Distribution and species composition of zooplankton (rotifers and crustaceans) in the Basin of the Middle Volga River, Russia

Abstract Background The presented dataset contains information on the distribution and species composition of zooplankton (rotifers and crustaceans) registered in the Basin of the Middle Volga River, Russia. The studies have been performed in the Kuibyshev Reservoir (Samara Oblast and the Republic of Tatarstan), the Saratov Reservoir (Samara Oblast), in several lakes (Raifskoe, Gniloe, Krugloe and Lenevo) in the Volzhsko-Kamsky State Biosphere Reserve (Republic of Tatarstan) and in Lake Aslikul, one of the largest lakes of the Middle Volga River Basin, located in the Asly-Kul Natural Park (Republic of Bashkortostan). The hydrobiological data were obtained and published from 1957 to 2020. In total, the dataset includes 5141 records of 111 zooplankton species (including 17 subspecies), belonging to 45 genera. These are mainly native species - 98.5%, while the naturalised, including invasive species, accounts for less than 1.5%. New information A total of 5141 records have been published on the taxonomic diversity and occurrence of zooplankton (rotifers and crustaceans) in the Middle Volga River Basin. Each record includes information about the place and date of finding the specimen, its taxonomy, occurrence and abundance and the collector. If the information about the find has been published, a link to the corresponding reference is provided. The presented dataset supplements the data on the distribution of zooplankton species in the European part of Russia. Data on zooplankton in the Middle Volga River Basin are published for the first time.


Introduction
Zooplankton is the main consumer of microalgae, which plays an important role in the transformation of organic matter (Gerasimova et al. 2021, Ivanova andKazantseva 2006). Planktonic animals mineralise organic matter and release metabolites to the external environment, which are further utilised by bacteria and algae. Zooplankton representatives are one of the few aquatic organisms that utilise microalgae, bacteria and detritus and then transfer this energy to higher trophic levels. They are food objects for larger invertebrates and fish (Gerasimova et al. 2021, Shurganova et al. 2021. This publication presents a prepared dataset on the current diversity and distribution of zooplankton species in the reservoirs of the Middle Volga River (European Russia). The dataset contains information on the locations and abundance of approximately 38% of species known for the Middle Volga River Basin (e.g. Timokhina 2000, Popov and Mukhortova 2016, Lazareva 2020, Shurganova et al. 2021. The dataset records refer mainly to native species (98.5%); naturalised species, including invasive species, account for no more than 1.5% ).

Project description
Title: Distribution and species composition of zooplankton (rotifers and crustaceans) in the Basin of the Middle Volga River, Russia Personnel: Oksana Mukhortova, Stepan Senator, Elena Unkovskaya Study area description: The studies covered the Kuibyshev Reservoir (Samara Oblast and the Republic of Tatarstan), the Saratov Reservoir (Samara Oblast), several lakes (Raifskoe, Gniloe, Krugloe and Lenevo) in the Volzhsko-Kamsky State Biosphere Reserve (Republic of Tatarstan) and Lake Aslikul, one of the largest lakes of the Middle Volga River Basin, located in the Asly-Kul Natural Park (Republic of Bashkortostan) (Fig. 1  Layout of the investigated water bodies.

2.
The water of all the lakes of the Volzhsko-Kamsky Biosphere Reserve belongs to the hydrocarbonate class of the calcium group with low and medium degree of mineralisation. The waters are characterised as "soft" and "moderately hard" (WQA). The environmental pH varies between the lakes, depths and seasons from slightly acidic (6.5-6.9) to slightly alkaline (7.5-8.3), reaching maximum values (8.8-10.0) during the period of phytoplankton and cyanobacteria blooms. The gas regime of the lakes in summer is typical for eutrophic water bodies: in the surface layers, the dissolved oxygen saturation varies as 72-226.9% and in the bottom layers, there is an oxygen deficiency (7-46%). 3.
The water in Lake Aslykul is slightly brackish with high salinity (1.94 g/l). The Sharlama Stream flows into the Lake. When the water level is high, the Asily-Udryak Stream flows out of the Lake.
The distribution of the number of records on the species composition and abundance of zooplankton for the Kuibyshev Reservoir (81% in total), the Saratov Reservoir (10%) and in the Lakes (5%) is presented below (Fig. 2).
Design description: The first step was to collate a general list of zooplankton species (including rotifers and crustaceans) found in the Middle Volga River Basin. All records are confirmed by collection samples stored at the Institute of Ecology of the Volga Basin, Russian Academy of Sciences. The earliest records on the species composition of zooplankton in the Middle Volga date back to 1957, the latest being obtained in 2020 (Fig.  3). The increase in the number of finds in 2020 is associated with an increase in interest in studying the species composition of zooplankton in various biotopes (littoral, pelagial), as well as ongoing monitoring by employees of the Institute of Ecology of the Volga Basin, Russian Academy of Sciences. The number of records for each of the studied water bodies.
In addition, some species are expanding their areas and actively conquer new reservoirs. For example, in the first decade of the XXI century, the rotifer Kellicottia bostoniensis (Rousselet, 1908), an invader species of American origin, has been found more frequently in Russia than before. By 2015, K. bostoniensis was found in more than 40 different types of water bodies and watercourses of the European part of Russia. This rotifer species is widespread; it has become a common species in forest lakes and rivers of the Baltic Sea Basin and the Volga-Baltic watershed area. In the Volga River Basin, it settled south to 55°N (lakes of the Basins of the Oka and Pra Rivers) and to the east almost up to 45° E (Kerzhenets River, Basin of the Cheboksary Reservoir) (Zhdanova et al. 2016). We noted the first finds of K. bostoniensis in Usinsky Bay of the Kuibyshev Reservoir in autumn of 2020.

Funding:
The study was carried out within the framework of the State Program for Basic Research for 2013-2021, projects nos. АААА-F17-117112040039-7 and АААА-F17-117112040040-3

Number of records in different observation periods.
Distribution and species composition of zooplankton (rotifers and crustaceans) ...
The dataset contains mainly native species (98.5%), while naturalied (including invasive) species account for no more than 1.5% (Fig. 4). An invasive species is an alien species that actively displaces native species. (Zhdanova et al. 2016, Lazareva 2020, Shurganova et al. 2021). Quality control: All samples were identified by the researchers working in the Institute of Ecology of the Volga Basin, Russian Academy of Sciences and were stored in the scientific collection of the Institute. The reliability of the taxonomic definitions was confirmed by taxonomists of A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences (Sinev and Gavrilko 2020, Sinev et al. 2020, Korovchinsky et al. 2021. The taxonomic nomenclature is given in accordance with the taxonomic system GBIF Backbone Taxonomy (GBIF Secretariat 2021). In order to publish the dataset on the GBIF network, the records have been adjusted according to the Darwin Core specifications (Wieczorek et al. 2012).

Sampling description:
Step description: 1. The materials presented in scientific monographs served as the initial data for developing a complete list of zooplankton species in the Middle Volga River Basin (Timokhina 2000, Chuikov 2000, Popov and Mukhortova 2016. 2. The obtained information is supplemented by the results of field sampling of zooplankton, which have been carried out regularly in the study area (Figs 5,6). In the Kuibyshev Reservoir, 10 dm of water were taken from standard depths (0-32-m water column) by a Dyachenko bathometer; the water samples were sieved for zooplankton through a plankton net with a 99-µm mesh, equipped with a 0.1-dm cod end.
From 1957-2006, the zooplankton sampling was also performed with a 10-dm Dzyuban bathometer and a Juday net (nylon sieve, 99-µm mesh, 0.1-dm cod end). In the Saratov Reservoir and lakes, zooplankton samples were taken also with a 4-dm Ruthner bathometer and concentrated through a plankton net as described above (Mordukhay-Boltovskoy 1975).
The percentage of records in the dataset of the samples collected with the Dyachenko bathometer is 92% of total number of samples, Dzyuban bathometer, 1%, Ruthner bathometer, 0.5% and Juday net, 7%. 3. The zooplankton species were identified using the taxonomic keys for local fauna (Kutikova 1970, Alekseev andTsalolikhin 2010).
4. The calculation of the zooplankton occurrence (ind.) was carried out according to an accepted standard (Bolotov 2012).

Geographic coverage
Description: The Volga River is located in the European part of Russia. It is one of the largest rivers worldwide, the world's largest river flowing into a closed body of water and the largest river in terms of water content, watershed area and length in Europe. The studies were carried out in the Middle Volga River Basin, namely, in the Kuibyshev Reservoir (Samara Oblast and the Republic of Tatarstan) and Saratov Reservoir (Samara Oblast), in several lakes (Raifskoe, Gniloe, Krugloe and Lenevo), located in the Volzhsko-Kamsky State Biosphere Reserve (Republic of Tatarstan) and in Lake Aslikul, one of the largest lakes of the Middle Volga River Basin, located in the Asly-Kul Natural Park (Republic of Bashkortostan).

1.
The length of the Kuibyshev Reservoir is 510 km, the greatest width is 40-44 km at the mouth of the Kama River, the water surface area is 6,450 km² (the second largest riverine reservoir in the world); 50.7% of the area is located in the Republic of Tatarstan); the total water volume is 58 km³, useful volume is 34 km³. The backwater level at the dam of the Zhigulevskaya HPP is 29 m (Figs 7, 8).

2.
The length of the Saratov Reservoir is 341 km, the maximum width is 0.8-12 km, the water area at a normal retaining level is 1,831 km², the total volume is 12.9 km³ and the depth is 8-28 m. The flow rate is 0.27-0.56 m/sec.

3.
Volzhsko-Kamsky State Biosphere Reserve was established in 1960; since 2005, it has been included in the UNESCO system of biosphere reserves. It is located in the Republic of Tatarstan, consisting of two sites: Raifsky (5,921 hectares, Zelenodolsky District) and Saralinsky (4,170 hectares, Laishevsky District). The water area of the reserve covers is 1,300 hectares.

4.
Lake Aslikul is the largest lake in the Middle Volga River Basin, located in the Asly-Kul Natural Park (Republic of Bashkortostan). The water surface area is 22-23.5 km², the lake volume is 0.119 km³, the average depth is 5.1 m. It has a karst-hole origin.
The occurrence of zooplankton in the samples collected varied from 1 ind. to 3161 ind. (on average ~ 87.01 ± 123 ind.).
The zooplankton abundance in different water bodies ranged from 0.33 to 1,046 thousand ind./m (on average ~ 13.79 ± 50.51 thousand ind./m ).

Temporal coverage
Notes: The presented dataset contains information on the occurrence of zooplankton species before the regulation of reservoirs since 1957, while the most recent observations are made in 2020 (Fig. 13).    geodeticDatum The ellipsoid, geodetic datum or spatial reference system (SRS) upon which the geographic coordinates given in decimalLatitude and decimalLongitude are based.

minimumDepthInMetres
The lesser depth of a range of depth below the local surface, in metres.

maximumDepthInMetres
The greater depth of a range of depth below the local surface, in metres.

verbatimDepth
The original description of the depth below the local surface. georeferencedBy A list (concatenated and separated) of names of people, groups or organisations who determined the georeference (spatial representation) for the Location. institutionCode The name (or acronym) in use by the institution having custody of the object(s) or information referred to in the record. language The language of the resource associatedReferences A list (concatenated and separated) of identifiers (publication, bibliographic reference, global unique identifier, URI) of literature associated with the Occurrence.
of Sciences) for her technical support and consultations. We are also grateful to the Global Biodiversity Information Facility, Finnish Biodiversity Information Facility for the "Call for data papers describing datasets from Russia" project.