Ostracods in the plankton of the Sivash Bay (the Sea of Azov) during its transformation from brackish to hypersaline state

Bay Sivash (the Sea of Azov), the largest lagoon in Europe, demonstrates a sharp ecosystem change due to anthropogenic impact. A pronounced salinity increase occurs as a result of the decision to stop supplying Dnieper water into the North-Crimean Canal. Salinity has increased from 20–22 g/L to 55–75 g/L in the lagoon. This led to the changes of biotic composition. The authors used data on ostracod species composition and abundance in samples of 2014–2016 samples with comparison with data from samples collected in 2004, before the salinity increase. The identification of ostracod species in the Sivash Bay was first made; five species was found in 2004–2016: Cyprideis torosa (Jones, 1850), Loxoconcha bulgarica Caraion, 1960, Loxoconcha aestuarii Marinov, 1963, Cytherois cepa Klie, 1937, Leptocythere devexa Schornikov, 1966. In June and August 2004, the average ostracod abundance in plankton was 22 ind./m 3 (CV = 1.506). In October 2014, the ostracod abundance in the plankton reached 210 ind./m 3 , and in August 2015 – average of 1273 ind./m 3 (CV = 1.220). In some plankton samples, ostracods dominated in numbers and reached 3850 ind./m 3 . In the salinity range of 50–80 g/L, a significant positive correlation was observed between total ostracod abundance in plankton and salinity.


Introduction
Lagoons, as a land-sea interface, play a very important role in the Biosphere functioning and human activities (Kjerfve 1994;Perez-Ruzafa et al. 2011).Their ecosystem changes, as a result of climate variability and anthropogenic impacts, occur on Earth everywhere (Conesa & Jimenez-Carceles 2007;Kennish & Paerl 2010;Jia et al. 2012;Newton et al. 2014).The largest bay of the Sea of Azovthe Sivash lagoon is a vivid example of this (Shadrin et al. 2016;Kolesnikova et al. 2017).Currently, the second transformation of the lagoon is taking place.The first transformation of the Sivash ecosystem occurred after the construction of the North-Crimean Canal (1963-1975), when the Dnieper water began to flow into the canal and used for irrigation with discharge of drainage water from irrigated fields in Sivash.For example, in 1985, 630 million m 3 of water was thrown into the lagoon.This led to the fact that in the hypersaline Sivash, average salinity decreased from 140 g/L to 23 g/L in 1989 and to 17 g/L in 1997; a fundamentally new ecosystem was formed there (Getmanenko et al. 1996;Grinchenko 2004;Zagorodnyaya 2006;Kireeva & Potekha 2013;Shadrin et al. 2016).In April 2014, Ukraine decided to stop supplying Dnieper water into the North-Crimean Canal.In October of the same year, no water was observed in the main branch of the canal, input of fresh water into the bay practically ceased to discharge, salinity in the lagoon increased substantially and significant changes in the structure of biodiversity occurred (Shadrin et al. 2016).An increase in the total number of Ostracoda and their share in meiobenthos and zooplankton were noted.Ostracoda began to play a prominent role among the leading groups of animals in the bay (Sergeeva et al. 2014;Shadrin et al. 2016).This is the reason to study the structure of this group in the lagoon, and this work is devoted to investigate the species structure and abundance of Ostracoda in the plankton of in Bay Sivash in 2014-2016 in comparison with 2004.

Research area
Sivash with an area of about 2560 km² is the largest lagoon of Europe.The sandy Arabat Spit, of 112 km long and from 250 m to 8 km wide, separates the bay from the Sea of Azov; on the opposite side it is bounded by the Crimean Peninsula (Fig. 1).The lagoon has water exchange with the Sea of Azov through the Straits -Genichesky and Promoina (Tonkii) in the north (Vorobyev 1940).In the first half of the 20th century, Sivash was a semi-enclosed shallow high-productive hypersaline lagoon, the largest among the hypersaline lagoons of the world (average salinity is 140 g/L, and in the southern partup to 200 g/L) (Vorobyev 1940;Zenkevitch 1963).After the construction of the North Crimean Canal it transformed in the brackish water body (Getmanenko et al. 1996;Grinchenko 2004;Zagorodnyaya 2006;Kireeva & Potekha 2013;Sergeeva et al. 2014).Studies, conducted in 2013-2016, showed that in 2013, before the closure of the canal, the salinity in the main part of the bay varied within the range of 10-25 g/L, and only in small detached parts could reach 40 g/L (Sergeeva et al. 2014;Shadrin et al. 2016).Similar values were noted in previous years also.In June 2014, salinity in the central part of the bay varied from 17 to 26 g/L, and by October it had grown to 50-65 g/L.In August 2015, salinity in the central and southern parts was 55-65 and 70-75 g/L, respectively.In August 2015, water temperature was of 1.5-2.0°C higher than in previous years, average 30.3 °C (CV = 0.03).In August 2015, intensive development of floating mats, formed by green filamentous algae Cladophora siwaschensis C. Meyer, was noted; mass of them was 140-415 g/m 2 (absolutely dry weight).They became a key element of the lagoon ecosystem with a high number of animals in them, an average of 1476 animals per 1 g of dry mat mass (CV = 0.080), or 210-613 thousand ind./m 2 .

Sampling data and methods
Plankton and floating mat samples were collected in 2014-2016 in the central and southern parts of Sivash (see Fig. 1).Standard sampling and analysis methods were used, which are described in (Shadrin et al. 2016).The total abundance of ostracods was determined and the ostracod species were identified in three samples of algal mats (August 2015), andin seven zooplankton ones (October 2014, August 2015, and June 2016).In addition to these materials, the results of processing 20 of the zooplankton samples collected in June and August 2004, in which ostracods were previously recorded (Zagorodnyaya 2006), were used.The samples were revised to clarify the abundance and identification of ostracod species.In total, about 300 ostracod specimens were identified.To identify the ostracod species, the authors used (Shornikov 1966(Shornikov , 1969)).The designation of instars, as in (Heip 1976).Counting, species identification, photographing and measuring the length of individuals were made with the use of a LOMO MBR-10 stereomicroscope (x32) and a Leica DM LS2 (х100-400).Photographs were made with a Canon PowerShot A520 camera.The data were subjected to standard statistical processing in EXCEL.

Results and Discussion
In the zooplankton samples of 2004, 2014-2016 five species of "alive" ostracods belonging to the same order Podocopida were identified: Cyprideis torosa (Jones, 1850) (family Cytherideidae), Loxoconcha bulgarica Caraion, 1960 andL. aestuarii Marinov, 1963 (family Loxoconchidae), Cytherois cepa Klie, 1937 (family Paradoxostomatidae), Leptocythere devexa Schornikov, 1966 (family Leptocytheridae) (Fig. 2).The size characteristics of these species, as well as their instars are presented in Table .In 2004, the representatives of three species of Ostracoda, C. torosa, L. bulgarica and L. devexa were found (see Fig. 1).In the autumn of 2014, four species were recorded in the plankton: C. torosa, L. bulgarica, L. aestuarii and C. cepa.In the summer of 2015, all five of the above species were present in the plankton, but L. devexaonly as empty shells.Single "alive" female of this species was found in 2004 samples.In 2015-2016, shells with limb remains of L. devexa were also found.Perhaps they were alive only until the salinity increase.But now there is not enough information to make conclusion about a change in the general list of ostracod species in the lagoon.
During all periods of research, C. torosa was the most common and abundant species in plankton, dominating or being the only species in most samples with a salinity of 55-75 g/L.In one of the plankton samples in summer 2015, L. bulgarica dominated, and C. torosa was not numerous.C. torosa was completely absent in all floating mat samples; L. bulgarica dominated in those samples.L. aestuarii and C. cepa were in the mat samples, but was not numerous.All species are common in the Sea of Azov, widespread C. torosa is the most common species in the Azov-Black Sea region, which is an inhabitant in it at least since the Miocene (Schornikov 1969; Dykan et al. 2009;Gliozzi et al. 2016).C. torosa, C. cepa and L. aestuarii are halotolerant species, the first can live up to salinity of 100 g/L, the secondup to 60 g/L and the thirdup to 45 g/L (Schornikov 1969;De Deckker & Lord 2017).L. bulgarica and L. devexa had not previously been observed in hypersaline waters.The highly halothorent ostracod species Eucypris mareotica (Fischer, 1855) Fischer, 1855 is common and abundant in the hypersaline lakes of the Crimea (Anufriieva & Shadrin 2012;Shadrin & Anufriieva 2013;Anufriieva, 2015;Shadrin et al., 2015), and it can be expected that this species will self-introduce into the hypersaline Sivash.
In June and August 2004, ostracods were extremely few; the average abundance in plankton was of 22 ind./m 3 (CV = 1.506).In October 2014, the ostracod abundance in the plankton reached 210 ind./m 3 , and in August 2015average of 1273 ind./m 3 (CV = 1.220).In some plankton samples, ostracods dominated in numbers and reached 3850 ind./m 3 .In the salinity range of 50-80 g/L, a significant positive correlation was observed between ostracod abundance in plankton and salinity (R = 0.818, p = 0.02); such trend was noted earlier also (Vorobyev 1940).Transition of benthic animals, including ostracod, to a planktonic life in hypersaline habitats is a well-known phenomenon, due to the increase in water density and frequent anoxic events near the bottom (Zagorodnyaya et al. 2008;Shadrin et al. 2017).It can partly explain an increase of ostracods in plankton during salinity growth.High ostracod abundance was in mats of green filamentous algae, which averaged 39,324 ind./m 3 (CV = 0.345).The increase in the total ostracod abundance occurred not only in plankton and mats, but also in benthos between 2013 and 2015 (Sergeeva et al. 2014;Shadrin et al. 2016).

Conclusion
The identification of ostracod species in the Sivash Bay was first made; five species was found in 2004, 2014-2016.Summarizing the obtained data, it can be concluded that in the conditions of salinity growth, the abundance and role of ostracods in the Sivash ecosystem have increased; approaching what was noted in the hypersaline Sivash before Bay Sivash freshening (Vorobyev 1940).At the same time, available data do not allow us to speak of a change in the species composition.Ostracods are abundant and play an important role in hypersaline waters; therefore, it is necessary to continue the study of changes in their composition and abundance in Sivash during salinity growth.

Table 1 .
Shell lengths (L sh ) of the five ostracod species and their instars (VIII -IV) (based on measurements of both "alive" specimens and empty shells).