Zooplankton impact on the organic matter flux of Siberian Arctic seas

Role of zooplankton in the organic matter flux was studied in the Kara and Laptev seas using sediment traps in the course of voyage 63 of the Academician Mstislav Keldysh research vessel in August–October 2015. The values of the total flux and that of organic matter were at least an order of magnitude higher than those obtained before the 2000s. A hypothesis is proposed on an increasing influence of the river runoff on the Kara Sea area under climatic changes. Zooplankton contribution into C_org flux varied broadly, averaging 50%. The highest flux values and contribution of zooplankton in them (up to 96%) were recorded in the frontal (gradient) zones.     

The dominant copepods <Emphasis Type="Italic">Senecella siberica</Emphasis> and <Emphasis Type="Italic">Limnocalanus macrurus</Emphasis> in the Ob Estuary: ecology in a high-gradient environment

Intensive transformation and sedimentation of suspended matter from riverine runoff occur in estuarine frontal zones. The mesozooplankton community plays an important role in these processes. In the Ob Estuary, the dominant copepods Limnocalanus macrurus and Senecella siberica form dense local aggregations, but only scarce data on the ecology of these species in the estuarine environment are available. We aimed at analyzing the main aspects of the ecology of the two species including their grazing impact on phytoplankton. The distribution (net tows), ingestion rates (gut fluorescence analysis), respiration and excretion rates (incubation experiments), diet composition, gonad development and size of the lipid sacs of these copepods in a high-gradient area of the Ob Estuary were studied during a cruise of the R/V Professor Stockman in September 2013. S. siberica predominantly inhabited the freshwater zone; L. macrurus was more abundant in the estuarine frontal zone. In L. macrurus, adult females and males dominated the population, the herbivorous feeding hardly met the metabolic demands, the specific lipid content was high, and the gonads were developed. In S. siberica, the fifth copepodite stage (CV) dominated. The feeding rate considerably exceeded the metabolic requirements, and the lipid content was variable. The gonads were undeveloped. The two species grazed one-fifth of the phytoplankton biomass and more than 100 % of primary production, with S. siberica responsible for the main part of the total grazing impact (up to 90 %). These results are discussed in connection with the hydrophysical parameters and phase of the population’s life cycle. The obtained results contribute to the knowledge about zooplankton ecology and the transformation of suspended matter in an estuarine high-gradient environment.     

Diapause in the life cycle of Calanoides carinatus (Kroyer), (Copepoda, Calanoida)

The distribution and stage composition of Calanoidescarinatus (Kroyer) are described together with changes in protein and lipid content, respiration rate and gonad development in copepodite stages IV and V (CV) and adults in the Northern Benguela current (17°S, 23°S and 25°S).During active upwelling the population consisted of two parts: the surface part over the shelf was represented by all development stages, while the deep part offshore was dominated (90–95%) by diapausal CVs. In the surface CVs the surplus assimilated energy was allocated to structural growth and maturation or to synthesis of reserve lipids. CVs with large oil sacs and high lipid content descended into deeper layers and formed diapausal stock; they were characterized by a dramatic decrease of respiration rate. Increase of gonad size in association with decrease of oil sac volume in diapausal CV suggests that reserve lipids were expended not only for respiration but also for gonad development. The moulting of dispausal CVs into adults took place in deep water.These results are discussed in relation to the life cycle of the C. carinatus population and the factors causing the formation and termination of the diapausal phase.     

Spatial and Temporal Variability in the Abundance and Size Structure of Larvae of the White Sea Herring (<Emphasis Type="Italic">Clupea pallasii marisalbi</Emphasis>) in Onega and Kandalaksha Bays of the White Sea

According to the data of ichthyological surveys conducted in Onega and Kandalaksha bays of the White Sea in June 2015, the abundance and pattern of spatial distribution of larvae of the White Sea herring Clupea pallasii marisalbi are comparable with those of 2012. Aggregations of herring larvae detected at a distance of 12?14 km from the coast in the apex part of Kandalaksha Bay are probably the result of their mass drift caused by fresh floodwater discharge. In coastal waters of the bays adjacent to the littoral part, the abundance of herring larvae above the depths less than 5 m varies considerably due to their drift under effect of alongshore and/or tidal currents. The White Sea herring larvae reach high abundance only in the inlets (Chupa, Knyazhaya, Belaya, and Maikova inlets) with the river runoff; their length increases with the distance from spawning grounds. In different years, the main bulk of herring larvae in Knyazhaya Inlet is concentrated at depths about 12–15 m at 6?8°C, or moved to the upper 5-m quasi-homogeneous layer when the water temperature at the depth was 0?1°C.     

Spatial Distribution of Picophytoplankton in the White Sea in the Beginning of Summer

Biomass of photosynthetic picoplankton (B_pic), its contribution to the total phytoplankton biomass (B_pic%), chlorophyll a concentration (Chl), and associated hydrophysical characteristics of the water masses in the White Sea were estimated in June 2015 at 47 stations located in the open parts and in the inlets of the Onega and Kandalaksha Bays and in the western part of the Basin. Spatial variability of mean values of B_pic in the photic layer (0.01–1.91 mg C/m³) was preconditioned by sub- and mesoscale heterogeneity of the hydrological conditions. The values of B_pic were higher near the frontal zones than those in the water masses characterized by quasi-homogeneous distribution of the thermohaline characteristics. The relative contribution of B_pic did not exceed 1% at half of the stations and varied from 1 to 8% for the rest of the studied water area. The value of B_pic% reached 40.5% in the Basin and did not exceed 2% during the phytoplankton bloom in Knyazhaya Inlet (Chl > 3 mg/m³) accompanied by the dominance of Skeletonema costatum sensu lato.     

Physiological characteristics of the antarctic copepod Calanoides acutus during late summer in the Weddell Sea

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.     

Feeding,metabolism and body composition of the Antarctic copepod Calanus propinquus Brady with special reference to its life cycle

Summary 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.     

Pyrosoma atlanticum (Tunicata, Thaliacea): grazing impact on phytoplankton standing stock and role in organic carbon flux

Pyrosomas are the large group of pelagic tunicates whose trophicrole in pelagic communities has not yet been sufficiently studied.We ran across a local area of high concentration of the mostwidespread and commonest species of pyrosomas, Pyrosoma atlanticum,450 miles off the Congo river mouth. The following was estimated:gut pigment content, defecation rate, organic carbon and pigmentcontent of fecal pellets, and sinking rate. Based on these dataand the measured number of pyrosomas colonies the grazing impacton phytoplankton and the fecal pellet flux were calculated.During the night swarms of 50–65 mm P.atlanticum removed53% of phytoplankton standing stock in the 0–10 m layer;sparsely distributed pyrosomas consumed only 4%. The grazingimpact in the 0–50 m layer was only 12.5 and <1% respectively.The fecal pellet flux resulting from nocturnal feeding of P.atlanticumwhile swarming made up 1.4–1.6 x 10⁶ pellets m^–210 h^–1 or 305–1035 mg C m^–2 10 h^–1 and1.4 x 10⁵ pellets m^–2 10 h^–1 or 87.4 mg C m^–210 h^–1 while non-swarming. Incubation experiments showedthe rapid degradation of fecal pellets at 23°C: the lossof pigment and carbon content was {small tilde}60–70%after 45 h. We believe that given the sinking rate of 70 m day^–1the main part of fecal material does not leave the upper watercolumn and is retained in the trophic web of the epipelagiclayer.