Aleksandr V. Drits

Zooplankton composition, distribution, and feeding rate in the Ob Estuary at the beginning of ice formation were studied for the first time. The data were collected during cruises nos. 69 and 92 of the Akademik Mstislav Keldysh research vessel in late September 2017 and in October 2023. A biomass was dominated by copepods Limnocalanus macrurus, Drepanopus bungei, and Pseudocalanus spp. Zooplankton species composition and spatial distribution depended on a hydrophysical structure. In 2023, a local zooplankton aggregation of D. bungei with an abundance of 22 000 ind m^–3 was found in the frontal zone. In the same area, in 2017, an increase in zooplankton biomass was related to a high abundance of L. macrurus. Due to a seasonal deterioration of trophic conditions in October 2023, a specific daily food ration of D. bungei decreased by almost fourfold, while that of Senecella siberica, by an order of magnitude compared with 2017. A daily ration of L. macrurus was lower than its respiration expenditures in both years. Only Jaschnovia tolli’s ration was significantly higher than its respiration expenditures in both years. Average values of daily grazing of phytoplankton biomass and production were 1–2% and 19% outside of the zooplankton aggregations, whereas they reached 7 and 300%, respectively, in these aggregations.

The composition, distribution, and grazing impact of mesozooplankton on phytoplankton in the area of the continental slope of the Laptev Sea and adjacent areas of the outer shelf and deep basin were studied. The data were obtained in August–September 2018 during cruise 72 of the R/V Akademik Mstislav Keldysh along two quasi-meridional transects. The zooplankton biomass (dry weight, DW) in the 0–bottom layer increased with depth from 3–5 g m^–2 on the shelf to 9–17 g m^–2 above the slope and 12–17 g m^–2 in deep-water areas. Over the shelf and slope, the proportion of Calanus glacialis in the total biomass was 80 and 45%, respectively, and it decreased to 32% in the deep-water area, while the proportion of C. hyperboreus increased to 39%. Contrary to the existing concept of an increased plankton concentration “belt” over the continental slope, no increase in the total zooplankton biomass was found in this area. The difference in the timing of the seasonal ice retreat was significant for the development of C. glacialis and had virtually no effect on the population dynamics of C. hyperboreus and Metridia longa. Daily rations when feeding on phytoplankton did not cover metabolic expenditures. Daily zooplankton grazing impact on phytoplankton of the size fraction >3 μm was 0.4–2% of phytoplankton biomass on the shelf, 7–10% above the slope, and 1.5–6% in the deep-water area.

—: A characteristic feature of the eastern Kara Sea, which is separated from its central part by the shallow Central Kara Rise, is a later ice breakup, a shorter ice-free period, and lower water temperature. Zooplankton in this part of the sea is virtually unstudied. The first data on the composition, spatial distribution of zooplankton, age structure and feeding of the dominant species, and their grazing impact on phytoplankton were obtained in the cruise 125 of the R/V Professor Shtokman (September 2013) and cruise 63 of the R/V Akademik Mstislav Keldysh (September 2015) to the eastern Kara Sea. 2015 was a typical year in terms of the ice regime for this region, while in 2013, seasonal ice coverage lasted longer than usual. Zooplankton biomass (17–112 mg dry weight m ^–3 ), to which copepods Calanus glacialis contributed the main portion, were close to the values in the central Kara Sea in September. The seasonal development of the C. glacialis population began much later than in the central Kara Sea, but development of Metridia longa was not influenced by the specifics of the ice regime and hydrophysical conditions in the eastern Kara Sea. The interannual differences in the timing of ice retreat had almost no effect on the zooplankton abundance, seasonal development of the dominant copepod populations, feeding activity, and grazing impact on phytoplankton.

In the estuaries of large Siberian rivers, ice coverage and the timing of ice retreat have varied in recent decades under the ongoing climate change. The seasonal development and functioning of the mesozooplankton community depend to a great extent on the timing of ice retreat. In the Arctic estuaries, the response of zooplankton to the timing of ice melt remains unclear. An earlier ice retreat was suggested to result in an advanced seasonal development of zooplankton, and higher biomass and feeding rates. Zooplankton composition, biomass, demography and grazing (assessed with the gut fluorescent approach) were studied in the Ob Estuary in July 2019 (“typical” ice retreat time). The obtained results were compared with the published data for July 2016 (ice retreat three weeks earlier). Zooplankton biomass in 2019 was considerably lower than in 2016, while species composition was similar; dominant populations were at an earlier stage of development. Herbivorous feeding of the dominant copepod, Limnocalanus macrurus, was also lower in 2019. The consequences of an earlier ice melt and increased temperature on seasonal population dynamics of the dominant brackish-water species are discussed. Our findings demonstrate that zooplankton communities in the Arctic estuaries are highly sensitive to the environmental changes associated with early sea-ice reduction.

Organic carbon of fecal pellets and small-sized (≤ 1mm) zooplankton was measured using a high- temperature combustion method. The method was adapted for measurements with Shimadzu TOC-VCPH analyzer equipped with a manual injection module. The range of organic carbon values was 30–10 000 ng ind^–1. The samples for analysis were collected during the cruises of the R/V “Akademik Mstislav Keldysh” in the Kara Sea in 2019–2021. Relationship between body carbon (C, µg ind^–1) and prosome length (L, mm) of zooplankton describes by the equation С = 4.24 L^1.84, r² = 0.85, n = 46. Carbon of field collected fecal pellet varied from 9.4 to 102.9 µg С mm^–3. The carbon to chlorophyll a ratio of fecal pellets differed by more than two orders of magnitude. The highest values (542.1 and 726.1) were obtained in the bays of the Novaya Zemlya Archipelago, the lowest (3.2–5.6) on the Kara sea shelf in June 2021 soon after ice melting. The perspectives of implication of high temperature combustion method for studies of zooplankton in situ feeding and to estimate the contribution of fecal pellets to POC vertical flux are discussed.

Application of High-Temperature Combustion Method for Measuring Content of Organic Carbon in Faecal Pellets and Small-Sized (≤1 mm) Zooplankton Применение метода высокотемпературного сжигания для измерения содержания органического углерода в мелкоразмерном (≤1 мм) зоопланктоне и фекальных пеллетах

Sedimentation rates in the northern Kara Sea between September 2019 and July 2020 were calculated from integrated monthly values for sedimentary matter, organic carbon, nitrogen, total silica, aluminum, fecal pellets, and phytoplankton using a multicup sediment trap moored at a depth of 110 m. The seasonal dynamics of the total flux and its components was characterized by three pronounced maxima, observed not only in the ice-free season in September, but also under ice in February and April. The flux of total matter and particulate organic carbon (POC) during these periods was 400–600 mg m^–2 day^–1 and 8–14 mgC m^–2 d^–1, respectively. The increase in the flux in winter and early spring was most likely due to the formation of polynyas. The contribution of the biogenic POC flux was 30% of the total POC flux in the winter months, 50% in the autumn, and up to 80% in May–June. The maximum flux of fecal pellets and larvacean “houses” (0.2 mgC m^–2 d^–1) was observed in autumn and did not exceed 0.02 mgC m^–2 d^–1 the rest of the year. The phytoplankton flux was about 0.05 mgC m^–2 d^–1 in autumn, and it decreased to –2 d^–1 in winter. An increase in the phytoplankton flux in May–June to its maximum values (0.2 mgC m^–2 d^–1) indicated the development of plankton algae long before ice melting. The calculated annual flux for total matter and POC (75 g m^–2 yr^–1 and 2 gC m^–2 yr^–1) was significantly lower than the published values for the Kara Sea shelf influenced by river runoff.

We studied the influence of river discharge and timing of ice retreat on sinking fluxes of total particulate matter and its components in the Laptev Sea in August–September 2017 and 2018. Vertical fluxes were measured with short‐term sediment traps located on the shelf adjusted to the Lena and Khatanga Rivers inflows (four shallow stations in 2017) and in Vilkitski Strait/Trough area (two deeper stations) in 2018. One station outside the Khatanga plume was repeated in both years. The effect of river discharge manifested by high total particle flux (1–1.6 g m⁻² d⁻¹), high Si (0.3 g m⁻² d⁻¹) and Al (0.1 g m⁻² d⁻¹) fluxes, a high proportion of terrigenous organic carbon (10.7%–27.5% total organic carbon) was clearly recognizable only in the shelf area closest to the Lena inflow. Large contribution of the lithogenic matter to the flux on the Laptev shelf is suggested to result also from sediment resuspension (caused by the tidal mixing and upwelling events), lateral offshore transport of sediment just above the seabed, and coastal abrasion. The magnitude of vertical fluxes of total particulate matter and organic carbon was two to three times higher in 2017 (late ice retreat) than in 2018 (early ice retreat). Centric diatoms dominated protist flux in 2017 while in 2018, dinoflagellates and tintinnids dominated. The contribution of protists to the biogenic organic carbon flux reached 62% in 2018 versus <1% in 2017. Zooplankton fecal pellet flux was not influenced by river discharge but was higher soon after ice melt.

We studied sedimentation processes in the Lofoten Basin of the Norwegian Sea based on data of the Automatic Deep-Sea Sedimentation Observatory (ADSO) deployed for a year (August 2017–July 2018). The article presents estimates of the vertical fluxes of total sedimentary material, organic carbon (C_org), calcium carbonate (CaCO₃), biogenic silica (SiO_2bio), aluminum, and the particles of planktonic origin (pteropods, foraminifera, fecal pellets, necrozooplankton). The seasonal pattern of total vertical flux is characterized by a unimodal distribution with a maximum in September 2017 (191 mg dry weight m^–2 day^–1) in the 0–500 m surface water layer. The total flux decreases markedly from November and reaches the minimum (2.6 mg m^–2 day^–1) in March–April 2018. The maximum C_org flux values (5.2 mg C m^–2 day^–1), CaCO₃ (178 mg CaCO₃ m^–2 day^–1), and SiO_2bio (9.1 mg SiO_2bio m^–2 day^–1) are observed in autumn. The value of fluxes and the portion of lithogenic material in it are higher at a depth of 2950 m due to redeposition from the nepheloid layer. The amplitude of flux variations during the year is noticeably smaller at this depth. The highest fluxes are related to the maass sedimentation of juvenile pteropods Limacina retroversa. The contribution of pteropods to vertical fluxes in autumn is 60–100% for CaCO₃ and 56–73% for C_org. The contribution of foraminifera to the total CaCO₃ flux does not exceed 6% and only reaches 60% at a depth of 550 m in October. The flux of fecal pellets, where ostracod pellets dominate, varies from –2 day^–1, making up from 0.1 to 90% of the C_org flux. The flux of zooplankton carcasses (ZCs) in the 0–550 m water layer varies from 0 to 0.45 mgC m^–2 day^–1, reaching the maximum value in February. The share of ZCs flux was 0–33% in the summer–autumn season, 12–52% in winter, and 23–41% in spring relative to the total (taking into account the ZCs) C_org flux. The ZCs flux varied from 0.04 to 0.22 mgC m^–2 day^–1 in the near-bottom water layer and amounted to 0.7–2% of the total C_org flux. The total contribution of all studied planktonic particles to the annual CaCO₃ flux was 16–47%, and their contribution to the annual C_org flux was 37–45%. Comparative analysis of our results and published data obtained in the 1990s demonstrates that the observed climatic changes (increasing temperature and salinity, as well as an increase in the thickness of the Atlantic water layer) in the Lofoten Basin did not result in significant changes in the quantitative characteristics of sedimentation process.