13. Response of the photosynthetic rate of macrophytes to increased CO2 concentrations in a brackish-water ecosystem

Pajusalu Liina*, Martin Georg, Põllumäe Arno, and Paalme Tiina

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

The future increasing CO2 concentrations together with eutrophication and the predicted warming of seawater will create multiple threats to the coastal ecosystems of the Baltic Sea. Macrophytes are important structural component in the shallow coastal Baltic Sea ecosystems. However, their response to acidification and climate change is not well understood. The aim of the current study was to assess whether partial pressure of carbon dioxide (pCO2) and different environmental factors exerted interactive effects on the photosynthesis of key macrophyte species. The second objective examines the short-term variability of carbonate chemistry in shallow-water macroalgal habitats. The field experiments were conducted in Kõiguste Bay (northern part of Gulf of Riga, the Baltic Sea) during vegetation seasons of 2011-2014. Separate mesocosms were maintained at different pCO2 levels: ~2000, ~1000 and ~200 µatm. We measured the short-term photosynthetic responses of four macroalgal species: Fucus vesiculosus, Furcellaria lumbricalis, Ulva intestinalis, Cladophora glomerata and seagrass: Zostera marina. Our results show that increased CO2 levels may enhance the photosynthetic rate of macrophyte species and suggest that predicted marine acidification of the Baltic Sea could have implications for interspecific competition and structure of benthic communities in a future high CO2 world. Daily pH fluctuations may be larger than 1 unit in a shallow-water macrophyte meadow in summer conditions. These daily pH changes may be of a larger magnitude than the scenario modeling suggests for the surface–water pH decrease in the Baltic Sea by 2100.