Spatial Variability of Macro- and Mesozooplankton in the Bransfield Strait in January 2020

Abstract

The spatial distribution of the biomass of macro- and mesozoopankton species, as well as the size and sex structure of the species Euphausiasuperba (krill), Salpathompsoni, and Ihlearacovitzai in Bransfield Strait during the Antarctic summer of 2020 were studied. Krill is at the heart of the food chain of the Antarctic ecosystem because numerous populations of seabirds, seals and whales feed on krill. In recent decades, the Antarctic ecosystem has been under significant changes caused by global warming, but the nature and extent of this impact on krill stocks, especially its juveniles in the Bransfield Strait, remains insufficiently studied. In recent decades, the Antarctic ecosystem has been experiencing significant changes associated with global warming, but the nature and extent of this impact on krill stocks, especially its juveniles in the Bransfield Strait, remains insufficiently studied. At the same time, this particular region is a highly productive zone of the Antarctic ecosystem and one of the areas where concentrations of Antarctic krill accumulate. The aim of this study was to study the spatial variability of the structure, numbers and biomass of zooplankton in the Bransfield Strait during the Antarctic summer of 2020. Zooplankton samples in Bransfield Strait were collected with Bongo net in January 2020. The samples were processed on the research vessel in the intervals between sampling according to the standard procedure. At all stations studied, the main zooplankton biomass was by salps (S. thompsoni and I. racovitzai). According to the index of coenotic significance, salps were of primary importance in the zooplanktocenosis, in which E. superba occupied the third place with biomass ranged from 6.3 to 96.3 mg/m3, and krill was meet only in the northwestern deep-water part of the Bransfield Strait. This is due to food competition between salps (S. thompsoni, I. racovitzai) and krill. The presence of krill in the deep-water part of the Bransfield Strait is explained by its introduction from the Bellingshausen Sea due to the prevalence of western winds. The biomass of other zooplankton species in all areas of the strait happened very small in comparison with that of salps and krill and was distributed uniformly. When comparing the ratio of the biomass of the above species obtained during the expedition with the data of the 90s and 2000s, there is a clear trend of an increase in salps and a decrease in E. superba, that is probably associated with the general trend of an increase in water temperature in the Antarctic waters. Thus, the example of Bransfield Strait clearly shows a negative correlation between the biomass of salps and krill.