Seasonal interactive effects of pCO₂ and irradiance on the ecophysiology of brown macroalga Fucus vesiculosus L.

Stochastic upwelling of seawater in the Baltic Sea from the deep, anoxic bottoms may bring low-pH water rich in CO₂ close to the surface. Such events may become more frequent with climate change and ongoing ocean acidification (OA). Photoautotrophs, such as macroalgae, which are important foundation species, have been proposed to benefit from increased carbon availability due to reduced energetic cost in carbon acquisition. However, the exact effects of CO₂ fertilization may depend on the ambient light environment, as photosynthesis rates depend on available irradiance. In this experimental study, interacting effects of CO₂ addition and irradiance on the habitat-forming macroalga Fucus vesiculosus were investigated during two seasons – winter and summer – in the northern Baltic Sea. Growth rates remained unaffected by CO₂ or irradiance during both seasons, suggesting that direct effects of elevated CO₂ on mature F. vesiculosus are small. Increases in CO₂ affected algal elemental ratios by increasing carbon and decreasing nitrogen content, with resulting changes in the C:N ratio, but only in winter. In summer, chlorophyll a content increased under low irradiance. Increases in CO₂ caused a decline in light-harvesting efficiency (decrease in F_v/F_m and α) under high irradiance in summer, and conversely increased α under low irradiance. High irradiance caused increases in the maximum relative electron transport rate (rETR_max) in summer, but not in winter. Differences between winter and summer indicate that F. vesiculosus responses to CO₂ and irradiance are season-specific. Increases in carbon content during winter could indicate slightly positive effects of CO₂ addition in the long run if the extra carbon gained may be capitalized in growth. The results of this study suggest that increases in CO₂, either through upwelling or OA, may have positive effects on F. vesiculosus, but these effects are probably small.