Climate change confers a potential advantage to fleshy Antarctic crustose macroalgae over calcified species.

• The Antarctic Clathromorphum obtectulum has no response to climate change factors.
• Hildenbrandia sp. increases growth in increased temperature and decreased pH.
• Hildenbrandia sp. may have a competitive advantage in near future conditions.

Climate change impacts were investigated in two species of crustose Antarctic macroalgae that may be natural competitors in their habitat. The seawater parameters oceanic pH and temperature were modified to near future projections for the western Antarctic Peninsula in microcosm experiments. Experiments included two crustose algae, the calcified coralline alga Clathromorphum obtectulum and the fleshy encrusting rhodophyte Hildenbrandia sp., and were run for six weeks. Treatments reflected near future ocean conditions under climate change predictions: increased temperature (3.5 °C × pH 8.1), increased pCO2 (1.5 °C × pH 7.8), combined factors (3.5 °C × pH 7.8), and ambient conditions (1.5 °C × pH 8.1). The physiological responses of the algae were evaluated through photosynthetic parameters (slope to saturation of photo centers (α), saturating irradiance (Ek), maximum electron transport rate (ETRmax), maximum quantum yield of photosystem II (Fv/Fm)), growth, chlorophyll a concentration, and for C. obtectulum calcium carbonate content and Mg/Ca ratio. No negative impacts of elevated temperature or increased pCO2 were observed in either species. The fleshy alga decreased in size in low pH and high temperature treatments alone, but increased growth significantly when these factors were combined. Photosynthetic parameters were depressed by increased temperature in the calcified species and pH in the fleshy species but no significant differences were observed in other parameters in either species. This indicates that Hildenbrandia sp. may have a competitive advantage for space in the subtidal environment in near future oceanic conditions. However because benthic ecology in this geographic region is not well understood it is uncertain how these results will ultimately impact the community.