Data on distribution, demographic structure and grazing of the dominant mesozooplankton species in the Yenisei estuary and adjacent shelf in early summer

The data article refers to the paper “Distribution and grazing of the dominant mesozooplankton species in the Yenisei estuary and adjacent shelf in early summer (July 2016)” (Drits et al., 2020). The data were collected along quasi-longitudinal transect “Yenisei estuary – Kara Sea shelf” on 24–28 July 2016. Here we present data on the spatial and vertical distribution, demographic structure and gut pigment content of the dominant zooplankton species as well as the grazing impact on autotrophic phytoplankton in the three distinguished zones: freshwater zone, frontal zone of the Yenisei plume and marine shelf zone. The related article (Drits et al., 2020) considers the structure and functioning of zooplankton community in relation to environmental characteristics such as temperature, salinity, phytoplankton abundance, timing of ice retreat. Information presented in this article can be used by marine biologists for studies of structure and functioning of estuarine pelagic communities, ecology of zooplankton in the Siberian seas. Besides the data could provide a baseline for the assessment of the ecological role played by climate change events (e.g., increased precipitation, permafrost thawing, elevated river discharge) on the Arctic ecosystems.


a b s t r a c t
The data article refers to the paper "Distribution and grazing of the dominant mesozooplankton species in the Yenisei estuary and adjacent shelf in early summer (July 2016)" (Drits et al., 2020). The data were collected along quasi-longitudinal transect "Yenisei estuary -Kara Sea shelf" on 24-28 July 2016. Here we present data on the spatial and vertical distribution, demographic structure and gut pigment content of the dominant zooplankton species as well as the grazing impact on autotrophic phytoplankton in the three distinguished zones: freshwater zone, frontal zone of the Yenisei plume and marine shelf zone. The related article (Drits et al., 2020) considers the structure and functioning of zooplankton community in relation to environmental characteristics such as temperature, salinity, phytoplankton abundance, timing of ice retreat. Information presented in this article can be used by marine biologists for studies of structure and functioning of estuarine pelagic communities, ecology of zooplankton in the Siberian seas. Besides the data could provide a baseline for the assessment of the ecological role played by climate change events (e.g., increased precipitation, permafrost thawing, elevated river discharge) on the Arctic ecosystems.

Value of the data
• There is virtually no data on distribution and demographic structure of zooplankton in the Yenisei estuary and adjacent shelf in the early summer (soon after the Yenisei discharge maximum). Our data fill this gap. • The data are among the few that describe zooplankton feeding and grazing impact on autotrophic phytoplankton in the Yenisei estuary and adjacent Kara sea shelf. • The data can be used to study the impact of seasonal changes and river run-off in the Arctic shelf seas on the structure and grazing of zooplankton as well as when studying other estuarine pelagic communities. • The data form a necessary step to assess the effects of climate change in the area of Yenisei influence.

Data
The data were obtained at 24 sampling stations investigating the distribution and grazing of dominant mesozooplankton species in the Yenisei estuary and adjacent Kara sea shelf in early summer (July 2016). The environmental characteristics of the stations are given in Table 1 . The freshwater zone (sts. 5345, 5344, 5343, 5342), frontal zone of the Yenisei plume (sts. 5340, 5339, 5337, 5335, 5333, 5350, 5351) and marine shelf zone outside the plume (5352 and 5353) were distinguished according to salinity and temperature characteristics.
Distribution of the biomass of the dominant zooplankton at the sampled stations is presented in Table 2 . Cladocera and mysids dominated zooplankton biomass in the freshwater zone. In the Yenisei plume zone, Limnocalanus macrurus dominated zooplankton biomass while C. glacialis dominated in the marine shelf zone ( Table 2 ).    Data on the vertical distribution of the dominant zooplankton are presented in Table 3 . In the freshwater zone, the dominant zooplankton group, Cladocera, concentrated in the upper water layer. In the Yenisei plume, mesozooplankton avoided the upper freshened layer with Calanus glacialis and L. macrurus concentrating in the pycnoline layer. Oikopleura vanhoeffeni was observed only in the deeper layer, while in the marine shelf zone, they inhabited the upper mixed layer ( Table 3 ). Horizontal and vertical distribution of the dominant zooplankton species was analyzed in relation to temperature, salinity, and Chl a concentration using Canonical correspondence analysis (CCA) [1] .
Data on the demographic structure/size structure of the dominant zooplankters are given in Table 4 . Dominance of the younger copepodites of Calanus glacialis and Pseudocalanus spp, as well as small size Oikopleura vanhoeffeni was noted at the marine shelf stations. The population of Limnocalanus macrurus consisted of adult specimens with almost equal numbers of females and males.
Gut pigment content of copepods depended on chlorophyll concentration in the layer of inhabitance ( Fig. 1 ). A significant correlation between gut pigment content and Chl a concentration was found for all copepods but the correlation coefficient was very low for L. macrurus.
In Oikopleura vanhoeffeni, gut pigment content depended significantly of the weight of the animals ( Fig. 2 ).

Collection and processing of samples
The material was collected during cruise # 66 of the RV "Akademik Mstislav Keldysh" to the Kara Sea in July 2016. A quasi-longitudinaltransect was sampled in the Yenisei estuary and adjacent shelf ( Fig. 3 ). At first, the transect was sampled from 74 °15 N to 71 °50 N on 24-25 July. Based on the obtained salinity and temperature data, several stations were selected and repeated along the reversed transect from 71 °50 N to 76 °N on 26-28 July. The coordinates of the sampling stations are in Tables 1. Data on temperature and salinity were obtained with a CTD Fig. 2. Dependence of Oikopleura vanhoeffeni gut pigment content (G, ng pigment ind −1 ) on dry weight (DW, mg ind −1 ).

Table 5
Grazing impact on autotrophic phytoplankton (mg Chl a m −2 day −1 ) by the populations of the dominant zooplankton species.

Station
Layer, m Cladocera

Limnocalanus macrurus
Oikopleura vanhoeffeni probe (Seabird Electronics SBE-32). Mesozooplankton was sampled using a Juday closing net (0.1 m 2 mouth area, 180 μm mesh size). Two layers were sampled at most of the stations along the SN transect, below and above the pycnocline, which was determined according to the CTD profiles. Additionally, we sampled the pycnocline layer at four of the reversed transect stations. For determination of zooplankton abundance and biomass, samples were immediately preserved in 4% borax-buffered formalin. Zooplankton were identified, staged, measured and counted in the laboratory under a stereomicroscope. For determination of zooplankton abundance and biomass, samples were immediately preserved in 4% borax-buffered formalin. Zooplankton were identified, staged, measured and counted in the laboratory under a stereomicroscope. Not numerous large specimens (the older stages of Calanus spp., Limnocalanus macrurus, Oikopleura vanhoeffeni and Chaetognatha) were counted in the whole samples, while more numerous forms were counted in subsamples so as not less than 50 specimens were recorded. Most taxonomic groups including Copepoda, Cladocera, Pteropoda, Chaetognatha, Larvacea, Mysidacaea were identified to the species/genus level. Prosome length was used to distinguish the closely related copepod species Calanus finmarchicus and C. glacialis according to [2] . For Calanus and Limnocalanus , all copepodite stages were distinguished. For Pseudocalanus spp. copepodites CI to CIV were pooled. The wet weight (WW) of each species was calculated using nomograms [3] . These tables ( www.twirpx.com/file/1588162/ ) allow the calculation of biovolume/wet weight of aquatic organisms based on their body shape and length. Dry weight (DW) of crustacean plankton was estimated as 0.16 WW [4] , DW of chaetognaths was calculated according to [5] , larvaceans -to [6] .

Feeding of zooplankton
Feeding rates of the dominant species ( Calanus glacialis, Limnocalanus macrurus, Pseudocalanus spp., Bosmina sp ., Daphnia spp ., Oikopleura vanhoeffeni ) were assessed with the gut fluorescence method [7] . To measure gut pigment content, 1 to 40 animals per replicate, depending on size/stage, were picked with forceps and placed in test tubes with 3 ml of 90% acetone. Two to five replicates for each species/stage were analyzed. Pigments were extracted for 24 h at 5 °C in the dark. Chl a and phaeopigment were measured by a standard fluorometric procedure [8] with a Trilogy Laboratory Fluorometer (Turner Designs).

Zooplankton grazing impact
Grazing impact of each of the dominant species (mg Chl a m −2 day −1 ) was estimated using the mean individual daily ingestion rates (ng Chl a ind −1 d −1 ) and the abundance of the given species (ind m −2 ).

Declaration of Competing Interest
None.