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Increased temperature and ocean acidification are two main factors threatening Arctic coastal ecosystems. To explore the consequences of increased average temperature and ocean acidification in Kongsfjorden, the performance of six common species of macroalgae, the chlorophyte Monostroma arcticum, the rhodophytes Phycodrys rubens, and Ptilota plumosa, and the phaeophytes Alaria esculenta, Desmarestia aculeata and Saccorhiza dermatodea, was tested after 9 days of culture at 4 and 10 °C in combination with current (ca. 390 ppmv) and future (1000 ppmv) levels of atmospheric CO under solar radiation in summer. Temperature affected mainly the photosynthetic performance as measured by PAM fluorescence, particularly the initial slope ( α) of ETR curves, the light saturation parameter ( E ), and F / F values, as well as the protein content, especially in the phaeophytes. On the other hand, CO affected mainly the internal accumulation of carbohydrates and lipids. The C:N balance was largely unaffected. External carbonic anhydrase activity was not inhibited at high CO, and nitrate reductase activity remained unaffected. The resulting growth rate was not altered by treatments in three out of the six species studied. On the other hand, P. rubens showed a positive effect of increasing temperature, D. aculeata was negatively influenced by CO, and S. dermatodea was positively affected by CO. Significant interactions between CO and temperature were found in 20 % of the analyses. Whether additive or synergistic, the co-occurrence of a higher temperature with other stressors such as elevated CO increases the probability of community changes by modifying the performance of these species. [ABSTRACT FROM AUTHOR]

The combined effects of ocean acidification and ultraviolet radiation (UVR) have been studied in the kelps Alaria esculenta and Saccharina latissima from Kongsfjorden (Svalbard), two major components of the Arctic macroalgal community, in order to assess their potential to thrive in a changing environment. Overall results revealed synergistic effects, however with a different amplitude in the respective species. Changes in growth, internal N, C:N ratio, pigments, optimum quantum yield (Fv/Fm) and electron transport rates (ETR) following CO 2 enrichment and/or UVR were generally more pronounced in S. latissima than in A. esculenta . The highest growth rates were recorded under simultaneous CO 2 enrichment and UVR in both species. UVR-mediated changes in pigment content were partially prevented under elevated CO 2 in both species. Similarly, UVR led to increased photosynthetic efficiency (α) and ETR only if CO 2 was not elevated in A. esculenta and even under high CO 2 in S. latissima . Increased CO 2 did not inhibit external carbonic anhydrase (eCA) activity in the short-term but in the mid-term, indicating a control through acclimation of photosynthesis rather than a direct inhibition of eCA by CO 2 . The higher benefit of simultaneous CO 2 enrichment and UVR for S. latissima respect to A. esculenta seems to involve higher C and N assimilation efficiency, as well as higher ETR, despite a more sensitive Fv/Fm. The differential responses shown by these two species indicate that ongoing ocean acidification and UVR could potentially change the dominance at lower depths (4–6 m), which will eventually drive changes at the community level in the Arctic coastal ecosystem. These results support an existing consideration of S. latissima as a winner species in the global change scenario. [ABSTRACT FROM AUTHOR]