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Freshwater crustaceans of Bykovsky Peninsula and neighboring territory ( Northern Yakutia , Russia )

лизировать структуру сообществ с целью выявить основные факторы среды, определяющие последнюю. Всего было идентифицировано 19 видов Copepoda (14 родов) и 16 видов Branchiopoda (12 родов). Среди них, семь видов были впервые найдены в этом регионе северной части Восточной Сибири, а именно два вида каляноид (Acanthodiaptomus denticornis, Arctodiaptomus wierzejskii), три вида циклопоид (Acanthocyclops venustus, Diacyclops crassicaudis, D. languidoides) и два вида кладоцер (Eurycercus cf. glacialis, Pleuroxus cf. trigonellus). Фауна района очень бедна и в основном представлена эврибионтными видами с широкими палеарктическими или голарктическими ареалами. Различия в структуре сообществ в различных водоемах могут быть объяснены различными абиотическими предпочтениями (в т.ч., в части удельной электропроводности/солености и температуры) доминантных видов.


Introduction
Numerous water bodies of Eastern Siberia, especially in its northern portion, are poorly studied in terms of species composition and community structure. Few papers were published on the microscopic crustaceans of this vast part of Russia [Sars, 1898;Rylov, 1928;Behning, 1942;Urban, 1949;Vekhov, 1989]. During the last decade several detailed publications on zooplankton from a few regions of northern Yakutia have appeared, i.e. Yana River basin [Sobakina, 2013;Trofimova et al., 2018], Indigirka River basin  and Anabar River basin Frolova et al., 2013]. All such papers were based on data from a very limited number of water bodies. Moreover, author's identifications need a substantial ABSTRACT. Numerous water bodies in the northern portion of Eastern Siberia are poorly studied both in their species composition and community structure. The aim of this study is to provide an inventory of the freshwater microcrustaceans of Bykovsky Peninsula and neighboring territory (North Yakutia, Russia) and to analyse the community structure in the studied water bodies with the aim of revealing separate environmental factors affecting it. In toto, we identified 19 copepod species (belonging to 14 genera) and 16 branchiopod species (12 genera). Seven species were first records from this area of NE Siberia, namely calanoid copepods Acanthodiaptomus denticornis, Arctodiaptomus wierzejskii cyclopoid copepods Acanthocyclops venustus, Diacyclops crassicaudis, D. languidoides; cladocerans Eurycercus cf. glacialis, Pleuroxus cf. trigonellus. The fauna of Bykovsky Peninsula and neighbouring territory is very poor and mainly represented by eurybiotic taxa with wide Palearctic or Holarctic ranges. Differences in community structure of different water bodies could be explained mainly by abiotic preferences (i.e. in conductivity/salinity and temperature) of dominant species.
Compared to other arctic territories of Russia, information on the pelagic fauna of lakes, ponds and wetlands in the Lena River Delta is rather detailed (e.g. Rylov, 1928;Kerer, 1968;Abramova, 1996Abramova, , 2003 re-evaluation keeping in mind recent progress in the taxonomy of different microcrustaceans [Korovchinsky et al., 2012;Kotov, 2015]. The best explored region is the Lena River Delta, one of the largest deltas in the world [Are, Reimbitz, 2000]. Its total area is almost 30.000 km 2 and there are more than 29.000 lakes within this area [Bolshiyanov et al., 2013]. Although zooplankton investigations in the Lena River Delta date back to a Russian Polar expeditions at the beginning of the 20th century (1901)(1902)(1903), the information on the structure and functioning of zooplankton communities within this large region is limited, although first data on the zooplankton were published already by Rylov [1928] and Behning [1942]. The general list of fauna, based on their data, included about 50 species and forms.
Further analysis of the material collected throughout the Lena River from Yakutsk prior to the beginning of the delta, and also in Olenekskaya and Tumatskaya channels of the delta, the bay of Tiksi, the Oleneksky Gulf and Neyelov's Gulf, expanded this list to 75 taxa [Pirozhnikov, Shulga, 1957]. The studies of the zooplankton in the Lena River Delta, first of all, covered the Рис. 1. Карта Восточной Сибири с отмеченным положением дельты Лены (A, стрелка), а также три района отбора проб (1-60) в окрестностях Тикси и на Быковском полуострове (B). Исходная карта взята с портала Google Earth (https://www.google.ru/intl/ru/ earth/).  Abramova, Sokolova, 1999;Abramova et al., 2009;Vishnyakova, Abramova, 2009;Nigamatzyanova et al., 2014;2016, etc.). The fauna of such water bodies is found to be much richer than that of the Lena River Delta [Kerer, 1968]. It was also shown that the fauna of standing waters of a seaside part of the delta is slightly poorer than that in central and southern portions [Serkina, 1969]. Our knowledge on the water communities of the Lena River delta has been slowly but steadily growing during the last decades. The most recent and comprehensive study of the zooplankton of the Lena River Delta has resulted in the list of 125 zooplankton taxa belonging to Rotifera (67) and Arthropoda (58) [Abramova et al., 2017].
The aim of this study is to provide an inventory of the freshwater microcrustacean fauna of Bykovsky Peninsula and neighboring territory, closely located to the Lena Delta (Fig. 1A). We believe that our study of this remote and uninvestigated area will improve the knowledge of the diversity of microcrustacean species in the water bodies across the northeastern part of Siberia. We also aimed to analyse the community structure in the studied water bodies and to evaluate the main environmental factors affecting it.
Preliminary species identification and counting was carried out in Bogorov counting chambers; the total numbers of Cladocera and Copepoda were recorded. The qualitative data was used to reveal dominant and rare species and the ratio of species in the community. Statistical analyses performed for qualitative data, i.e. considered only presence or absence of species. High power microscope Olympus CX-41 was used for accurate crustacean identification followed both standard taxonomic treatises [Rylov, 1948;Borutsky, 1952;Dahms et al., 2006;Alekseev, Tsalolikhin, 2010] and recent taxonomic revisions [Sinev, 2002[Sinev, , 2009Garibian et al., 2018].
For estimation of the contribution of the identified species to the potential species pool of the studied area, the accumulation curves of the number of species were plotted depending on the number of samples analyzed. We used the computer package EstimateS [Colwell, 2013] to estimate species richness of the cladocerans and copepods in the region.
A canonical correspondence analysis (CCA) was used to reveal the impact of environmental variables on the invertebrate community. The analysis was performed using PAST software [Hammer et al., 2001] for qualitative data on the water bodies of Bykovsky Peninsula (sites 14-60). The whole species list, including both rare and dominant species was applied.
To represent the faunistic relationships among water bodies from different localities in low-dimensional space, a non-metric multidimensional scaling (nMDS) ordination was performed using PRIMER software [Clarke, Gorley, 2001]. The faunal similarity was estimated using the Kulczynski index (K) for the qualitative data: K = (M/N1 + M/N2)/2, where N1 and N2 are the total numbers of taxa present in the compared lists and M is the number of common taxa. This index is independent of joint absence and is moderately sensitive to differences in the total length of the compared lists, making it preferential for potentially incomplete data [Clarke, Warwick, 2001].
Microcrustacean diversity in the studied water bodies was low, and averaged 4.7 species (ranging from 0 to 10). The most common and abundant species in the planktonic samples were the cladocerans Daphnia pulex and Chydorus cf. sphaericus and copepod Heterocope borealis. They occurred in more than 20 localities each (34-45%). D. middendorfiana, Canthocamptus glacialis, Megacyclops gigas, M. viridis, D. languidoides, Cyclops cf. strenuus and A. venustus were also quite frequent in the samples and occupied in 19-24% of the investigated water bodies. Almost half of the species (9 copepods and 6 cladocerans) were rare and occurred only in one to four water bodies. Our analysis reveals that neither sample-based rarefaction curve nor the best species richness estimator (Chao 1) curve reaches a plateau ( Fig. 2A). Therefore, the cladoceran and copepod biodiversity of the region is still incompletely studied, but only few additional taxa could be found here, and a preliminary analysis on the biodiversity could be made in such a situation.
Majority of observed species (>80%) has wide Palearctic and Holarctic distribution areas (Table 3). Only five of the species (Heterocope borealis, Leptodiaptomus angustilobius, Canthocamptus glacialis, Daphnia middendorffiana, Eurycercus cf. glacialis) are limited to the Arctic area and are specific only for high latitudes of the northern hemisphere.
The investigated lake analysis revealed that the zooplankton communities of most lakes and ponds are relatively similar both in community structure and species composition. Most of the lakes are characterized by a combination of high abundances of small and large crustaceans. Thus, large copepods like Heterocope borealis and large cladocerans Daphnia pulex and D. middendorfiana coexist with small cyclopoid copepods and small chydorids. The same trend was observed for the thermokarst lakes of the Lena River Delta region [Abramova et al., 2017]. The species composition of big foodplain lakes in the Lena River Delta region differs from the small ones. Several rather big lakes of Bykovsky Peninsula with rare species stay a little bit separately too (Fig. 2C).
CCA indicated complex relationships between assemblage composition and habitat variables measured in this study (Fig. 2B, Table 4). Vectors superimposed on the CCA ordination plot graphically represented the correlations between environmental variables and assemblage structures. First two canonical axes explained together 56% of the response table's inertia. The first axis accounted for 36% of the total variation in zooplankton assemblage and mainly correlated with water salinity, conductivity; this axis discriminates sharply the two localities with the maximal values of salinity, and with the noticeable role of Eurycercus cf. glacialis and Megacyclops gigas. The salinity varied in the range from 0 to 344 ppm; the conductivity ranged from 0 to 688 µS. The second axis (20% of the total variation) was mainly related to water temperature (varied from 7.8 to 14.8°C). Along this axis, the stations were arranged gradually from cold-water bodies preferred by Eurytemora lacustris, Pleuroxus trigonellus, Mesocyclops leuckartii and Acroperus harpae, to the warm waters, where Arctodiaptomus bacillifer, Leptodiaptomus angustilobus, Polyphemus cf. pediculus and Bosmina cf. longispina were the characteristic species.
The comparison of the samples by the nMDS (Fig.  2C) method does not show a significant separation of the crustacean assemblages in the water bodies of Tiksi vicinities, subjected to a moderate anthropogenic im- pact, and in undisturbed water bodies of the Bykovsky Peninsula. The analyzed samples compose a single group, that indicates the high similarity of the Cladocera and Copepoda species complexes in the compared lakes and ponds.
Among the copepods, calanoids A. denticornis and A. wierzejskii, recorded for the first time for the NE Siberia, are eurybiotic species [Borutsky et al., 1991;Alekseev, Tsalolikhin, 2010]. Both species are eurythermic and euryhaline, living in freshwater and brackish water bodies of various hydrological types and size. A. denticornis is also characteristic for mountain lakes at altitudes from 500 to 2500 m [Borutsky et al., 1991]. A. wierzejskii is distributed in the Palaearctic in a large latitudinal range from the Bolshezemelsky Tundra to Central Asia, and A. denticornis has a Holarctic circumpolar range [Borutsky et al., 1991;Fefilova, 2015]. A. venustus and D. languidoides, as well as D. crassicaudis, are typical for the shallow-water swampy water bodies of the northern regions; to the south these species are found in springs or highland water bodies [Alekseev, Tsalolikhin, 2010;Rylov, 1948]. The first two species are Palearctic, and the third has Holarctic circumpolar range [Alekseev, Tsalolikhin, 2010;Fefilova, 2015].
In general, most taxa recorded in this study are Palearctic or Holarctic with wide latitudinal ranges (Table 3); the latter are confirmed by genetic studies for several members of cryptic species complexes [Crease et al., 2012;. Most taxa revealed in the region of this study have wide temperature ranges [Rylov, 1948;Borutsky et al., 1991;Alekseev, Tsalolikhin, 2010;Fefilova, 2015]. The exception is Acanthocyclops venustus, which, although widespread in the Palearctic, is exceptionally cold-water, and is noted only in springs and wells south of the Polar Circle [Alekseev, Tsalolikhin, 2010]. Similar trend in the composition of species areas has been previously noted for plankton crustaceans of other high-altitude regions: Svalbard , Iceland [Novichkova at al., 2014], Shokalsky island [Novichkova, Chertoprud, 2017] and Bering Island [Novichkova, Chertoprud, 2016].
Although the observed crustacean communities are relatively similar to those from the thermokarst lakes of the Lena River Delta, we found some exceptions from this rule. It was previously shown that the small cladocerans Bosmina longirostris and Chydorus sphaericus are typical inhabitants of the large thermokarst lakes on Samoylov Island, together with the larger Daphnia pulex and two Anostraca species, Polyarthemia forcipata and Branchinecta paludosa. Among calanoid copepods, Eudiaptomus graciloides, L. angustilobus and H. borealis are the dominant copepods in the thermokarst lakes in August [Abramova et al., 2017]. Most of these species are also typical for the observed water bodies of the neighbourhood of Tiksi and Bykovsky Peninsula. Cyclopoid copepods occur in the samples regularly but are much less numerous.
The results of CCA indicated that assemblage structure was correlated mainly with conductivity/salinity and temperature. The correlation between the species composition of the zooplankton community and temperature and mineralization of water for small water bodies of high latitudes is noted in many previous publications [Novichkova, Chertoprud, 2017;Walseng et al., 2018;Jensen et al., 2019]. At the same time, the variability of mineralization of the water body usually correlates with the distance from the sea and/or nesting places of geese on the banks of the water body Jensen et al., 2019, etc.]. In turn, the temperature factor is usually related to the altitude and distance to the focuses of glaciation . It is known that inter-annual temperature fluctuations causing alternation of warm and cold summer seasons in high latitudes can significantly influence the composition of the fauna of Arctic water bodies [Novichkova, Chertoprud, 2017;Dimante-Deimantovica et al., 2018]. Due to the accumulation of the resting stages of microcrustaceans in bottom sediments of water bodies, species of more southern regions may temporarily appear in communities in case of favorable temperature conditions [Novichkova, Chertoprud, 2017].
According to our data, Eurycercus cf. glacialis and Megacyclops gigas are characteristic of water bodies with relatively high mineralization and conductivity (not more than 344 ppm and 688 ìS, responsibly). Both these species are able to inhabit a wide range of water bodies and have been previously documented for such lakes [Rylov, 1948;. The latter species occurs even in brackish-water lakes [Rylov, 1948]. However, variations in the hydrochemical parameters of the comparing lakes of the Bikovsky Peninsula are not significant, all the puddles and lakes considered are freshwater. The range of differences in temperature is also relatively small -only 7 °C. It is necessary to take into account that the temperature of water in Arctic water bodies can vary significantly during the day, which is connected both with their rapid warming due to small depth and mainly dark color of the bottom, and severe weather variability [Grigoriev, 1956]. Relatively warm waters are inhabited by Arctodiaptomus bacillifer, Leptodiaptomus angustilobus, Polyphemus pediculus and Bosmina cf. longispina. Apart from L. angustilobus all these species prefer quite warm water. Thus, the optimum temperature for the development of P. pediculus is 17-21 °C [Butorina, 1971]. Following recent climatic changes B. cf. longispina has propagated from temperate latitudes to Arctic water bodies . A. bacillifer is distributed in the wide temperature ranges and inhabits water bodies of both, steppe and tundra zones [Borutsky et al., 1991;Sergeeva, Yevdokimov, 2016]. The only typical coldwater species in this group is L. angustilobus inhabiting tunrda water bodies [Borutsky et al., 1991]. For cold water bodies, Eurytemora lacustris, Pleuroxus trigonellus, Mesocyclops leuckartii and Acroperus harpae are typical. However, only E. lacustris is a real cold-water taxon. This species is considered a 'classical' glacial relict species in central Europe [Maier et al., 2011]. E. lacustris requires cold (around 10 °C) and well-oxygenated (>1 mg O 2 L -1 ) waters [Patalas, Patalas 1966;Kasprzak et al., 2005;Karpowicz, Kalinowska, 2018]. Oppositely, M. leuckartii, is relatively warm-water, it develops in 15-20 °C [Rylov, 1948], but has a wider distribution range -from tropics to Arctic [Fefilova, 2015]. Two cladoceran species (P. trigonellus, A. har-pae) inhabit wide temperature range of water bodies, dwelling on the bottom and in the phytal zone [Alekseev, Tsalolikhin, 2010]. Thus, the dominance of certain species in a particular water body in some cases may result from stochastic processes and be determined by the founder effect and subsequent water body monopolization [De Meeester et al., 2002]. Probably, the local dominance of cold-water species in warmed water bodies could be explained by such reasons.
According to the non-metric multidimensional scaling, the species composition of Cladocera and Copepoda does not differ in the water bodies from Tiksi and from Bykovsky Peninsula. The domination structure in the microcrustacean species assemblages of two compared regions is almost similar, while the number of species differs significantly (11 in Tiksi and 29 in Bykovsky Peninsula). The overlap between the species lists of these areas is only 81%. Only two species, Acanthocyclops denticornis and Diacyclops crassicaudis, were found exclusively in the water bodies within the settlement. The reduced species richness and simplification of the species assemblages structure is characteristic for the water bodies exposed to pollution . In general, zooplankton communities of high altitudes are very poor and driven, first of all, by the temperature factor and availability of food resources [Novichkova, Chertoprud, 2017;Walseng et al., 2018;Jensen et al., 2019]. Against this backdrop, a moderate anthropogenic impact has almost no effect on the distribution of dominant microctustacean species, but leads to a significant reduction in taxonomic diversity due to the decrease of the number of rare species.