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.

The ecosystems of the Arctic Ocean and their expected changes in a context of Global climate processes are crucially dependent on the freshwater input. The freshwater signal is assumed to be the main structuring factor for the marine fauna on the shallow shelf of the Siberian Arctic seas. The Laptev Sea, as a part of the world’s widest continental shelves surrounding the Arctic Ocean, is a key area for understanding the land–ocean interaction in high latitude regions. The largest freshwater input is provided by the deltaic Lena river followed by the estuarine Khatanga river. The plumes of these rivers differ considerably in their hydrophysical characteristics, suggesting differential impacts on ecosystems of the adjacent shelf. The key component of pelagic ecosystems is zooplankton, which transfers energy from primary producers to higher trophic levels and modifies sedimentations processes. This study is focused on the influence of river discharge on zooplankton in the Laptev Sea at the end of productive season. We studied zooplankton biomass, species composition, distribution patterns of the dominant species and assessed herbivorous feeding rates and grazing pressure of these species along a transect from the inner Khatanga Gulf northward to the continental slope, and a transect in the Lena plume influenced area in August-September 2017. Despite large spatial extension of the Khatanga plume, the impact of river discharge on zooplankton species composition was restricted mainly to the inner Gulf where the brackish species shaped the community. Contrary to the Khatanga input, the Lena freshwater inflow was highly variable and under certain conditions (discharge rate, wind forcing) governed the structure of zooplankton community over a vast shelf area. Distribution patterns of zooplankton biomass, ingestion rates and grazing impact of the dominant species on phytoplankton over the shelf influenced by the Khatanga and Lena plumes were similar. Analysis of our results on demographic structure of Calanus glacialis suggests that seasonal development of the population was largely controlled by the time of ice retreat. Observed climatic changes in the Arctic were not reflected in the total zooplankton biomass and composition of the dominant species.

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.

На основе материалов, собранных 18–26.06.2012 г. на трёх полигонах в Онежском (губа Ухта) и Кандалакшском (губы Чупа и Княжая) заливах Белого моря, рассматривается влияние концентрации и размерно-видовой структуры кормового зоопланктона, а также количества потенциальных хищников на численность и скорость роста личинок беломорской сельди Clupea pallasii marisalbi. Суммарная численность зоопланктона в губах Ухта, Чупа и Княжая (соответственно 25041, 39161–76555 и 18459–33870 экз/м3) заметно превышает минимальные значения, необходимые для роста и развития личинок. Корреляционный анализ размерного состава личинок сельди и размерно-видового состава сетного мезозоопланктона указывает на то, что пищевые предпочтения личинок разных размерных групп достоверно различаются. Скорость роста у личинок в Кандалакшском заливе составляет 0.20–0.25 мм/сут. Выявлена обратная связь численности личинок беломорской сельди и гидромедуз: в губе Чупа при большой концентрации желетелого зоопланктона численность личинок была значительно ниже, а её сокращение через 5 сут. оказалось более существенным, чем при относительно невысокой концентрации медуз в губе Княжая. Это свидетельствует о том, что пищевые условия не всегда являются единственным и решающим фактором формирования урожайного или неурожайного поколений сельди.

Four species of planktonic calanoid copepods that co-occur in the California Current System (Eucalanus californicus Johnson, Rhincalanus nasutus Giesbrecht, Calanus pacificus californicus Brodsky, and Metridia pacifica Brodsky) were investigated for evidence of seasonal dormancy in the San Diego Trough. Indices used to differentiate actively growing from dormant animals included developmental stage structure and vertical distribution; activity of aerobic metabolic enzymes (Citrate Synthase and the Electron Transfer System complex); investment in depot lipids (wax esters and triacylglycerols); in situ grazing activity from gut fluorescence; and egg production rates in simulated in situ conditions. None of the 4 species exhibited a canonical calanoid pattern of winter dormancy - i.e., synchronous developmental arrest as copepodid stage V, descent into deep waters, reduced metabolism, and lack of winter reproduction. Instead, Calanus pacificus californicus has a biphasic life history in this region, with an actively reproducing segment of the population in surface waters overlying a deep dormant segment in winter. Eucalanus californicus is dormant as both adult females and copepodid V's, although winter females respond relatively rapidly to elevated food and temperature conditions; they begin feeding and producing eggs within 2-3 days. Rhincalanus nasutus appears to enter dormancy as adult females, although the evidence is equivocal. Metridia pacifica shows no evidence of dormancy, with sustained active feeding, diel vertical migration behavior, and elevated activity of metabolic enzymes in December as well as in June. The four species also differ markedly in water content, classes of storage lipids, and specific activity of Citrate Synthase. These results suggest that copepod dormancy traits and structural composition reflect diverse adaptations to regional environmental conditions rather than a uniform, canonical series of traits that remain invariant among taxa and fixed across a species' range. Such interspecific and regional differences in life history traits need to be incorporated in models simulating Eastern Boundary Current pelagic ecosystem dynamics. (C) 1998 Elsevier Science Ltd. All rights reserved.

Ingestion, respiration and excretion rates as well as lipid and protein body content of the dominant Antarctic copepod Calanoides acutus (CIV to adult females) were studied during the period covering the end of phytoplankton bloom (February) to the beginning of transition to overwintering (March-April). Daily rations measured with gut fluorescence varied from 2.2 to 2.7% in surface C. acutus. Weight-specific respiration and excretion rates in deep C. acutus decreased by a factor of 11 and 3.5–3.8 compared to their surface counterparts. High lipid (up to 455 µg ind−1) and protein (198 µg ind−l) content was observed in surface C. acutus CV in February; a month later the animals with similar lipid and protein content were found in the depth (500–1000 m layer). Their lipid reserves were enough to overwinter and probably to ascend, molt and reproduce. At the same time some of the deep CVs had much lower protein and lipid content and could survive only for 4–5 months. Our own and literature data led to the conclusion that females of C. acutus reach adulthood at the age of more than one year while development of males could be completed in one year.

In the Weddell Sea, several biochemical and physiological characteristics of copepodite stage III to adult females of Calanus propinquus from the surface layer (0–100 m) and the deep layer (500–1,000 m) were compared at a time of high phytoplankton abundance (February) and a month later, when chlorophyll concentration was much lower and older copepodite stages had migrated to their overwintering depth. Daily rations of surface animals, estimated by the gut fluorescence method, varied from 0.8% to 7.8% body carbon (increasing with phytoplankton concentration). Respiration rate (calculated from measurements of electron-transport-system activity) and lipid and protein body content of animals inhabiting the surface layer decreased from February to April. Deep water animals (stage V and adult females) were characterized by high lipid and protein content; their respiration and excretion rates were much lower than in surface copepods. Our calculation showed that they could survive without an additional source of energy for 8–9 months. Based on our own and published data on bio-chemistry and physiology the possibility of a two-year life cycle is suggested.

Protein, lipid, phosphorus, and organic carbon contents, as well as electron transport system (ETS) activity, lactatedehydrogenase activity, and gut evacuation rate, were measured in four interzonal species of Pacific copepods:Calanus australis, C. pacificus, Eucalanus inermis, andE. elongatus f.hyalinus, collected at the upwelling areas off Peru (8°S) and California (27°N), and in the middle of the North Pacific (30°N), from February to April 1987. The two Eucalanidae species —E. inermis andE. elongatus — have distinctive biochemical and elemental body composition and rates of main physiological processes. Relative protein, lipid, phosphorus, and organic carbon contents (µg mg−1 wet weight) in these species were, respectively, ca. 1/7 to 1/10, 1/5 to 1/20, 1/5 to 1/10, and 1/5 those inCalanus spp. Likewise, oxygen uptake rate per unit of wet weight (based on ETS activity) inE. inermis andE. elongatus was 5 to 10% of that in calanids; a similar difference was found in phosphorus excretion rate. In addition, gut evacuation rates inE. inermis andE. elongatus were ca. one-fifth of those inCalanus spp. Based on these data, we considered the eucalanids as belonging to a distinctive physiological group, figuratively named “jelly-body” copepods. In contrast with calanids, active lactatedehydrogenase has been found in the bodies ofE. inermis andE. elongatus, apparently allowing them to survive for a long time in layers of extremely low oxygen content (<0.2 ml l−1). The adaptive value of physiological features in these eucalanids and “typical” calanids is compared.