Consequences of the Impacts of a Harmful Algal Bloom on Benthic Invertebrates in the Eastern Kamchatka Shelf

Data of bottom trawl surveys conducted in the shelf zone of Kronotsky and Avachinsky Bays and off the southeastern coast of the Kamchatka Peninsula at depths of 65–210 m in 2018 and 2022, before and after the harmful microalgal bloom that occurred off the Kamchatka coast in the fall of 2020, were compared. The results indicate a significant decrease in the species diversity, population density, and biomass of benthic organisms in the lower sublittoral zone in 2022 compared to 2018. The least affected area was the northernmost region (Kronotsky Bay), while the most severely affected area was the southernmost region off the southeastern Kamchatka coast south of Cape Povorotny, where the biomass and species diversity dramatically decreased. This is consistent with the satellite monitoring data on chlorophyll a concentration off the Kamchatka coast in September 2020. According to these data, the area of the highest chlorophyll a concentration was the greatest in Avachinsky Bay and off the southeastern Kamchatka coast, where the strongest reduction in biodiversity of marine invertebrates was recorded.