ISSN 1995-4255, Contemporary Problems of Ecology, 2020, Vol. 13, No. 6, pp. 631–642. © Pleiades Publishing, Ltd., 2020. Russian Text © The Author(s), 2020, published in Sibirskii Ekologicheskii Zhurnal, 2020, No. 6, pp. 753–767.

Structure and Dynamics of Mollusk Communities of Small Oxbow Lakes and the Determining Factors (the Khoper River Valley, Penza Oblast) I. V. Bashinskiya, *, T. G. Stoykob, V. A. Senkevichb, A. O. Svininc, E. A. Katsmana, and V. V. Osipovd, e aA.N. Severtsov Institute of Ecology and Evolution RAS, Moscow, 119037 Russia bPenza State University, Penza, 440026 Russia cMari State University, Yoshkar-Ola, 424000 Russia dThe State Nature Reserve Privolzhskaya Lesostep, Penza, 440031 Russia eSaratov branch of VNIRO, Saratov, 410002 Russia *e-mail: [email protected] Received September 28, 2019; revised January 28, 2020; accepted February 6, 2020

Abstract—The structure and dynamics of mollusk communities, as well as environmental factors influencing them, were analyzed in small oxbow lakes of the forest-steppe zone (a case of the Khoper river valley, Penza region, Russian Federation). 19 of mollusks were found in the studied water bodies. The oxbow lakes were characterized by relatively low alpha-diversity (number of species 4 ± 2) and relatively high beta-diver- sity (Jacquard index 0.26 ± 0.18), which is typical for floodplain ecosystems. Lake communities were domi- nated by planorbis (40%), Lymnaea stagnalis (15%), spirorbis (14%), Lymnaea saridalensis (9%), (8%). The results of our analysis showed that the group of factors that determined the type of water body (stability, light intensity, water temperature, oxygen concentrations) had the greatest impact on the structure of malacocenoses. The seasonal dynamics of mollusk communities was determined by the num- ber of adjoining water bodies, water temperature, and dissolved oxygen concentrations. The number of adjoining oxbows was the major factor influencing mollusk species diversity, as well as the total number and biomass of mollusks. Oxbow isolation was favorable for mollusk communities. The factors influencing the highest number of mollusks species (seven) were the light intensity and the presence of fish. Other significant factors were the stability of lakes (shown for five species of mollusks), the number of adjoining oxbows and water transparency (four species). Aquatic vegetation positively correlated with the abundance of only one species – Anisus vortex. Our data suggest that the presence of fish could influence the abundance of Lymnaea saridalensis. The mollusks, in their turn, apparently, had a positive effect on the abundance of leeches in water bodies. Mollusk species most sensitive to the external factors were Lymnaea stagnalis, Lymnaea saridalensis, Aplexa turrita, and , most tolerant were Bathyomphalus crassus, Anisus vor- tex, Planorbis planorbis and Planorbarius corneus.

Keywords: mollusks, environmental factors, oxbow lakes, forest-steppe DOI: 10.1134/S1995425520060037

INTRODUCTION These problems are especially pronounced in forest- Small water bodies are usually underestimated by steppe landscapes, where there is a high anthropo- scientists, though in recent years many studies were genic impact along with a shortage of lentic ecosys- published, that showed the significance of these water tems (Izmaylova and Drabkova, 2016). objects for freshwater biodiversity (Davies et al., 2008; Freshwater mollusks are one of the main compo- Lemmens et al., 2013; Hill et al., 2017). Small ponds nents of aquatic ecosystems. They play many ecosys- have large fluctuations of abiotic parameters due to tem functions – filter water, improve oxygen regime, low depth and area (Coops et al., 2003), and that leads and contribute to trophic webs as food resources to special conditions for aquatic organisms. In flood- (Monakov, 1998; Protasov, 2006). They are able to plain habitats the key factor is water regime—“the decrease eutrophic level of shallow water bodies, flood pulse” (Junk et al., 1989), but nowadays the dis- because of impact on phytoplankton and epiphyton turbance of hydrological cycles is observed throughout (Yang et al., 2020). Mollusks are biological indicators Europe, and water exchange between floodplain water of environmental conditions of water bodies and bodies and river is disrupted (Paillex et al., 2013). aquatic ecosystems (e.g. Foeckler et al., 2006).

631 632 BASHINSKIY et al.

There is a large body of knowledge about the ecol- MATERIAL AND METHODS ogy of freshwater mollusks (Hubendick, 1958; Dillon, Aquatic mollusks were studied in the conservation 2000). In various studies different key factors were zone of the State Nature Reserve Privlozhskaya considered – substrate properties (Chertoprud and Lesostep’ (segment Ostrovtsovskaya Lesostep’, Udalov, 1996; Spyra, 2018), temperature of environ- N52°48′58.4″, E44°27′40.4″), in eight oxbow lakes of ment (Berezkina and Starobogatov, 1988; Dillon, the Khoper river, from April to September in 2016– 2000), vegetation (Brönmark, 1988; Wolters et al., 2017 (Fig. 1). In total, the oxbow system consisted of 2019), some hydrochemical parameters, including about 35 water bodies, which were divided in two water hardness and calcium content (Carlsson, 2001; groups. The “open” part (oxbows I, II, III, VIII) was Heino and Muotka, 2006), trophic stage (Lassen, situated on the edge of the forest and partly sur- 1975; Lorencová and Horsák, 2019). Some of these rounded by agricultural fields. Runoff of melt water studies, for example those of the influence of substrate was disrupted by anthropogenic mound, water bodies and trophic level on molluck ecology, contain differ- were mainly temporal and seasonal. The second, ent conclusions about the role of these factors. For a “canopy” part (IV, V, VI, VII) was surrounded by for- number of factors, their influence is not straightfor- est, almost all water bodies were stable, anthropogenic ward, and depends on other conditions – for example, impact on water regime and shoreline was low. Two vegetation affects mollusks in the presence of preda- parts of the oxbow system differed with statistical sig- tors (Brönmark, 1988). A similar context dependence nificance by light intensity, temperature and water was noted for predation (presence of leeches) (Brön- level fluctuations (Bashinskiy et al., 2019). mark and Malmqvist, 1986) and competition for food with amphibian tadpoles (Brönmark et al., 1991). In Due to small area of water bodies, one sampling addition, most of the studies have analyzed the factors site per oxbow was chosen to study mollusks. From influencing the distribution of mollusks on a wide April to September 2016–2017 one sample per month geographic scale. At the same time, it is known that was taken. Total amount of samples was 78. Mollusks differences at the local level can be determined by very were sampled according standard methods (Zhadin, specific factors (Lodge et al., 1987). 1952; Guide to…, 1992) in littoral zone among macro- phytes and sites without plants, from the depth of Thus, despite many studies, there is still no com- 0.25–0.6 m, manually, using a hydrobiological bottom plete understanding of the effects of different factors scrapper (width 0.16 m) from the area of 0.48 m2. Mate- on freshwater mollusk populations. Even in the recent rial was fixed in 95% ethanol, which was replaced by works the key abiotic factors were described using very 70% ethanol after a week (Starobogatov et al., 2004). different parameters, so there is no clarity in habita- For some groups of mollusks classified in taxo- tion patterns of mollusks (Zealand and Jeffries, 2009; nomic units (orders, families, ), the standard Hoverman et al., 2011; Spyra, 2018; Lorencová and morphometric measurements were performed for tax- Horsák, 2019). Besides, on the territory of Russia and onomic definitions (Starobogatov et al., 2004; Kru- neighboring countries there is not enough studies on glov, 2005; Kijashko et al., 2016): height of shell, ecology of mollusks. Usually, mollusks are considered height of spire, height of aperture and height of last in the framework of broad benthic studies, or the anal- whorl; width of shell, width of spire, width of aperture, ysis of factors is carried out at a large regional level width of last whorl. The main index (ratio of height (e.g. Chertoprud and Udalov, 1996; Uvaeva and and width) and the index of ratio of heights of spire Gural, 2008; Nekhaev, 2011; Mikhailov, 2014). and aperture (Kruglov, 2005) were counted. Linear Oxbows of the upper flow of the Khoper river are suit- measurements were carried out for organisms over able model sites, as they have been isolated from the 10 mm using a caliper with an accuracy of 0.1 mm, and main river for a long time, and face various aspects of for organisms less than 10 mm – using an MBS-9 bin- anthropogenic impact. This leads to great variety of ocular microscope. To clarify the taxonomic rank of environmental conditions on a small territory (Bash- some gastropods of the family Lymnaeidae, anatomical inskiy et al., 2019). The present study showed that dissections were carried out according to generally used mollusks were one of the most abundant group of protocols (Starobogatov et al., 2004; Kruglov, 2005). organisms (14% of total macroinvertebrate diversity). To analyze the grade of development of freshwater However, a detailed study of the impact of environ- mollusks, in addition to species richness, abundance mental factors in oxbows on mollusks has not yet been (N, ind./m2) and biomass (B, g/m2), the following performed. indicators were taken into account: Our work aimed to study the influence of environ- —the index of frequency of occurrence (Shitikov mental factors on mollusks communities within a et al., 2003) calculated by the formula: Рi = mi/M, floodplain complex of small water bodies, which have where mi – number of samples in which i-species was dynamic heterogeneous conditions. To achieve this found, М – total amount of samples. Species with an goal, species composition, abundance and biomass of occurrence of ≥50% were considered to be widespread mollusks communities, their dynamics, and abiotic (determining). Species, the occurrence of which did not and biotic factors have been analyzed. exceed 10%, were considered rare (Methods to…, 1975).

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20° 40°

60° Helsinki

Moskow 20° VI Kiev V

40° р. Хопер VI

III

опер VII р. Х

I II

VIII 3 км 200 м

Fig. 1. Study area.

—the Paliy-Kownacki index (diN for abundance octoculata (Linnaeus, 1758), Glossiphonia complanata and diB for biomass) (Paliy, 1961; Kownacki, 1971), to (Linnaeus, 1758), G. heteroclita (Linnaeus, 1758), assess the complexes of dominant species. It was cal- Haemopis sanguisuga (Linnaeus, 1758), Helobdella culated using the formulas: diN = 100PiNi/NS and diB = stagnalis (Linnaeus, 1758). 100PiBi/BS, where Рi – frequency of occurrence, Ni – For ichtyological study funnel traps with 5 mm cells, number of individuals of i-species, NS – total number 0.7 m length, 120 mm enter diameter; and with 2 mm of individuals, Bi – biomass of individual i-species, cells, 0.35 m length, 60 mm enter diameter were used. BS—total biomass of individuals. Descriptions of studied factors is presented in The Jaccard and Morisita indices were used as Table 1. measures of similarity. Nonparametric Spearman rank correlation (RS) The following environmental factors were studied – coefficient was used for correlation analysis. For esti- sizes of water bodies (were measured with Garmin mation of differences between samples Mann-Whit- GPSmap 60Cx), depth, transparency (the Secchi ney (U) test was performed, because it is suitable for disk), water temperature and pH (Hanna Instruments small samples. All calculations were performed using Portable pH/ORP/EC/Temp “Water Test” Meter MS Excel, STATISTICA 7, Past 3 software. HI-98204), amount of dissolved oxygen (Hanna Instruments Dissolved Oxygen Meter HI-9142), light RESULTS intensity (luxemeter Testo 540). The stability status of water body was described (seasonal, temporal, perma- During our study 19 species of mollusks were found nent). Each month for each oxbow lake the number of in the oxbow lakes. Among them two species were adjoining water bodies was determined. The concen- small bivalve – Musculium creplini (Dunker, 1845) trations of biogenic compounds (nitrates, nitrites, (abbreviation – Mc) and Amesoda sp. (Am) (shell was ammonium, phosphates) were measured using a pho- broken), others were belonging to the class Gastrop- tocolorimeter Ecotest-2020-4-01-RS twice a year, in oda. Among gastropods, three species of prosobranch the beginning and at the end of summer. from the family Bithyniidae was slightly pre- sented – Opisthorchophorus troschelii (Paasch, 1842) For describing of vegetation, plant coverage of (Ot), Codiella leachii (Sheppard, 1823) (Cl) and Val- water bodies (percentage of water surface) by emergent vatidae – Cincinna macrostoma Morch, 1864 (Cm); and submerged plants was assessed. others were pulmonates. Among snails For analyses of predator pressure, two groups of species from six families were present: Acroloxidae – water organisms – leeches and fish – were surveyed. Acroluxus lacustris (Linnaeus, 1758) (Acl); Physiidae – Leeches were sampled with bottom scraper along with Aplexa turrita (O.F. Müller, 1774) (At); Lymnaeidae – the mollusks sampling. During the study we assessed Lymnaea stagnalis (Linnaeus, 1758) (Lst), L. corvus total abundance of five leech species – Erpobdella (Gmelin, 1791) (Lc), L. palustris (O.F. Müller, 1774)

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Table 1. Characteristics of studied oxbow lakes. The mean values ± standard deviation are given. Number of samples is shown in superscript “Open” oxbows Number I II III VIII Stability Temporal Permanent Permanent Permanent Surface area, thous. m2 1.2 ± 1.0(6) 1.3 ± 0.8(6) 1.8 ± 2.3(3) 14.2 ± 9.6(3) Adjoining oxbows, pcs. 0–2 0–2 0–1 0–1 Т °C 19.0 ± 7.3(12) 18.9 ± 7.2(12) 14.7 ± 8.3(4) 13.8 ± 4.7(6) (7) (7) (3) (1) О2, mg/L 5.2 ± 2.0 6.1 ± 2.5 2.7 ± 2.5 3.4 рН 6.9 ± 0.3(5) 7.2 ± 0.7(5) 7.2 ± 0.5(3) no data − 6.4 ± 7.4(4) 13.7 ± 13.2(4) 0 44.6 ± 3.4(2) NO3 , mg/L − 2.8 ± 0.7(4) 1.5 ± 1.4(4) 0 2.3 ± (1) NO2 , mg/L + 4 ± 3.1(4) 4.5 ± 1.5(4) 0 6.9 ± 0(2) N,H4 mg/L 3− 6.9 ± 6.7(4) 8.2 ± 7.3(4) 0 3.3 ± 0.3(2) PO4 , mg/L Transparency, m 0.5 ± 0.1(12) 0.4 ± 0.2(12) 0.3 ± 0.1(12) 0.4 ± 0.1(6) Light intensity, thous. lux 20.6 6.4 6.9 18.6 Plant coverage, % 75 60 20 90 Leeches, ind./m2 16.2 ± 17.0(12) 0.5 ± 1.3(12) 0(12) 0(12) Fish None None None Present “Canopy” oxbows Number IV V VI VII Stability Temporal Permanent Permanent Permanent Surface area, thous. m2 0.6 ± 0.2(4) 1.0 ± 0.5(6) 1.9 ± 1.4(6) 0.5 ± 0.3(6) Adjoining oxbows, pcs. 0 1–2 0–2 0–1 Т °C 15.0 ± 6.2(9) 17.9 ± 6.4(12) 17.5 ± 5.9(12) 14.2 ± 5.9(12) (6) (7) (7) (7) О2, mg/L 3.1 ± 2.3 5.6 ± 1.7 5.1 ± 2.4 3.3 ± 2.1 рН 6.9 ± 0.3(5) 7.4 ± 0.2(5) 7.1 ± 0.3(5) 6.8 ± 0.1(5) − 5.8 ± 6.6(3) 7.7 ± 7.2(4) 11.7 ± 11.2(4) 8.0 ± 6.9(4) NO3 , mg/L − 3.3 ± 0.7(3) 3.0 ± 2.1(4) 2.3 ± 1.7(4) 2.4 ± 2.2(4) NO2 , mg/L + 3.4 ± 0.9(3) 2.4 ± 1.9(4) 3.4 ± 2.9(4) 2.3 ± 3.0(4) N,H4 mg/L 3− 6.1 ± 4.5(3) 5.2 ± 3.9(4) 4.8 ± 2.0(4) 6.4 ± 4.8(4) PO4 , mg/L Transparency, m 0.3 ± 0.1(9) 0.4 ± 0.1(12) 0.4 ± 0.2(11) 0.3 ± 0.1(12) Light intensity, thous. lux 3.1 5.6 9.7 5.2 Plant coverage, %60668095 Leeches, ind./m2 0(12) 0.2 ± 0.6(12) 0.7 ± 1.6(12) 0(12) Fish None Present Present None

(Lp), L. saridalensis Mozley, 1934 (Ls), L. peregra spread species were P. planorbis (82%) and L. stagnalis (O.F. Müller, 1774) (Lp); Helisomatidae – Planorbar- (60%). Seven species were less common – P. corneus ius corneus (Linnaeus, 1758) (Pc); – Pla- (31%), A. spirorbis (27%), A. vortex (26%), B. crassus norbis planorbis (Linnaeus, 1758) (Pp), Bathyomphalus (26%), L. saridalensis (23%), A. turrita (22%) and crassus (Da Costa, 1778) (Bc), Anisus spirorbis (Lin- O. troschelii (19%); other species were rare. naeus, 1758) (As), A. vortex (Linnaeus, 1758) (Av), Distribution peculiarities of mollusks in different A. leucostoma (Millet, 1813) (Al) and Segmentidae – oxbows and structure parameters of communities are Segmentina nitida (O.F. Müller, 1774) (Sn). Wide shown in Table 2.

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Table 2. Structure parameters of two malacocenoses with mollusks communities from eight oxbow lakes of the Khoper river. Abbreviations were given in text “Open” oxbows Oxbow number (n) I (12) II (12) III (4) VIII (6) Species number, S 98113 Mean N (ind./m2) 39.7 ± 7.1 20.3 ± 4.9 13.9 ± 7.9 57.2 ± 32.9 Mean B (g/m2) 22.4 ± 8.6 7.7 ± 3.4 0.2 ± 0.1 5.7 ± 2.7 Dominants in N Ls, Lst, Pp Ls,Pp As Ot Dominants in B Ls, Lst Ls, Lst As Lst, Pc

Determining species (Рi > 50%) Ls, Lst, Pp

Rare species (Рi < 10%) Al, Acl, Av, At, Cl, Lc, Sn Abundance, ind./m2 32.8 ± 9.8 Biomass, g/m2 9.0 ± 4.7 “Canopy” oxbows Oxbow number (n) IV (9) V (11) VI (12) VII (12) Species number, S 10 11 11 9 Mean N (ind./m2) 205.2 ± 91.2 7.5 ± 1.2 30.5 ± 10.4 27.3 ± 7.4 Mean B (g/m2) 23.8 ± 11.5 16.5 ± 0.5 3.9 ± 1.1 5.18 ± 2.1 Dominants in N Pp Lst, Pp Av, Pp Pp Dominants in B Pp Lst, Pp Lst, Pp Lst, Pp

Determining species (Рi > 50%) Lst, Pp

Rare species (Рi < 10%) Am, Mc, Cl, Lc, Lp Abundance, ind./m2 67.8 ± 46.1 Biomass, g/m2 8.6 ± 5.1

Most usual were the communities with domi- they are usually found in small non-floodplain lake- nance of P. planorbis (40%), L. stagnalis (15%), type water bodies, including temporary ones (Kho- A. spirorbis (14%), L. saridalensis (9%), A. vortex (8%). khutkin and Vinarskiy, 2013). In this oxbow, A. spiror- Less common were communities with prevalence of bis was more numerous; in other “canopy” oxbows, it O. troschelii (4%), B. crassus (3%), C. macrostoma (3%), was rarely found, while A. vortex was found almost M. creplini (3%), A. leucostoma (1%), Aplexa turrita (1%). constantly. This species is more often observed in large Other species were not dominant in any sample. non-floodplain lakes, small and medium-sized rivers, Distribution of mollusks communities according and is rare in temporary water bodies (Zhadin, 1952; the structure parameters are shown on Fig. 2. Malaco- Khokhutkin and Vinarsky, 2013). cenoses of “canopy” water bodies were grouped more Seasonal changes of biodiversity parameters of dif- densely, whereas communities of “open” oxbows were ferent water bodies are shown on Fig. 3. dispersed wider. Alpha-diversity of oxbow lakes was 4 ± 2 species, In “open” water bodies species number varied from that could be considered as relatively low value, that is 1 to 13. In oxbow lake I among individuals of L. stag- twice less then noted in water bodies of Europe (Brön- nalis, one shell with subulate spire was found, that is mark, 1985; Jurkiewicz-Karnkowska, 2008). The spe- typical for snails L. fragilis. Probably, we observed cies richness of malacocenoses of the two parts of the abnormal development of a great pond . In oxbow system was significantly different (Mann- oxbow III, that was dried out by the end of June, only Whitney test, p = 0.02). In the “open” oxbows, the one species was observed – A. spirorbis. The highest largest number of species was recorded in May in both species richness was in oxbow VIII. All species of pro- years of observation. In 2016 in the “canopy” water sobranch snails and rare S. nitida were found there. bodies, the peak of diversity was observed later (in In the “canopy” oxbows, the species abundance is June) than in 2017 (in April). less variable – from 9 to 11. In oxbow IV, two taxo- The beta diversity of malacocenoses was relatively nomically close species A. leucostoma and A. spirorbis high; the community similarity according to the Jac- were found. The ecology of both species is similar; card index was 0.26 ± 0.18. The “open” oxbows were

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IV VVII VI VIII III III IV V VI VIIIII VIII III

0.90 0.90

0.75 0.75

0.60 0.60

0.45 0.45 Jaccard Index Jaccard Morisita Index

0.30 0.30

0.15 0.15

0 0 Fig. 2. Dendrogram of similarity of mollusk communities structure of two malacocenoses (I, II, III, VII—“open” and IV, V, VI, VII—“canopy”).

(a) (b) (c) 18 0.7 14 0.8 12 0.7 16 0.6 12 0.6 10 0.6 14 0.5 10 0.5 12 0.4 0.4 8 0.4 10 8 0.3 0.2 6 0.3 8 6 0.2 0.2 6 0 4 4 4 0.1 0.1 –0.2 2 2 0 2 0 0 –0.1 0 –0.4 0 –0.1 July July July July  July July May May May May May May June June June June June June April April April April April April August August August August  August August September September September 2016 2017 September 2016 2017 September 2016 2017 September Fig. 3. Seasonal changes in total alpha (species number) and beta (Jaccard index) diversity in: a) all oxbows; b) “open”, c) “canopy”. more similar than “canopy” ones (0.32 ± 0.26 and Changes in the abundance and biomass of mollusks 0.29 ± 0.15, respectively), but the Mann-Whitney test of different oxbows are shown on Fig. 4. did not reveal significant differences (p > 0.05). The In the “canopy” part, two peaks of abundance and maximum differences in the mollusk fauna of water biomass were observed in both years – at the begin- bodies were observed in June, while the similarity ning and at the end of summer, and this, apparently, increased closer to autumn. There was a strong nega- determined the seasonal dynamics of mollusk com- tive statistically significant correlation between the munities of the entire oxbow system. In the “open” indicators of alpha and beta diversity in “open” part, a progressive change in the structural parameters oxbows (RS –0.68, p < 0.05), and a moderate positive of communities was observed in both years, with the correlation between the total number of species in the exception of a sharp peak of abundance in May 2017. oxbow system and the beta diversity of “canopy” water This was due to the intensive drying of oxbow III, so bodies (RS 0.6, p < 0.05). the mollusks were concentrated on a smaller area.

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N, ind./m2 (a) B, g/m2 N, ind./m2 (b) B, g/m2 N, ind./m2 (c) B, g/m2 9 1.6 6 2.5 14 1.6 B 8 1.4 5 12 N 1.4 7 2.0 1.2 10 1.2 6 1.0 4 1.0 5 1.5 8 0.8 3 0.8 4 6 1.0 0.6 3 0.6 2 4 2 0.4 0.4 1 0.5 1 0.2 2 0.2 0 0 0 0 0 0 July July July July July July May May June June May May May May June June June June April April April April April April August August August August August August September September September September 2016 2017 September 2016 2017 September 2016 2017

Fig. 4. Seasonal changes in total abundance and biomass of mollusks: (a) all oxbows; (b) “open”, (c) “canopy”.

For the nine most abundant species of mollusks, abundance) and light intensity (RS 0.27, p < 0.05 with the maximum abundance was observed for P. planorbis biomass). 2 (15.9 ± 60.3 ind./m ), A. spirorbis (7.4 ± 23.8), L. stag- If we analyze the mean values of indicators for each nalis (5.0 ± 8.2) and A. leucostoma (3.2 ± 14.8). For month, reflecting the seasonal dynamics, then there is P. planorbis, the maximum relative abundance was a strong correlations of the abundance with the noted among all the mollusks of the studied water 2 amount of dissolved oxygen (RS –0.86, p < 0,05), and bodies – 520.8 ind./m . The lowest abundance values of biomass with water temperature (R 0.78, p < 0.01). were typical for A. lacustris (0.05 ± 0.47) and L. palus- S tris (0.04 ± 0.35). A moderate correlation was found between the abun- dance of mollusks and the number of adjoining The seasonal variation of the abundance was deter- oxbows (RS –0.58, p < 0.05) and water temperature mined by the dominants, the structure of which is (RS 0.64, p < 0.05). shown in Fig. 5. To assess the influence of various factors on the In general, P. planorbis dominated in the oxbow structure of mollusk communities, their correlations system, with peaks in mid-summer and early autumn. with the values of the Paliy-Kownatski dominance indi- In spring, the dominance of A. spirorbis was observed, ces were considered. The factors that influenced the which sharply decreased by mid-summer. Starting largest number of species of mollusks were light inten- from the second half of summer, the role of A. leu- sity and the presence of fish (seven mollusk species for costoma (both years) and A. vortex (in 2017) in the each). The statistically significant values of the correla- communities increased. The proportion of pond snails tion coefficients of light intensity and dominance indi- was relatively stable throughout the year, however ces of abundance/biomass (p < 0.05 and p < 0.01*) was L. saridalensis dominated in early or mid-summer and for A. turrita –0.48*/–0.50*, A. leucostoma –0.49*/– L. stagnalis – in the second half of summer. Seasonal 0.48*, L. saridalensis 0.37*/0.36*, L. stagnalis changes in the structure of malacocenoses of the two 0.30*/0.27, P. planorbis –0.28/–0.24, C. macrostoma types of oxbow lakes were similar, however, the 0.25/0.25, O. troschelii 0.25/0.23. Correlation with “open” part was distinguished by a greater proportion fish presence was observed for B. crassus of pond snails (especially in 2016), and the “canopy” (0.44*/0.42*), A. spirorbis (–0.41*/–0.41*), L. sarid- part was characterized by a higher dominance of rams- alensis (–0.36*/–0.34*), A. turrita (–0.33*/–0.34*), horn snails of the genus Anisus (especially in 2017). C. macrostoma (0.3*/0.3*), A. vortex (0.3*/0.29), Analysis of environmental factors showed that the L. peregra (0.24/0.23). Other significant factors were alpha diversity of malacocenoses moderately cor- stability of water body (A. spirorbis –0,76*/–0,75*, –0.70*/–0.69*, –0.33*/– related with the number of adjacent oxbows (R –0.41, A. leucostoma P. corneus S 0.39*, A. turrita –0.28/–0.33, L. saridalensis p < 0.01). This factor also influenced the total abun- 0.24/0.24, L. stagnalis 0.25/none), number of con- dance of mollusks (RS –0.53, p < 0.01) and the total nected oxbows (O. troschelii –0.36*/–0.35*, A. leu- biomass (RS –0.23, p < 0.05). Other statistically signif- costoma –0.35*/–0.35*, P. corneus –0.33*/–0.34*, icant factors were the type of water body (RS –0.24, p < A. turrita –0.31*/–0.33*) and water transparency 0.01 with abundance, RS 0.26, p < 0.05 with biomass), (L. corvus 0.28/0.28, L. saridalensis 0.25/0.27, Anisus dissolved oxygen concentration (RS –0.45, p < 0.01 with spirorbis –0.24/–0.24, A. leucostoma –0.23/–0.22).

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100% (a) Sn 80% Pp Pp Lst 60% Ls Pc Ot 40% Lst

20% As Ls As Al Av Al 0% 100% (b) Pp Sn Pp 80% Pc

60% Lst At Ot 40% Pc Ls Ls Lst 20% As Cm Al As Av 0% 100% Sn (c) Pp 80% Pp Lst Ot 60% Pc Lst At 40% At Av As Av 20% As Al Al 0% July July May May June June April April August August 2016 2017 September September

Fig. 5. Seasonal dynamic of communities’ structure: (a) all oxbows; (b) “open”, (c) “canopy”. Abbreviations were given in text.

The significant correlations were found for dissolved A. spirorbis (5), A. leucostoma (5). The smallest num- oxygen (A. turrita –0.43*/–0.49*, A. leucostoma – ber of correlations for common species (Pi> 15% of 0.31/–0.31, L. stagnalis none/0.31), abundance of samples) was noted for B. crassus (1). leeches (L. stagnalis 0.36*/0.30*, L. saridalensis 0.32*/0.31*), water temperature (L. saridalensis 0.31*/0.29*, A. spirorbis –0.25/–0.25), aquatic vege- DISCUSSION tation (A. spirorbis –0.59/–0.59, A. vortex 0.55/0.58), Of the 19 found mollusk species, almost half are concentrations of ammonium (O. troschelii 0.49/0.49), typical for seasonal and temporary water bodies рН (P. planorbis –0.40/–0.35), surface area (C. mac- (A. leucostoma, A. spirorbis, A. turrita, C. macrostoma, rostoma 0.38/0.38). It is important to note that with L. corvus, L. palustris, L. peregra, L. corvus, P. planor- the exception of strong correlations with the stability bis), or those that have connections with permanent of the water bodies, the rest of the factors showed ones (A. vortex); some live on vegetation in lentic and moderate correlations. lotic (A. lacustris, P. corneus), as well as in heavily over- Significant correlations of the dominance indices grown water bodies (L. stagnalis, C. leachii, O. trosche- were not found only with the concentrations of three lii) and in shallows of river floodplains (L. saridalen- biogenic compounds (nitrites, nitrates, and phos- sis). River oxbows are one of the typical habitats of the phates). The largest number of correlations was found rare species S. nitida (Khokhutkin et al., 2009; Kiyas- for L. saridalensis (6), L. stagnalis (5), A. turrita (5), hko et al., 2016).

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The results of our analysis showed that the struc- in the dominance of A. turrita, P. corneus, and O. tro- ture of malacocenoses was influenced to the greatest schelii in more isolated water bodies indicated that extent by a group of factors that determined the type of these species preferred ecosystems with less dynamic water body (stability of water level, light intensity, conditions. The isolation of permanent oxbows in a water temperature, dissolved oxygen content). In our dynamic heterogeneous floodplain complex indicates previous studies, it was described that under the influ- the stability of the water body and its longer history. ence of anthropogenic activity and disturbed water The large number of adjoining water bodies indicates exchange with the river, the development of oxbows the presence of seasonal and interannual differences proceeded in two directions – some water bodies depending on hydrological conditions, and, accord- remained relatively stable, while other became more ingly, a greater amplitude of variability of factors. dynamic (Bashinskiy et al., 2019). The data on mala- Despite the fact that most of the found species cocenoses confirmed such a division, first of all, in the breathe atmospheric air, correlations were found with case of stable “canopy” oxbows. The “open” part of the content of dissolved oxygen. This finding partly the oxbow system may have a more diverse division indicates the indirect influence of other factors (sta- according to the similarity of the structural parameters bility of the water body, isolation). For example, a of mollusk communities, which confirms its more het- strong significant negative correlation for and erogeneous conditions. A. turitta A. spirobis indicated their distribution in temporary To a large extent, the distribution of mollusks in water bodies. different water bodies was associated with the stability Hydrochemical factors had less effect on malaco- of water bodies. Strong negative correlations were cenoses. This indicates the relative instability of the found with ramshorns and , A. spirobis A. leucostoma studied oxbow lakes, because a decrease of the influ- for which habitation in temporary water bodies is typ- ence of hydrochemistry is characteristic for temporary ical. In unstable seasonal water bodies, the dominance and drying up water bodies (Jurkiewicz-Karnkowska, of and was also observed. With an P. corneus A. turrita 2011). In addition, it was previously noted that hydro- increase in stability, the share of pond snails L. sarid- chemical factors often play a key role in oligotrophic and has grown. alensis L. stagnalis water bodies, while morphometric parameters are The light intensity and temperature conditions more important in eutrophic water bodies (Brönmark, were of great importance for malacocenoses. It is 1985). Our data indicate the absence of oligotrophic known that a change in environmental temperature by conditions in the studied water bodies. Nevertheless, only a few degrees can have a very significant effect on the factors determining trophicity showed moderate survival and the reproduction of mollusks (Dillon, correlations with five species of mollusks – A. spiror- 2000). In our case, the heating of water bodies affected bis, A. leucostoma, L. saridalensis, L. corvus, O. tro- the characteristics of eight species. The greatest was a schelii. At the same time, two species of pond snails positive correlation of water heating with the indices of had significant positive correlations with water trans- dominance of L. saridalensis and L. stagnalis, and a parency, and two species of Anisus had negative cor- lesser one – with those of O. troschelii and C. mac- relations, which indicates their predominance in more rostoma. Strong heating and, correspondingly, inten- eutrophic water bodies. For A. spirorbis, it is known sive drying were not favorable for A. turrita, A. leu- that this species can have positive correlations with costoma, A. spirobis and P. planorbis. phosphate concentrations (Spyra, 2018). In our case, phosphate compounds had no effect on the mollusk Another important factor for floodplain ecosys- communities; however, different correlations with tems is the isolation of water bodies, which contributes nutrients from the nitrogen group may indirectly indi- to an increase in beta diversity with a concomitant cate the effect of different trophic status of water bod- decrease in alpha diversity (Amoros and Bornette, ies. In addition, as mentioned above, the dominance 2002). In our study, we also observed a strong negative of is typical for “canopy” oxbows, – correlation between these two indicators in the “open” Anisus Lymnaea for “open” ones, where a greater abundance of phyto- part of the oxbow system. The taxonomic variability of plankton was observed (Bashinskiy et al., 2019). Most mollusk communities was highest among the water likely, these nutrients influence mollusks indirectly, bodies with low numbers of mollusk species. Our data through their effects on vegetation (Dillon, 2000). indicate that the isolation of water bodies led to an Higher ammonium concentrations likely led to an increase of the dominance of five species of mollusks increase in the dominance of O. troschelii. (A. turrita, A. leucostoma, P. corneus, O. troschelii). The positive effect of the disruption of water exchange with Aquatic vegetation had a strong positive correlation other water bodies on mollusk communities has with only one species of mollusks, A. vortex, the phy- already been noted earlier (Jurkiewicz-Karnkowska, tophilicity of which was also noted earlier (Uvaeva, 2008). In our case, for more abundant species, we can Gural, 2008). It is known that the abundance of mol- speak not so much of a favorable effect, but of a higher lusks correlates with the abundance of macrophytes, intensity of drying in separately located oxbows, and a primarily in the presence of predators (Brönmark, subsequent increase in mollusk density. The increase 1988). Since small water bodies are characterized by

CONTEMPORARY PROBLEMS OF ECOLOGY Vol. 13 No. 6 2020 640 BASHINSKIY et al. the absence of large consumers (primarily fish), the and the species less influenced by these factors influence of vegetation on mollusks may not take (B. crassus, A. vortex, P. planorbis, P. corneus). place in such ecosystems. This can explain why we did not find correlations between the characteristics of vegetation and mollusks in the studied oxbows, which CONCLUSIONS were mostly fish-free. In addition, as in the case of hydrochemical factors, the influence of vegetation can (1) 19 species of mollusks were found in the oxbow be an indirect manifestation of other factors (Zealand lakes. The studied water bodies had relatively low indi- and Jeffries, 2009). For one type of ramshorns cators of alpha diversity (the number of species was (A. spirorbis), negative correlation with vegetation was 4± 2), with relatively high values of beta diversity noted. This species prefers temporary and seasonal (Jaccard index 0.26 ± 0.18), which is typical for flood- water bodies, where the weak development of aquatic plain ecosystems. vegetation is typical. The presence and abundance of predators can be (2) Data on mollusk communities confirmed the considered an important biotic factor. As noted above, division of the development of oxbow ecosystems in most oxbow lakes are characterized by the absence of two directions (stable and dynamic), primarily in rela- ichthyofauna. In three water bodies fish were repre- tion to permanent “canopy” oxbows. The “open” part sented by benthic-eating species – Carassius carassius of the oxbow system had a more fractional division (Linnaeus, 1758), C. auratus (Linnaeus, 1758) and according to the similarity of mollusk communities, Misgurnus fossilis (Linnaeus, 1758). In our studies, we which confirmed its more dynamic and heterogeneous found moderate positive correlations between the conditions. presence of fish and the indices of dominance of A. vortex, B. crassus, L. peregra and C. macrostoma, (3) The highest occurrence frequency and relative which indirectly indicates the preference of these spe- abundance was noted for P. planorbis. Communities cies for permanent water bodies. Negative correlations dominated by this species were found most often (40% found for A. spirorbis, A. turrita, L. saridalensis can be of all habitats), the proportion of the species was max- considered a more informative indicator. The first two imum in mid-summer and early autumn. In the species are also characterized by habitation in seasonal spring, the dominance of A. spirorbis was observed, water bodies, where fishes are absent. On the other which sharply decreased by the mid-summer. From hand, the pond snail was found everywhere, so our the second half of summer, the role of A. leucostoma data allow us to say that the presence of fish affects its and A. vortex in the communities increased. L. saridal- abundance. In “canopy” oxbows inhabited by fish, ensis dominated more in early or mid-summer, while L. saridalensis conceded its share in the community to L. stagnalis dominated in the second half of summer. some other species (Fig. 5), including P. planorbis, three Anisus species, and another pond snail L. stagna- (4) Seasonal dynamics of mollusk communities lis. The presence of a significant correlation with only was determined by the number of adjoining oxbow one species of mollusks indicates the possible prefer- lakes, water temperature, and dissolved oxygen con- ences of fish for this particular species. tent. The number of adjoining oxbows to the greatest extent determined the species diversity of oxbows, as In addition to fish, in our work we considered five well as the total abundance and biomass – isolation species of leeches that feed on mollusks. Moderate positive significant correlations were found between was more favorable for mollusk communities. Other the total abundance of leeches and two species of mol- statistically significant factors were water body stabil- lusks, L. saridalensis and L. stagnalis. The abundance ity and light intensity. of individual species of leeches positively correlated (5) To the greatest extent, the structure of malaco- with the indices of dominance of most species of mol- cenoses was influenced by a group of factors that lusks (except for all species of Anisus, A. turrita, P. pla- determine the type of water body (stability, light inten- norbis, P. corneus). There was also a significant cor- sity, water temperature, oxygen content). The factors relation between the abundance of and E. octoculata that influenced the largest number of species of mol- H. stagnalis and the species richness of malacocenoses (R 0.31, p < 0.01 and R 0.27, p < 0.05, respectively). lusks were light intensity and the presence of fish S S (seven species for each). Other significant factors were Our data indicate an inverse relationship – the num- ber of specialized leeches increased with a greater the stability of the water body (five species), the num- abundance and diversity of mollusks. ber of adjoining oxbows and the transparency of the water (four species for each). If we talk about the tolerance of different species of mollusks to various environmental factors, then we (6) The most sensitive species of mollusks to the can distinguish species that differ in a significant rela- studied factors were L. stagnalis, L. saridalensis, A. tur- tionship with environmental conditions (L. stagnalis, rita, A. spirorbis and A. leucostoma, the most tolerant – L. saridalensis, A. turrita, A. spirorbis, A. leucostoma), B. crassus, A. vortex, P. planorbis, P. corneus.

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