Assessment of the state for macrobenthos on solid grounds in the up per sublittoral in certain areas of Avachinsky Bay impacted by harmful algal bloom in autumn 2020, a year after the natural disaster

Results of a quantitative diving survey of macrobenthos in the Avachinsky Bay at southeastern Kamchatka in October 2021 are presented. The survey was conducted in a year after the harmful algal bloom (HAB) with mass death of marine organisms in certain areas of the coastal zone. The depth range of 4–14 m was surveyed. In total, 92 species of macrobenthos were identified, the average biomass of macrobenthos was assessed in 2768.7 ± 2115.4 g/m 2 , and the average abundance — in 2449.9 ± 1502.1 ind./m 2 . The species structure of macrobenthos was similar to the results of survey conducted by Kamchatka researchers in the spring of 2021. Significant reduction in the number of species and biomass was detected then for sponges, anemones, gastropods, echinoderms and ascidia and confirmed again by our data. In October 2021, the portion of these taxonomic groups in the total biomass of macrobenthos on solid soils did not exceed 1–2 % that is evident consequence of significant changes in structure of the bottom community caused by mass death of benthic organisms during HAB in the fall of 2020. A year after the HAB, bivalves and polychaetes, as well as red and brown algae occupied the dominant positions in the community in terms of the species number and abundance, whereas pacific mussel Mytilus trossulus dominated by biomass (this species was dominant before the natural disaster and preserved this position) and subdominants were Hedophyllum bongardianum , Thalassiophyllum clathrus (Ochrophyta), Schisobranchia insignis (Polychaeta) and Ptilota asplenioides (Rhodophyta). Increasing in abundance of Amphipoda and Decapoda crustaceans was noted in 2021 relative to 2020.