Abnormal decrease in abundance and biomass of population of Calanus euxinus (copepoda) in the deep-sea areas of the Black Sea in autumn 2019 – what is happening with the ecosystem?
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Abstract
During plankton sampling carried out in the deep and shelf areas of the Black Sea near Crimea in October 2019 г. (110th cruise of the RV “Professor Vodyanitsky”), the quantitative distribution, abundance, biomass, size and age structure, lipid reserves of the population of Сalanus euxinus Hulsemann, 1991 were studied. Zooplankton samples were collected with the Bogorov – Rass net (inlet area of 0.5 m², mesh size of 300 μm) using vertical net hauls from the bottom or the lower boundary of the oxygen zone to the sea surface. The size and age structures of С. euxinus were determined using a stereomicroscope in the laboratory. Storage lipid content in the population of С. euxinus was estimated basing on the specific oil sac volume (% of the body volume) in the late copepodite stages.
It was shown that ongoing warming in the Azov-Black Sea region in 2019 led to a decrease in abundance and biomass of С. euxinus in the deep-sea areas up to 5.3 ± 0.5 ind.·m-2 and 3.7 ± 0.3 g·m-2, respectively, and also to the changes in the population structure. Partly, these phenomena could be related to a weakening of winter convective mixing in the sea. Probably, a decrease in river inflow after severe drought in 2018–2019 which dramatically reduced the supply of nutrients to the sea was more critical for C. euxinus. Due to a weak transfer of nitrates, phosphates and especially silicates with the river flow, abundant phytoplankton (particularly, diatoms critically needing silicates) in 2019 did not get sufficient mineral sources for development and, correspondingly, could not satisfy the trophic requirements of C. euxinus at the level of previous years. An increase in lipid content of V copepodites in autumn 2019 is explicable by an abnormal low density of population of C. euxinus that year, probably, because of extremely high biomass of planktivorous jellyfish, especially medusa Aurelia aurita, in winter–autumn 2017–2018.