A. B. Nikishina

The role of zooplankton in the transformation of the organic matter in the Ob estuary, on the shelf, and in the deep regions of the Kara Sea

The data for the present study were collected at 20 sampling stations in the Kara Sea along the transect from the Ob estuary to the deep sea St. Anna Trough in September 2007. Based on the hydrophysical features, the distribution of the Chl a, and the primary production, we distinguished six habitats: the river, estuary, inner and outer shelf, continental slope, and trough. The impact of the small-size (<0.5 mm) and large-size (>0.5 mm) fractions of the zooplankton on the phytoplankton’s organic carbon in the different regions of the Kara Sea was estimated. The ingestion rate was assessed using the analysis of the gut fluorescence content and the gut evacuation rate. The zooplankton grazed 1–2% of the phytoplankton biomass in the river and estuary; 3.5% over the shelf; and 6 and 10% in the regions of the trough and slope, respectively. The grazing impact of the small-sized zooplankton increased from the river zone to the deep regions (from 1 to 90%) along with their share in the total zooplankton abundance (from 18 to 95%). From 72 to 86% of the primary production was grazed over the shelf and slope. The primary production did not cover the feeding requirements of the zooplankton in the estuarine regions and St. Anna Trough in the autumn. In the estuarine regions, the major portion of the organic matter settles on the bottom due to the strong inflow of the allochthonous matter and the relatively low zooplankton grazing.

Different effects of increased water temperature on egg production of Calanus finmarchicus and C. glacialis

Two copepod species, Calanus finmarchicus (a widespread North Atlantic species) and C. glacialis (an Arctic species), are dominant in the zooplankton of Arctic seas. We hypothesized that the anticipated warming in the Arctic might have different effects on the arctic and boreal species. The effect of temperature on egg production rate (EPR) in these species at temperatures of 0, 2.5, 5, 7.5, and 10°C under contrasting feeding conditions was assessed in 5-day-long experiments. The EPR of the fed C. finmarchicus increased with temperature over the entire tested range. On the contrary, the EPR of C. glacialis increased only in the range of 0–5°C and dropped with further temperature growth. The difference in the influence of temperature on reproduction of these two species is statistically significant. Feeding conditions have a considerable effect on the C. finmarchicus EPR. The EPRs of the female C. glacialis that fed or starved for 5 days displayed no significant difference. These results suggest that the C. finmarchicus EPR increases with temperature under favorable feeding conditions, whereas the C. glacialis EPR decreases at a temperature over 5°C independently of the feeding conditions. This allows for prediction of the shift in abundances of these two species in pelagic communities of Arctic seas in the case of a warming scenario.

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.

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.

Evaluation of ecosystem status in the shelf-slope zone of the northeastern Black Sea based on the trophic index (TRIX)

The water conditions and trophic status in the shelf-slope zone and bays of the northeastern Black Sea were evaluated on the basis of monitoring data from 2007–2014. It has been shown that the concentrations of nutrients and chlorophyll “a” in the studied area are at the level of the pristine period (of the late 1970s). The concentration of mineral nitrogen in the surface water layer varied from 0.19 to 5.64 μM. The concentration of phosphates differed from analytical zero to 0.56 μM. The concentration of chlorophyll “a” in different seasons ranged from 0.24 to 0.89 μg/L. The trophic index characterizes the status of the marine shelf ecosystem near Gelendzhik city as “excellent” even in the bays. Significant year-to-year differences in the index were not detected. The range between the values of the trophic index in the bays and open sea was low (3.7 and 3.2, respectively).

Mesozooplankton grazing impact on phytoplankton in the northern regions of the Kara Sea in autumn

The role of mass zooplankton species, copepods Pseudocalanus spp. and Oithona spp. and pteropods Limacina helicina, in grazing impact on phytoplankon in the upper 50 m layer was investigated. Material was collected at 14 stations along two sections crossing the outer shelf, continental slope and adjacent deep-water regions in the western and eastern branches of St. Anna trough (Kara Sea) in September 2011. Feeding rate was measured with fluorescence method. It was shown that total grazing impact on phytoplankton varied in a wide range from 0.2 to 8.7% of biomass and from 2 to 230% of daily primary production. The main contribution to phytoplankton grazing was made by small copepods Oithona spp. and juvenile pteropods. Maximum values of grazing were found in the frontal zones over the slope, minimum–over the outer continental shelf. Significant mesoscale spatial heterogeneity in total zooplankton grazing due to the changes in zooplankton composition and abundance was observed.