Substantial short-term increases in brown macroalgal cover and total macroalgal biomass with decreased sea ice on the Western Antarctic Peninsula

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.