Anthropogenic carbon dioxide (CO2) emissions to the atmosphere are causing reduction in the global ocean pH, also known as ocean acidification. This change alters the equilibrium of different forms of dissolved inorganic carbon in seawater that macroalgae use for their photosynthesis. In the Baltic Sea, benthic macroalgae live in a highly variable environment caused by seasonality and rapid changes in meteorological conditions. The effect of increasing water CO2 concentration on the net photosynthesis of the red macroalgae Furcellaria lumbricalis (Hudson) Lamouroux was tested in short-term mesocosm experiments conducted in Kõiguste Bay (N Gulf of Riga) in June-July 2012 and 2013. Separate mesocosms were maintained at different pCO2 levels: ca. 2,000, ca. 1,000 and ca. 200 µatm. In parallel, different environmental factors were measured such as nutrients, light and water temperature. Thus, the current study also investigated whether elevated pCO2 and different environmental factors exerted interactive effects on the photosynthetic rate of F. lumbricalis. In addition, laboratory experiments were carried out to determine the optimal temperature for photosynthesis of F. lumbricalis. The results of our field experiments demonstrated that elevated pCO2 levels may remarkably enhance the photosynthetic rate of F. lumbricalis. However, the magnitude of this effect is altered by different environmental factors, mainly by changes in water temperature.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2016) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation is 2016-12-29.

Supplement to: Pajusalu, Liina; Martin, Georg; Paalme, Tiina; Põllumäe, Arno (2016): The effect of CO2 enrichment on net photosynthesis of the red alga Furcellaria lumbricalis in a brackish water environment. PeerJ, 4, e2505