorganic carbon. In addition, habitat mapping data was used to interpret the potential implications of the elevated CO2 to this coastal ecosystem. The results suggested that the primary productivity of macrophytes is often limited by carbon availability, and the increasing CO2 concentrations in the brackish Baltic Sea are expected to enhance photosynthetic production. While all species tested showed evidence of carbon concentrating mechanisms (CCMs), differential levels of CCM activity indicate varying levels of competitive fitness in a future high-CO2 environment. Overall, macrophytes which inhabit the shallowest and deepest parts of the vegetated zone are expected to experience physiological benefits under future CO2 conditions, while intermediate communities dominated by the perennial brown alga Fucus vesiculosus may experience loss of fitness. These fitness differences have implications for competitive interaction and species range under future climate.