A. V. Drits

Effects of local hydrophysical conditions on the spatial variability of phytoplankton in the White Sea

The species composition and biomass of phytoplankton, chlorophyll a concentration (Chl), and hydrophysical characteristics of water masses have been studied in Onega and Kandalaksha bays (Chupa Inlet and Knyazhaya Inlet) of the White Sea at 16 stations June 17–26, 2012. Structural analysis of the phytoplankton community according to the Bray-Curtis similarity index has revealed two groups of stations. The first group united stations in stratified waters in Kandalaksha Bay; all stations located in Onega Bay formed another group. In turn, the stations in Onega Bay were separated into two subgroups corresponding to mixed (MWs) and stratified (SWs) waters. The total phytoplankton biomass and the biomass of diatoms and small unidentified flagellates were higher in Onega Bay. The biomass of dinoflagellates and cryptophytes, as well as Chl, did not differ significantly in Kandalaksha and Onega bays. In Onega Bay, the total phytoplankton biomass, Chl, and contribution of dinoflagellates to the total biomass were higher in SWs than in MWs. The contribution of diatoms was higher in SWs. The study addresses the role of the frontal zones in shaping the structure and distribution of the phytoplankton community.

Role of the Noctiluca scintillans population in the trophic dynamics of the Black Sea plankton over the spring period

Noctiluca scintillans is a common and numerous component of the heterotrophic plankton living in the Black Sea. It can play a significant role in the trophic dynamics of the pelagial community. The ingestion rate for the Black Sea Noctiluca is experimentally measured in this article, and its influence on the plankton community over the spring period is estimated. The average grazing of phytoplankton by the N. scintillans population made up about 0.8% in March. In local swarms, it was up to 11%. Noctiluca grazed 4.6% of the primary production per day in June. The grazing of Calanus euxinus eggs by the Noctiluca population varied from 1% to 42% in March. Then, it was 16% in April and 23% in June. The grazing of Acartia clausi eggs was from 1% to 7% in March–April (up to 80% in Noctiluca swarms), from 60% to 817% in May, and 45% in June. The grazing of Calanus faecal pellets was less than 1% in March–April and more than 20% in the beginning of June. It is the first time when the cumulative daily ration (subject to its various food components) for N. scintillans has been measured. The size of the ration was significantly higher than the energy expenditures on metabolism both in March (0.21 μg of C/ind/day) and in the beginning of June (0.28 μg of C/ind/day). Thus, the rapid growth of the population was enabled.

Small scale distribution of the White Sea herring larvae (Clupea pallasii marisalbi) in relation to hydrophysical features

The small-scale quantitative distribution of herring larvae and the corresponding hydrophysical structure were investigated in three spawning areas of the White Sea in Onega Bay (Uchta inlet) and Kandalaksha Gulf (Chupa and Knyazhaya inlets) from June 17 through 26, 2012. The mean number of herring larvae varied from 1.8 to 2.5 ind/m2 in the Uchta inlet, from 2.8 to 7.1 ind/m2 in the Chupa inlet, and from 59.6 to 162.9 ind/m2 in the Knyazhaya inlet. The strongly pronounced association of herring larvae with coastal areas where hydrophysical fronts play a key role was found. The role of frontal zones in the retention of herring larvae in favorable habitats and preventing their spreading beyond these habitats was revealed. The spatial scale of the larvae retention areas in the White Sea coastal zone can be estimated as tens of km2.

Feeding, respiration, and excretion of the Black Sea Noctiluca scintillans MacCartney in summer

Studies were conducted at the end of June 2011 in the coastal region of the northeastern part of the Black Sea. The bulk of the Noctiluca scintillans population was observed in the thermocline and reached a density of 40000 ind./m3. Analysis of digestive vacuoles content showed that Noctiluca could consume cells of Neoceratium tripos and N. furca, which had been considered inedible for Black Sea zooplankton, as well as temporary cysts of dinoflagellates, presumably of the toxic genus Alexandrium. The Noctiluca population consumed in total 10–30% of the abundance of temporary cysts, 2–29% of primary production, and 2–9% of potential Calanus euxinus egg production. For the first time, the excretion rates of ammonium nitrogen and mineral phosphorus were measured for N. scintillans. Our calculations showed that in summer, excretion by Noctiluca contributed from 4 to 18% and from 15 to 53% of phytoplankton total nitrogen and phosphorus requirements, respectively. The specific growth rate of Noctiluca (0.17–0.35) in summer, estimated from data on the daily food intake and respiration rate, was close to the values obtained in spring.

Marine environmental monitoring in the shelf zone of the Black Sea: Assessment of the current state of the pelagic ecosystem

The state of the shelf pelagic ecosystem has been assessed based on multidisciplinary monitoring performed in the northeastern Black Sea in 2005–2014. Seasonal and interannual variations in sea surface temperature (SST) and chlorophyll a (Chl-a) concentration have been analyzed along with the concentration of nutrients (silicate, nitrogen, and phosphate), biomass, and taxonomic compositions of phytoplankton, zooplankton, and gelatinous macroplankton. The linear trend shows an increase in the annual average SST by 0.9°C over the last decade. An increase in the winter SST is accompanied by a decrease in the concentration of silicates in spring (p < 0.05) and an increase in summer SST, by a decrease in Chl-a concentration and biomass of diatoms in the period of summer to fall (p < 0.05). A decrease in the phosphate concentration also has a negative effect on the development of diatoms (p < 0.01). The decrease in diatom biomass caused a decrease in herbivorous zooplankton biomass in the second half of the year (p = 0.05). Correlation analysis shows no significant dependence between the biomass of gelatinous top predators and mesozooplankton biomass. The assessed current state of the shelf pelagic ecosystem is regarded as stable; however, trends of a decrease in biomass and a change in the taxonomic composition of phytoplankton and zooplankton are observed during the last 2 years; the latter is likely to result from both direct and indirect effects of temperature increase in the upper sea layer.

Feeding of dominant zooplankton species and their grazing impact on autotrophic phytoplankton in the Yenisei Estuary in autumn

Feeding of dominant mesozooplankton species was investigated in freshwater zone, estuarine frontal zone of Yenisei Estuary and adjacent inner shelf area during autumn season. Ingestion rate was estimated based on gut fluorescence measurements. It was shown that in spite on the end of productive season daily ingestion of phytoplankton for the most of the investigated freshwater, brackish and marine zooplankton species was higher than their metabolic requirements. Total phytoplankton consumption by zooplankton differed in different zones. In freshwater zone under high level of autotrophic phytoplankton biomass and primary production zooplankton grazing impact was low: 1.5% of standing stock and 20% of primary production. In the estuarine frontal zone 3.2–14.3% of standing stock and 150–290% of primary production was grazed by zooplankton per day, in adjacent inner shelf: 1.4–7.0% and 130%, accordingly. Based on comparatively analysis of obtained data and results of investigation of zooplankton feeding in Ob Estuary during the same season some general patterns of the zooplankton role in organic matter biotransformation in the large arctic rivers estuarine areas were revealed.

Spatial distribution and feeding of dominant zooplankton species in the Ob River estuary

The distribution and feeding of dominant mesozooplankton species were studied in the estuary of the Ob River and adjacent inner Kara Sea shelf waters in September 2013. It was shown that the spatial distributions of Cyclops sp., Senecella siberica, Limnocalanus macrurus, Mysis oculata, Drepanopus bungei, Jashnovia tolli and Pseudocalanus sp. are related to the specific characteristics of the hydrographic regime in the estuarine frontal zone. The distributions of Cyclops sp., Senecella siberica, and Pseudocalanus sp. are mainly limited by salinity, while other species inhabit an area with a wide range of salinity values without clear preferences. Peaks of their abundance could be either consolidated or distanced in space. The populations of Jashnovia tolli, Drepanopus bungei, and Pseudocalanus sp. permanently inhabit the layer under the pycnohalocline; the populations of Cyclops sp. and Mysis oculata inhabit the upper mixed layer. Limnocalanus macrurus demonstrates a different vertical distribution pattern: the copepod undertakes diel vertical migrations in the southern part of the estuarine frontal zone; in its northern part, the population is concentrated below the pycnocline during day and night. The differences in the distributions of the studied species determine their feeding behavior and their role in phytoplankton grazing. The most intense utilization of biomass and production of autotrophic phytoplankton by zooplankton occur in the freshened water zone and the adjacent southern periphery of the estuarine frontal zone: the total daily phytoplankton consumption makes up 10–18% of the biomass and 60–380% of primary production. Daily zooplankton consumption of phytoplankton in the estuarine frontal zone decreases to 2–7% of the biomass and to 14% of primary production; in inner shelf waters, the values do not exceed 1% for both phytoplankton biomass and production.

Effect of trophic conditions on the growth and abundance of larvae of herring Clupea pallasii marisalbi in areas of their mass aggregations in Kandalaksha and Onega bays of the White Sea

On the basis of materials collected during June 18–26, 2012, in three polygons in Onega (Ukhta Inlet) and Kandalaksha (Chupa and Knyazhaya inlets) bays of the White Sea, the effect of concentration and size-species structure of the zooplankton and of the amount of potential predators on the numbers and growth rate of larvae of the White Sea herring Clupea pallasii marisalbi are considered. The total numbers of zooplankton in Ukhta, Chupa, and Knyazhaya inlets (25041, 39162–76555, and 18459–33870 ind/m3, respectively) noticeably exceeds minimal values necessary for the growth and development of larvae. Correlation analysis of the size composition of larval herring and size-species composition of net mesoplankton indicates that food preferences of larvae of different size groups significantly differ. The growth rate of larvae in Kandalaksha Bay is 0.20–0.25 mm/day. A feedback between the numbers of larvae of the White Sea herring and hydromedusas was revealed: in Chupa Inlet at a greater concentration of jelly-bodied zooplankton, the numbers of larvae were considerably lower, and their reduction at 5 days turned out to be more significant than at a relatively low concentration of medusas in Knyazhaya Inlet. This indicates that food conditions are not always a single and decisive factor responsible for the formation of strong or weak-year classes of herring.

Accumulation of Chemical Elements in the Dominant Species of Copepods in the Ob Estuary and the Adjacent Shelf of the Kara Sea

Studies were carried out in the Ob River estuary and at the adjacent shelf of the Kara Sea. The concentrations of organic carbon, lipids, major elements (Na, Mg, P, S, K, and Ca), trace elements (Li, Be, B, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Y, Mo, Ag, Cd, Sb, Cs, Ba, Hg, Tl, Pb, Bi, Th, and U), and rare-earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were determined in the dominant species of mesozooplankton (Senecella siberica, Limnocalanus macrurus, and Calanus spp.). The similarities and differences are shown for the chemical compositions of the specimens. Calanus spp. are characterized by a large Li accumulation with concentrations ~350 times higher than those in S. siberica and L. macrurus. The total accumulation of chemical elements per unit volume is higher in L. macrurus than in S. siberica and Calanus spp., amounting to 6.63, 0.69, and 0.41 mg, respectively. The intensity of biological accumulation and the spatial disposition of the area of maximum accumulation of elements in the zooplankton community within the boundaries of the Ob River estuary depend on the hydrophysical conditions. Postmortem variations in the concentrations of chemical elements in dead L. macrurus are characterized by a multidirectional nature. The revealed distinctions of the chemical compositions in live and dead L. macrurus represent the features of lifetime and postmortem concentrations of elements.

Spatial and Temporal Variability in the Abundance and Size Structure of Larvae of the White Sea Herring (Clupea pallasii marisalbi) 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.