water Communication Winter Zooplankton in a Small Arctic Lake: Abundance and Vertical Distribution Vladimir G. Dvoretsky and Alexander G. Dvoretsky * Murmansk Marine Biological Institute (MMBI), 183010 Murmansk, Russia; [email protected] * Correspondence: [email protected] Abstract: Zooplankton assemblages are of great importance in aquatic food webs because they link lower (microplankton) and higher trophic levels (top predators). Small water bodies in the Arctic regions of Russia are less studied in winter because of severe ice conditions. For this reason, we analyzed the winter zooplankton community in Lake Kulonga (western coast of Kola Bay, Barents Sea). A total of 9 taxa were found in the samples. The total abundance varied from 200 to 1320 ind. m−3, averaging 705 ind. m−3. The total zooplankton biomass was 1.8–72.8 mg of wet mass m−3 with an average of 30 mg m−3. These parameters were lower than in other Russian Arctic and sub- arctic lakes in summer. Old copepodites of Cyclops spp. dominated the zooplankton community at deep-water stations in terms of the total abundance consisting of 24–33%. The copepod Macrocyclops albidus prevailed in terms of the total zooplankton biomass comprising 30–33% at deep-water stations while Cyclops scutifer and copepodites Cyclops spp. had the highest biomass at shallow water stations. Vertical distribution demonstrated different patterns at neighboring stations, probably as a result of differences in the density of fish predators. Keywords: freshwater zooplankton; copepods; vertical distribution; Lake Kulonga; Arctic Citation: Dvoretsky, V.G.; Dvoretsky, A.G. Winter Zooplankton in a Small 1. Introduction Arctic Lake: Abundance and Vertical The significance of plankton for freshwater, brackish, and marine ecological systems Distribution. Water 2021, 13, 912. is unquestionable [1–4]. Zooplankton organisms present one of the main constituents in https://doi.org/10.3390/w13070912 aquatic food webs, they link primary producers with the higher trophic levels both in fresh- water and marine environments [5–8]. Planktonic animals serve as a major food source Academic Editor: Artem Y. Sinev for various freshwater fish determining the total production in water bodies. Because many Copepoda, Cladocera, and Rotifera are filter-feeding organisms with relatively short Received: 24 February 2021 lifespans and their taxonomical identification is easier than in the case of phytoplankton, Accepted: 24 March 2021 Published: 26 March 2021 zooplankton assemblages may be considered as good biological indicators of environ- mental conditions and shifts of hydrological regimes in freshwater ecosystems [9]. Their Publisher’s Note: MDPI stays neutral composition and abundance may reflect changes in aquatic environments under modern with regard to jurisdictional claims in climatic fluctuations and anthropogenic impacts [10,11]. Zooplankton of the sub-Arctic published maps and institutional affil- and Arctic lakes are well studied, but the bulk of the data are obtained during summer sea- iations. sons [12–16]. In general, the trophic structure of zooplankton in Arctic lakes is considered to be simple compared to boreal waters [13,14]. Polar areas are characterized by significant seasonal changes in temperature, light regime, and aquatic production. Furthermore, the polar freshwater ecosystems are strongly affected by low temperatures and changing ice cover. The winter period in the Arctic is Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. a less investigated season because the ice cover makes the lakes more difficult to study. This article is an open access article A traditional opinion about Arctic freshwater and marine ecology is that most biological distributed under the terms and processes at high latitudes are reduced to a minimum during the polar night due to the conditions of the Creative Commons reduction in light and low food availability. However, some previous investigations have Attribution (CC BY) license (https:// shown that both freshwater and marine environments demonstrate life activity despite low creativecommons.org/licenses/by/ productivity during winter conditions [17]. It has been found that the light penetrating 4.0/). into the water from the ice and snow may provide an energy input that is high enough for Water 2021, 13, 912. https://doi.org/10.3390/w13070912 https://www.mdpi.com/journal/water Water 2021, 13, 912 2 of 15 several planktonic animals to be active [7,17]. Recently, Norwegian authors have shown active vertical migrations of zooplankton [18] and bioluminescence levels indicative of functional activity in plankton biota [19]. Further findings have confirmed that Arctic organisms can respond to light levels undetectable by the human eye [20]. Small lakes are characterized by the short distance between the pelagic and the bottom habitat and the environmental factors weekly affecting the horizontal and vertical biotic gradients in these aquatic systems [5,21–23]. Lake Kulonga is situated on the western coast of Kola Bay (Murmansk oblast, Barents Sea region). Plankton fauna of the lake and adjacent small water bodies are less studied and, therefore, their investigations may be of interest for understanding plankton ecology in Arctic regions. Recent climatic changes in the Arctic, especially warming processes, lead to a reduction of Arctic sea ice cover and thickness as well as seasonal snow cover duration in lake systems [24]. These shifts can affect local aquatic organisms, their interactions, and more precisely ecosystem processes. Therefore, studying winter under-ice plankton may help in better understanding the ecosystem responses to these environmental fluctuations. The western coast of Kola Bay near the village of Belokamenka (69◦405100 N, 33◦100900 E) is an area selected to establish a complex for manufacturing large marine structures (CMLS) to supply Russian oil and gas industries in the Barents and Kara Seas. This complex now includes production and warehouse halls, docks, terminals, local infrastructure, and roads. Construction work began in 2018 and this process is still ongoing. Such a large-scale project and its further expansion would require substantial amounts of freshwater. Lake Kulonga is considered a potential source of water. Anthropogenic impacts on the local ecosystem can change the total productivity in the lake and affect the zooplankton community, which can be considered as a good indicator of potentially negative processes. The aim of this paper is to report the species composition, abundance, biomass, and the vertical distribution of the zooplankton community in Lake Kulonga in winter as a basis for further water quality assessment of this lake under anthropogenic pressure. 2. Materials and Methods Lake Kulonga is an oligotrophic lake and it has an ice-tectonic origin, as do many other lakes of the Murmansk region. The coast of the lake, in general, is low but there are some hills. An exception is the northwestern coast where shores are abrupt and composed of rocks or their products (stones and boulders). Due to such a complex origin, maximum depths in the lake are registered in the north-western part, the area of tectonic breaks. In the mid-part of the lake, depth does not exceed 15 m but in the south-eastern part, it is about 9 m [25]. The study site is a forest-tundra area with birch as a dominating flora. There are single pines, spruces, and willows. This lake is 2.61 km2 in area and 4.8 km long, averages 1.57 km in width, and has a maximum depth of 33 m. The coastline is weakly indented, and its total length is about 14.8 km [25]. During the study period, the lake was covered with ice and snow and an ice auger was used to drill holes in the ice (Figure1). Sampling was conducted on 3–4 March 2016 at four stations with three replicates at each station (Figure2). In general, weather conditions (air temperature, wind speed, and humidity) were similar during the sampling events. Light intensity was similar at stations 1, 2, 4 (moderate cloudiness) while higher light intensity was recorded at station 3 due to low cloudiness. Depth was measured with a marked rope attached to a sampling bottle. Temperatures at the surface and bottom were measured with a thermometer (±0.1 ◦C). Depth at stations varied from 2 (St. 4) to 24 m (St. 2) (Table1). Ice during the study period was covered with snow. Snow thickness was measured to be 15–40 cm in the open area and 50–55 cm near the coast. Ice thickness was 60–65 cm. Water 2021, 13, x FOR PEER REVIEW 3 of 15 Water 2021, 13, 912 3 of 15 covered with snow. Snow thickness was measured to be 15–40 cm in the open area and 97 50–55 cm near the coast. Ice thickness was 60–65 cm. 98 99 Figure 1. LakLakee Kulonga, Kulonga, March March 2016. 2016. North North-eastern-eastern coast, coast, snow snow thickness thickness 40 40cm cm (a) (; aa); hole a hole for forzooplankton zooplankton sampling, sampling, ice 100 icethickness thickness 63 cm 63 cm(b). ( b). 101 Water 2021, 13, x FOR PEER REVIEW 4 of 15 Water 2021, 13, 912 4 of 15 102 FigureFigure 2. Location 2. Location of zooplankton of zooplankton sampling sampling stations stations (red circles) (red circles) in Lake in Kulonga, Lake Kulonga, March 2016. March 2016. 103 Purple square indicates location of Murmansk. 104 Purple square indicates location of Murmansk. SamplingSampling was was done done using using a 5 aL 5Niskin L Niskin bottle bottle (General (General Oceanic, Oceanic, Miami, Miami, Florida, Florida, FL, FL, 105 USA)USA) at every at every 5 m 5 min inthe the whole whole water water layer. layer. Samples Samples were were filtered filtered through aa sievesieve (50 (50µ m) 106 µm)and and preserved preserved immediately immediately with with 4% 4% formalin. formalin. A total A total of 39 of samples 39 samples were w collected.ere collected.