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Abstract Increases in atmospheric CO₂have led to more CO₂entering the world’s oceans, decreasing the pH in a process called ’ocean acidification’. Low pH has been linked to impacts on macroalgal growth and stress, which can alter palatability to herbivores. Two common and ecologically important macroalgal species from the western Antarctic Peninsula, the unpalatableDesmarestia menziesiiand the palatablePalmaria decipiens, were maintained under three pH treatments: ambient (pH 8.1), near future (7.7) and distant future (7.3) for 52 days and 18 days, respectively. Discs ofP. decipiensor artificial foods containing extracts ofD. menziesiifrom each treatment were presented to the amphipodGondogeneia antarcticain feeding choice experiments. Additionally,G. antarcticaexposed to the different treatments for 55 days were used in a feeding assay with untreatedP. decipiens. ForD. menziesii, extracts from the ambient treatment were eaten significantly more by weight than the other treatments. Similarly,P. decipiensdiscs from the ambient and pH 7.7 treatments were eaten more than those from the pH 7.3 treatment. There was no significant difference in the consumption by treatedG. antarctica. These results suggest that ocean acidification may decrease the palatability of these macroalgae to consumers but not alter consumption byG. antarctica. 

Abstract Ocean acidification refers to a decrease in the pH of the world’s oceans from the oceanic uptake of human-derived atmospheric CO₂. Low pH is known to decrease the calcification and survival of many calcifying invertebrates. Shallow, hard bottom communities along the Western Antarctic Peninsula often have incredibly large numbers of invertebrate mesograzers that shelter on and are mutualists with the dominant brown macroalgae. The common amphipod speciesDjerboa furcipes,Gondogeneia antarctica,andProstebbingia graciliswere collected from the immediate vicinity of Palmer Station, Antarctica (64°46′S, 64°03′W) in January–February 2023 and maintained under three different pH treatments simulating ambient conditions (approximately pH 8.0), near-future conditions for 2100 (pH 7.7), and distant future conditions (pH 7.3) for 8 weeks. Molt number and mortality were monitored throughout the course of the experiment. After the 8 week exposure, amphipods were analyzed for their biochemical compositions including the Mg/Ca ratio of their exoskeletons. There was no significant difference in biochemical composition or survival among the pH treatments for any of the amphipod species. All three species, however, had significantly fewer total numbers of molts in the pH 7.3 treatment than in the ambient treatment. These results suggest that amphipods may be able to maintain their survival in decreased pH by reallocating energy into compensatory behaviors, such as acid–base regulation, and away from energy expensive processes like molting. 

The pH of the world’s oceans has decreased since the Industrial Revolution due to the oceanic uptake of increased atmospheric CO₂in a process called ocean acidification. Low pH has been linked to negative impacts on the calcification, growth, and survival of calcifying invertebrates. Along the Western Antarctic Peninsula, dominant brown macroalgae often shelter large numbers of diverse invertebrate mesograzers, many of which are calcified. Mesograzer assemblages in this region are often composed of large numbers of amphipods which have key roles in Antarctic macroalgal communities. Understanding the impacts of acidification on amphipods is vital for understanding how these communities will be impacted by climate change. To assess how long-term acidification may influence the survival of different members in these assemblages, mesograzers, particularly amphipods, associated with the brown algaDesmarestia menziesiiwere collected from the immediate vicinity of Palmer Station, Antarctica (S64°46′, W64°03′) in January 2020 and maintained under three different pH treatments simulating ambient conditions (approximately pH 8.1), near-future conditions for 2100 (pH 7.7), and distant future conditions (pH 7.3) for 52 days then enumerated. Total assemblage number and the relative proportion of each species in the assemblage were found to be similar across the pH treatments. These results suggest that amphipod assemblages associated withD. menziesiimay be resistant to long-term exposure to decreased pH. 

There are numerous reports of macroalgal abundance along the northern portion of the Western Antarctic Peninsula (WAP) but little information about short-term variation in macroalgal abundance or in community structure. Here, we video-recorded replicate vertical transects between 5 and 40 m at 4 sites separated by <30 km near Anvers Island in 2019 and 2023, with 2 of the sites also recorded in 2020. Total macroalgal cover increased between 2019 and 2023 in all individual transects sampled in both years, with substantial increases in perennial brown overstory macroalgal cover. Nonparametric multivariate analyses of the communities identified significant differences in the macroalgal assemblages among all sites, and between 2019 and 2023 at 3 of the sites, but there were no significant differences in the macroinvertebrate assemblages across sites or years. Combined percent cover and destructive biomass quadrat sampling of a limited number of quadrats enabled estimations of macroalgal biomass changes from the video data. Although the absolute magnitudes reported here should be treated as preliminary estimates, biomass increases between 2019 and 2023 were clearly substantial because they were primarily from increases in the large overstory brown macroalgae. Sea ice concentrations were decreasing substantially across this time interval and were likely a causal factor in the increased macroalgal cover and biomass.