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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. 

Coastal food webs that are supported by multiple primary producer sources are considered to be more stable against perturbations. Here, we investigated how declining macroalgal abundance and diversity might influence coastal food web structure along an annual sea ice cover gradient along the Western Antarctic Peninsula (WAP). The most common benthic invertebrate consumers, macroalgae, and surface particulate organic matter were collected at 15 stations along the WAP. Stable carbon and nitrogen isotope values of primary producers changed negligibly in relation to the sea ice cover gradient, while isotope values of most invertebrate feeding groups increased with higher sea ice cover, although at low explanatory power. Food web length became shorter and consumer trophic niche width smaller in regions with higher sea ice cover. Changes in food web structure were mostly associated with shifts in trophic position of lower trophic levels. Food web structure in higher ice-covered regions resembled that of more generalist feeders with a loss of specialist species, concurrent with an increased reliance on a more reworked detrital food source. These results suggest that a number of benthic invertebrates are able to adjust to differences in basal energy sources. Conversely, these food webs dominated by generalist feeders are likely less efficient in energy transfer, which can create less-stable systems with lower adaptive capacity to disturbance. The predicted sea ice loss along the WAP may ultimately lead to a longer food web with higher macroalgal abundance, more specialist species, and wider consumer trophic niches in the currently more ice-covered regions. 

Previous research in southeast Alaska on the effects of sea otters Enhydra lutris in seagrass Zostera marina communities identified many but not all of the trophic relationships that were predicted by a sea otter-mediated trophic cascade. To further resolve these trophic connections, we compared biomass, carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope (SI), and fatty acid (FA) data from 16 taxa at 3 sites with high and 3 sites with low sea otter density (8.2 and 0.1 sea otters km -2 , respectively). We found lower crab and clam biomass in the high sea otter region but did not detect a difference in biomass of other seagrass community taxa or the overall community isotopic niche space between sea otter regions. Only staghorn sculpin differed in δ 13 C between regions, and Fucus , sugar kelp, butter clams, dock shrimp, and shiner perch differed in δ 15 N. FA analysis indicated multivariate dissimilarity in 11 of the 15 conspecifics between sea otter regions. FA analysis found essential FAs, which consumers must obtain from their diet, including 20:5ω3 (EPA) and 22:6ω3 (DHA), were common in discriminating conspecifics between sea otter regions, suggesting differences in consumer diets. Further FA analysis indicated that many consumers rely on diverse diets, regardless of sea otter region, potentially buffering these consumers from sea otter-mediated changes to diet availability. While sea otters are major consumers in this system, further studies are needed to understand the mechanisms responsible for the differences in biomarkers between regions with and without sea otters.