The Caspian-Volga-Baltic Invasion Corridor

Home , Inva, Kama (river), Kuybyshev Reservoir, Lake Kubenskoye, Mologa, Mologa (river)

THE CASPIAN-VOLGA-BALTIC INVASION CORRIDOR

YURY V. SLYNKO1*, LIUDMILA G. KORNEVA1, IRINA K. RIVIER1, VLADIMIR G. PAPCHENKOV1, GRIGORY H. SCHERBINA1, MARINA I. ORLOVA2 & THOMAS W. THERRIAULT3

1Institute of Biology of Inland Waters, Russian Academy of Science, Borok, Russia 2Zoological Institute, Russian Academy of Science, St. Petersburg, Russia 3Great Lakes Institute for Environmental Research, University of Windsor, Windsor, Canada *Corresponding author syv@ibiw.yaroslavl.ru

ABSTRACT

The north-south transfer of species in the Volga River basin is not new, but the scale and nature of invasions changed along the Volga-Baltic corridor following transformation of the Volga River from a riverine environment to one of a series of cascading reservoirs. Southward penetration of northern species was facilitated by the formation of a cold-water hypolimnion in the Volga reservoirs. Following reservoir impoundment, 106 invasive species have been found in the Volga River basin, a process that occurred over two different time periods. The first period of invasions occurred between 1940 and 1970, and involved many northern species (77% of total species) moving downstream by passive dispersal. The second period of invasions is still on going and involves invasions by many Ponto-Caspian species (51% of total species) while new invasions by northern species has decreased substantially (7% of total species). The proportion of exotic species (i.e., invaders originating from basins not adjacent to the Volga basin) increased from 7% during the first period to 41% during the second period. Since the late 1970s, water temperatures in the Volga basin have continued to increase and it is postulated that many invasions during the second period are related to global climatic change.

1 Introduction: the formation of the European part of the Russian intercontinental waterway

We define an aquatic invasion corridor as a system of waterways connecting previously geographically isolated river and sea basins, thereby allowing the active or passive dispersal of aquatic species beyond their historical, natural ranges. Also, available human-mediated vectors play a catalytic role in species dispersal along these invasion corridors. The Ponto-Volga-Baltic invasion corridor was formed along Europe's largest meridional river, the Volga River. The hydrology of the Volga has remained unchanged since the end of the Valday glaciation period (approximately 10,000 BP) and the entire catchment area is part of the Caspian basin (Obidientova 1977). The Volga River basin is divided into three main parts, the Upper, Middle and Lower Volga. The Volga-Ahtuba flood plain and the delta are considered separate parts of the Lower Volga (The Volga and its Life 1978). Berg (1962) attributed the entire basin to the Ponto-Caspian-Aral province of the Mediterranean sub-region while the basin itself belongs to the Palaeo-Arctic region. However, according to Starobogatov (1970) the entire Volga basin belongs to the European-Siberian sub-region and the Lower and Middle Volga are included in the Volga-Ural province, while the Upper Volga is included in the Baltic province. Indigenous biota of the Volga was formed by the end of Valday glacial period. Until recently, the Volga fauna consisted primarily of freshwater Palaeo-Arctic species with a unknown number of Ponto-Caspian species found in the Lower Volga. Even before the building of dams, the Volga River was an important transportation route in Russia. This fact greatly contributed to the uniting of territories from the Baltic to Black and Caspian Seas and the creation of a united Russia. Since the time of Peter the Great, attempts have been made to build a waterway connecting the Volga River and the Baltic, White, Caspian, Azov and Black Seas. Between 1703 and 1709 the Vyshnevolotsk Waterway was built to connect one Volga River tributary, the Tvertsa River, with the Msta River. Connections were also established with Ilmen and Ladoga Lakes and the Baltic Sea. In 1718 Vyshnevolotsk Reservoir was impounded, however this waterway never developed into a significant European transportation route. In 1810 the Mariinskaya Waterway was built, connecting the Volga River and the Baltic Sea through the Neva, Sheksna, Kovzha and Vytegra Rivers and through Ladoga and Onega Lakes. In 1811 another waterway, the Tikhvinskaya, was created to connect the Volga River with the Baltic Sea via Rivers Neva, Mologa and Ladoga Lake. In 1829 the Severo- Dvinskiy Canal was opened connecting the Volga River with the White Sea basin via the Sheksna, Suk- hona, Northern Dvina Rivers and Lake Kubenskoye. However, due to considerable seasonal and yearly water level fluctuations, regular ship traffic was not possible in this system until the 1940s. Full regulation of the Volga River flow was not possible until reconstruction resulted in the formation of the United Deep Waterway System of canals that was completed by the former USSR in the 1930s. New inter- and intra-basin canals and a cascade of reservoirs were constructed to maintain a constant depth of 3.5 m, a depth necessary for ship traffic. As a result, the speed of traffic, and consequently the amount of transported cargo, increased. In addition, the White-Baltic and Moscow Canals were built in 1933 and 1937 respectively. The latter formed a circular intra-basin waterway in Central Russia by connecting the upper reaches of the Volga and Oka Rivers. In 1952 the Volga-Don Canal was opened pro- viding a direct connection between the Volga-Caspian Sea basin and the Azov and Baltic Seas. In 1964 the Volga-Baltic Waterway was fully reconstructed. A series of reservoirs along the Volga River were impounded including Uglich (1940), Rybinsk (1947), Verkhnevolzhskoye (1947), Gorki (1957), Kuybyshev (the largest; 1957), Volgograd (1960), Saratov (1968) and Cheboksary (1981) Reser- voirs. Impoundment of these reservoirs resulted in full regulation of the entire river channel except for the Volga-Ahtuba flood plain (The Volga and its Life 1978). In addition, Tsimlyansk (on the River Don), Sheksna, and Vytegra Reservoirs, the system of Kama River Reservoirs and several smaller man-made lakes were impounded to maintain the Moscow Canal. As a result of this development, the Volga River became the largest transcontinental waterway in Rus- sia. The river is 3,530 km long and its catchment area is in total 1,360 km2. There are 12 large and more than 300 medium and small reservoirs resulting in a total surface area of 25,660 km2 and a total volume of 180.5 km3, of which 85 km3 is considered usable (Avakyan & Shirokov 1994). The Caspian-Volga- Baltic route is responsible for more than 70% of all cargo transported by the Russian river fleet. With respect to cargo transportation, Volga River ship traffic is second only to the St. Lawrence Seaway (> 300 million tons per year by the mid 1990s; The Problem of Territorial Re-distribution... 1985). About 40% of the fleet transporting goods along the Volga River from the Ponto-Caspian region to the Baltic and White Seas belongs to the "sea-river" class. In 2000 an agreement was signed between Russia, India, Iran and Oman creating a united transportation corridor, termed the "North-South" corridor, in order to provide a transportation route between the Persian Gulf and Indian Ocean to the Baltic Sea (Enactment of the Government of Russian Federation 2000). A primary focus of this global venture is the Volga- Baltic Waterway, a waterway that is expected to handle 50 times more cargo than at present. In 2001 this transportation corridor entered the first stage of active development.

2 Early stage of biological invasions

Since the creation of the canal and the intensified ship traffic during the 18th and 19 th centuries, increased invasions by several Caspian species have been noted. This is particularly true for species that prefer solid substrates and either foul ship hulls or are able to actively migrate in the littoral zone. For example, in the late 19th century the bivalve mollusc Dreissena polymorpha and the crayfish Astacus leptodactylus were found in the Northern Dvina River (Starobogatov & Andreeva 1994) and in the Baltic Sea (Nowak 1951). During the early 20th century, a mysid, Paramysis ullskyi and three amphipod species (Pontogammarus sarsi, Dikerogammarus haemobaphes and Corophiurn curvispinum) were found near the mouth of the Mologa River, a tributary of the Volga River (The Volga and its Life 1978). However, regulation of river flow between the 1940s and 1960s resulted in an even more pronounced invasion pattern. 3 Recent "North-South" invasions

The early invasion sequence consisted of plant and animal species of northwestern limnophilic origins primarily due to the impoundment of the Rybinsk Reservoir, which started in the 1940s. The Volga- Baltic and Severo-Dvinsky Waterways provided major corridors for these invasions (The Volga and its Life 1978). The predominant donor waterbodies were large lakes in the Pskov, Leningrad, Vologda and Archangelsk regions (including Lakes Ladoga and Onega), and estuaries of large rivers in the Baltic and White Sea basins. Successful southward invasions by northern limnophilic species is determined primarily by environmental conditions including deceleration of river flow, formation of large lacustrine water bodies along the river channel (i.e. Rybinsk, Ivankovo and Kuybyshev Reservoirs), pronounced thermal stratification of water masses and existence of a cold water hypolimnion. Therefore passively moving pelagic limnophilic species of northern origin (algae, zooplankton and pelagic fish) have dominated among invaders between the 1940s and 1960s.

3.1 PHYTOPLANKTON AND AQUATIC VASCULAR PLANTS

Between the late 1950s and early 1960s phytoplankton species of the genus Stephanodiscus (S. binderanus, S. hantzschii and S. minutulus) increased in abundance from north to south and now dominate the spring-summer algal community of the Volga River (Korneva 1999). Anthropogenic transformation of river discharge led to an increase in the proportion of limnophilic species, including S. binder anus (Korneva 2001), a cosmopolitan indicator of highly eutrophic waters. The abundance of this species in Ivankovo, Rybinsk and Gorki Reservoirs was several orders of magnitu- de higher than in the unregulated part of the Volga River even during the period after its first appearance (1955-1957). In the Upper Volga reservoirs (Ivankovo, Uglich, Rybinsk and Gorki), the maximum abundance of this species reached 5,880,000 cells 1-1 and attained a biomass of 2.78 g m-3 in 1989-1991 (unpubl. data). In the Middle Volga reservoirs (Kuybyshev and Cheboksary) maximum abundance reached 704,000 cells 1-1 and biomass reached 1.67 g m-3. In contrast, maximum abundance was 62,000 cells 1-1 and biomass was 0.109 g m-3 in Saratov and Volgograd Reservoirs and unregulated parts of the river. This finding supports the hypothesis that the southward decreasing abundance of S. binderanus, first identified during the 1950s, is still apparent. By the end of the 1960s, after all main Volga-Kama reservoirs were impounded, the proportion of small-sized species including S. hantzschii and S. minutulus increased. These species are cosmopolitan, alkaliphilic and tolerant to high concentrations of readily degradable organic matter (a-mesosaprobes) (Korneva & Genkal 2000). In 1969-1972 the average number of S. hantzschii (syn: S. tennis) decreased from 250,000 to 81,000 cells 1-1 in the Upper Volga, to 65,000 cells Г in the Middle Volga and to 0 cells 1-1 in the Lower Volga. Among higher aquatic plants only a northern species, Potamogeton wolffgangii, spread far to the south, into Kuybyshev Reservoir and the Kama River mouth (Papchenkov 1997).

3.2 ZOOPLANKTON

The Volga River zooplankton was enriched by 18 species of northern invaders: Heterocope appendiculata, Eudiaptomus gracilis, E. graciloides, Eurytemora lacustris, Cyclops kolensis, Limnosida frontosa, Daphnia cristata, D. longiremis, Bythotrephes longimanus, Synchaeta verrucosa, S. lakowitziana, Cono- chiloides natans, Keratella hiemalis, Bosmina coregoni, B. longispina, B. kessleri, B. obtusirostris, and B. crassicornis (The Volga and its Life 1978). Impoundment of Uglich and Ivankovo Reservoirs facilitated invasion by planktonic crustaceans belonging to the central limnofaunistic complex. The proportion of "northern invaders" at these sites was relatively low: B. longimanus and H. appendiculata were repre- sented by single specimens and C. kolensis was rare. In the cascades' northernmost reservoir (Rybinsk), conditions were more favourable for the establishment of the northwestern limnofaunistic complex. Bet- ween 1946-1948 (i.e., about 5 years after impoundment) the regular inflow of northern invaders to this lacustrine waterbody allowed the development of large populations of D. cristata, B. longispina, B. longimanus, E. gracilis, E. graciloides, and H. appendiculata (The Rybinsk reservoir 1972). These new zooplankton species of northern origins formed the base of a new food chain for planktivorous fish (The Volga and its Life 1978; Rivier 1993). In the Volga River cascade, zooplankton of northern origins formed two complexes: the winter complex that actively develops under ice cover and during early spring (8 forms of genus Notholca, Synchaeta verrucosa, S. lakowitziana, Conochiloides natans, Keratella hiemalis, Cyclops kolensis, Daphnia longiremis, D. cristata, etc.) and the summer stenolimnophilic complex that develops following spring turno- ver (Heterocope appendiculata, Limnosida frontosa, Bosmina longispina, Bythotrephes longimanus, etc). In the Upper Volga, especially in Rybinsk Reservoir, the winter complex dominates from December until June, while in Volgograd Reservoir; the duration of the winter complex is considerably shorter (Rivier 1986).

3.3 FISH SPECIES

Two fish species belonging to the Arctic fish complex, the cisco Coregonus albula and the smelt Osme- rus eperlanus, naturalized in Volga River soon after the impoundment of Rybinsk Reservoir (Ivanova 1982). Prior to reservoir impoundment both species occurred only as relict populations in some lakes in the Upper Volga basin (Lakes Seliger, Peno, Volgo, and Plescheevo). Both species appeared in Rybinsk Reservoir from Lake Beloye and from there they spread widely through Rybinsk, Uglich and Ivankovo Reservoirs. From the Upper Volga basin both species moved downstream along the Volga River and by the late 1970s smelt had reached Saratov Reservoir and cisco had reached Volgograd Reservoir (Yakov- leva 1975; Nebol'sina 1975). Until 1995-1998 smelt dominated the pelagic fish complex of the three main reservoirs in the Upper Volga basin and was an important food item for pelagic predators including zander, Volga zander, asp, perch and burbot (Ivanova 1982). In 1989, in Beloye Lake (River Kem' mouth) the nine-spined stickleback, Pungitius pungitius was identified as a new invader. In 2001 single specimens of this species were found in Rybinsk Reservoir (River Ukhra mouth). In 2001 the river flounder (Platichthys flesus) was identified in Gorki Reservoir. Probably this species will penetrate the Upper Volga reservoirs from the north via the Volga-Baltic corridor.

4 Recent "South-North" invasions

River flow and powerful rapids in the Samara Luka region limited the upstream expansion of Ponto- Caspian species. Impoundment of lower reservoirs in the Volga cascade and construction of locks led to the removal of rapids as natural barriers for species dispersal to upstream locations and transformed the Volga River into a deep-water waterway. The presence of a continuous waterway in conjunction with the opening of the Volga-Don Canal facilitated increased shipping activities. Stabilization of reservoir ecosystems and intensification of ship traffic increased the rate of invasions by Ponto-Caspian species to the White and Baltic Seas basins. Turning the Volga River and other large rivers within the basin into a cascade of limnic, interconnected waterbodies resulted in the formation conditions similar to those during late Pleistocene Khvalyn transgression of the Caspian Sea, when many Ponto-Caspian species were found at latitudes comparable to Moscow (Mordukhai-Boltovskoi 1960). Removal of natural hydroche- mical barriers such as flood plain terraces led to increased mineralisation (Tyuryukanov et al. 1996) and nutrient inputs to the Volga Reservoir (The Volga and its Life 1978) and has facilitated the dispersal and establishment of Ponto-Caspian species upstream in the Volga River. Global warming is considered to be another important factor facilitating the northward range expansion of southern species. For example, in Rybinsk Reservoir the average water temperature has increased since the 1960s from 12 °C to 15 °C (unpubl. data). Since the late 1970s there has been both qualitative and quantitative increases in Ponto- Caspian species (i.e., the "southern" flow of invaders and successful naturalization of some of the exotic subtropical and tropical species became obvious (Fig. 1, 2). In contrast, the "northern" flow of invasions by euryhaline zoobenthos and benthophilic fish species of the estuarine complexes of the Caspian, Azov and Black Seas have dominated for the last four decades.

4.1 PHYTOPLANKTON

By the late 1960s, a new euryhaline alga, Sceletonema subsalsum, was found along the entire Volga Ri- ver (Genkal & Kuzmin 1980) but was most abundant at downstream locations. Maximum abundance and biomass of this species is usually observed during the summer-spring period at water temperatures between 10 and 22 °C. This cosmopolitan species prefers stagnant waters with high concentrations of organic matter (Korneva 2001). Mass development of S. subsalsum was noted in 1954-1964 during the summer period in the near-mouth and central parts of the Northern Caspian Sea. This species is a common component of the algal communities in the Caspian and Azov Seas (Makarova 1969). In the 1960s it spread northward from the brackish seas to the Volga River reservoirs and it successfully conquered its own ecological niche. At the same time, a Caspian Sea species, Thalassiosira incerta, rapidly spread through the Volga River reservoirs. This species is euryhaline and eurythermal and its abundance decrea- ses from south to north. Currently, this species is found in all reservoirs of the Volga cascade (Genkal 1992). However, it is numerous only in the Lower and Middle Volga reservoirs (unpubl. data for 1989- 1991). In the late 1960s, other brackish water species of the genus Thalassiosira (T. pseudonana, T. guillardii and T. weissflogii) appeared in algal communities of the Volga River (The Volga and its Li- fe 1978). These species are scarce in reservoirs and can be identified only by using electron microscopy. Since the mid 1980s the distribution of the brackish water Caspian species Actinocyclus normanii was noted in the Volga reservoirs (Korneva 2001). Its appearance is explained by climatic changes that have resulted in increased Volga River discharge, increased Caspian Sea levels, decreased salinity in its northern part and changes in water ionic composition. Further dispersal of S. subsalsum T. incerta and A. nordmanni northward is now restricted north of Kuybyshev Reservoir.

4.2 AQUATIC VASCULAR PLANTS

Since the 1970s-1980s the northward spread of nonindigenous vascular plants became more intense. Among these invasive plants are such typical southern hydrophytes as Lemna gibba, Vallisneria spiralis and Zannichellia palustris, helophytes Alisma gramineum, Bolboschoenus koshewnikowii, Phragmites altissimus, Scirpus tabernae-montanii, and Typha laxmannii. Except for P. altissimus and T. laxmannii all these species are already present in the Upper Volga (e.g., A. gramineum became dominant in Ry- binsk Reservoir). Duckweed, L. gibba, is numerous in the Yaroslavl part of the Volga basin although 10-15 years ago it was rare. Relatively rapid northward spread along small rivers and ponds also has been demonstrated by Z. palustris. B. koshewnikowii and S. tabernaenontanii. V. spiralis, native to the Volga delta, was reported from cooling ponds of thermal electric power plants; now it can be found far to the north, into the Yaroslavl region (Lisitsyna et al. 1993). T. laxmannii, previously inhabited salt marshes, meadows and banks in the southern region of the Volga River, has since appeared near the city of Kazan (Papchenkov 1985) and the Ponto-Caspian species P. altissimus has been found near the city of Ulyanovsk.

4.3 ZOOPLANKTON

Ponto-Caspian species are a relatively minor component of zooplankton communities of the Volga River. Their distribution northwards takes place slowly and such species have only been found in Kuybyshev Reservoir. In mid summer, Heterocope caspia is numerous in this reservoir. Two species from different limnofaunistic provinces, H. appendiculata and H. caspia also inhabit this reservoir (The Kuybyshev Reservoir 1983). Single specimens of Cornigerius m. maeoticus were reported in 1991 and 1995 (A.F. Timokhina pers. comm.). The Caspian species Calanipeda aquaedulcis now inhabits the Lower Volga, including Volgograd Reservoir (Vyushkova & Gurova 1968). In addition, two cercopagid species also were found in this reservoir: Cercopagis pengoi from the Sea of Azov and the confamiliar "northern invader" Bythotrephes longimanus. Both species co-occur and have similar feeding types, but their trophic relationships remain to be determined. Volgograd and Tsimlyansk Reservoirs are inhabited by two podonid subspecies of Azov Sea origin: Podonevadne trigona ovum and Cornigerius m. maeoticus (Vyushkova 1971). In the Caspian Sea other subspecies are found including P. t. trigona and С. m. hir- cus suggesting that polyphemoids of the Sea of Azov origin have invaded the Volga Reservoir cascade.

4.4 ZOOBENTHOS

In addition to two mussel species, Dreissena polymorpha and D. bugensis, natural expansion of two oligochaetes, Potamothrix vejdovskyi and P. heusheri were reported. These species first appeared in the Upper Volga reservoirs in 1969 (Semernoy 1974). The isopod Jaera sarsi moved along the Kama River upstream to the Belaya River mouth (Lyakhov & Mordukhay-Boltovskoy 1973). The Caspian gastropod Theodoxus pallasi was found between the cities of Volgograd and Saratov, at the southernmost extent of its range (The Volga and its Life 1978). The Ponto-Azovian estuarine gastropod Lithoglyphus naticoides invaded the Volga delta in 1971 (Biserova 1996) and has since been observed in Gorki and Kuybyshev Reservoirs (unpubl. data). Several amphipods typical of the Lower Volga River and the Volga Delta expanded their ranges upstream including Pontogammarus robustoides (found near Volgograd), and P. crassus and Stenogammarus deminutus, both found between Volgograd and Saratov (Belavskaya & Vyushkova 1970). Recently, up to 14 Ponto-Caspian gammarid species inhabited the Kuybyshev Reser- voir (Pirogov et al. 1990). Gammarids (13 species) have also penetrated the Kama cascade of reservoirs. Many of these benthic macroinverte-brate species have reached high abundance and biomass and play a significant role in the functioning of Volga reservoir ecosystems. The zebra mussel D. polymorpha was repeatedly found in the Upper Volga basin during the last century. However, its mass development was noted in this system only following impoundment of the Rybinsk Reservoir (Ovchinnikova 1954). Found for the first time in 1954, this species occupied the entire waterbody by 1968 and by 1980-1990 Dreissena-dominated communities occupied about 25% of the bottom area in the deep-water part of Rybinsk Reservoir. In the early 1980s average biomass of D. polymorpha reached 594 g m-2 and by the early 1990s it had increased to 1,341 g m2. The zebra mussel biomass in Rybinsk Reservoir was in total 732,623 tons in 1990. First reports of zebra mussels in Ivankovo Reservoir were made in 1953 (Fenyuk 1959). In the other Volga reservoirs, the invasion of this species was faster: by the second year after its first appearance it became dominant in many reservoirs (The Volga and its Life 1978; The Kuybyshev reservoir 1983). By the early 1990s, D. polymorpha dominated benthic communities of canal sections of reservoirs and were often found in deep water habitats of the limnic parts of these reservoirs (Scherbina et al. 1997). Being a powerful filter-feeder zebra mussels intercept considerable organic matter preventing its sedimentation in the profundal zones of lakes and river canals. Metabolic by-products of the mussels (agglutinates and feces) are a primary food source for many detritivores. The mussels play an important role as prey items for molluscivorous fish. The roach is the most active consumer of zebra mussels in reservoirs and the growth rate and condition factor of this species increase considerably when feeding on zebra mussels. Another freshwater dreissenid, the quagga mussel Dreissena bugensis, was until recently only found in European estuaries, reservoirs and man-made canals of the Black Sea basin (Mills et al. 1996). In the Volga basin (Kuybyshev Reservoir), this species was found for the first time in 1992 (Antonov 1993). Since 1994 established populations of the quagga mussel were identified in the Volga delta and in low- brackish, shallow waters of the Northern Caspian Sea. Presumably, invasion of this species took place in the 1980s by ship traffic from Black Sea estuaries via the Volga-Don waterway (Orlova et al., 1998). The quagga mussel was found for the first time in Rybinsk Reservoir during 1997 (Orlova et al. 2000). A group of Ponto-Caspian species that were unable to reach upstream locations of the Middle Volga River, were able to "jump" over the Upper Volga and the Volga-Baltic system of canals directly to the eastern Gulf of Finland, the terminal site of this invasion corridor based on ship traffic. Either within ballast water or on ship hulls, the hydroid polyp Cordylophora caspia and the oligochaetes Paranais frici, Potamothrix vejdovskyi, and P. heusheri penetrated the Neva Estuary and the eastern Gulf of Fin- land and are now established (Panov et al. 1999). D. polymorpha and the fish hook water flea Cercopagis pengoi became dominant species in the eastern Gulf of Finland in the 1990s. The Chinese mitten crab Eriocheir sinensis was noted during the 1990s in the Volga River delta. In 2001 adult specimens were found for the first time in Cheboksary and Rybinsk Reservoirs (unpubl. data), suggesting crab invasions in Volga River reservoirs originating from both southern (River Don and Volga River delta) and northern locations (Gulf of Finland).

4.5 FISH SPECIES

The increasing rate of invasions is evident for some Ponto-Caspian fish species. In the mid 1960s, the pipefish Syngnathus nigrolineatus penetrated the Volga basin through the Volga-Don Canal. From Tsimlyansk Reservoir this species expanded its range both down- and upstream in the Volga River. It has since naturalized in all Lower Volga reservoirs, the Volga-Ahtuba flood plain, the Volga Delta in the coastal part of the Northern Caspian Sea, and in Kuybyshev Reservoir (Sharonov 1971). Several Caspian species, including several species of gobies, two species of shad and the small southern stickleback Pungitius platygaster have move northwards. The Caspian shad Alosa caspia was noted in the Saratov Reservoir, the goby Benthophilus stellatus in the Gorki Reservoir, two other goby species, Neogobius iljini and N. melanostomus and the small southern stickleback have reached the Rybinsk Reservoir (Slynko et al. 2000). The Caspian kilka Clupeonella cultriventris has shown the most striking rate and intensity of range expansion, first noted in the late 1950s this species had naturalized in Kuybyshev Reservoir by the mid-1960s. In the mid-1970s, it was found in the Gorki Reservoir and in the early 1990s it was found in the Rybinsk Reservoir. Kilka became the dominant species of pelagic planktivorous fish assemblages in almost all reservoirs of the Volga cascade, including the Rybinsk Reservoir. The northward expansion of the kilka is still continuing. In 2000 established populations were identified in Ivankovo Reservoir and in 2001 it dominated fish communities from Sheksna Reservoir to Beloye Lake. Some fish species have expanded their ranges beyond the Volga reservoir system. For example, in 1994 in Ivankovo Reservoir we found the Caspian goby Neogobius melanostomus, while this species is absent from downstream reservoirs (Uglich and Rybinsk). This finding suggests that this species moved from the Middle Volga through the River Oka - River Moscow - Moscow Canal system. The same pattern has been observed for the bitterling Rhodeus sericeus which moved beyond its original range downstream to the Volga River and its delta (The Astrakhan Reserve 1991) and upstream to the Ivanko- vo Reservoir (Slynko et al. 2000). Detection in 2001 in Kuybishev Reservoir of a river flounder, supports the hypothesis that invasions in the Volga reservoir system occur from both northern and southern locations.

5 Invasions of thermophilic species

In addition to species invasions of waterbodies adjacent to the Ponto-Volga-Baltic corridor, the invasion by species originating from tropical and subtropical, eutrophic and highly mineralised waters has increased, mainly due to the discharge of large volumes of thermal effluents. Factors facilitating the distribution of termophilic species accidentally or intentionally acclimatized in different regions of the corridor have also increased. During 1989-1991 in Kuybyshev and Cheboksary Reservoirs, two new Thalas- siosira species (T. faurii and T. gessneri) were identified (Genkal & Korneva 2002). The first is known from Central African lakes while the second is known from North American rivers. In 1990, Hemiaulus sp., a marine species was also identified in the Kuybyshev Reservoir. Several species of thermophilic vascular plants have spread in the Volga basin: Potamogeton biformis (originating from Kazakhstan and spread upstream to the Oka River mouth), Butomus junceus (from the southern part of West Siberia) (Lisitsyna & Papchenkov 2000), Persicaria hypanica (from the Ukraine and now spreading upstream through the Volga to Rybinsk Reservoir). Also, the North American invaders Elodea canadensis and Epilobium adenocaulon have developed large abundances in this system (Skvortsov 1995). During the last decade rapid range expansion was noted for Bidens frondosa, native to North America, and is currently approaching Rybinsk Reservoir. Other rapid invaders include Zizania latifolia, a Far Eastern species introduced in the European part of Russia in 1934 (Matveev & Solovyeva 1997). During the 1950s-1970s numerous attempts were made to introduce this species to water-bodies of game territories, shallow parts of newly impounded reservoirs and ponds. These attempts were usually unsuccessful, resulting in a lack of further interest. However, in the 1980s-1990s, this species exhibited "self-expansion" from regions of its intentional introductions. In Ivankovo, Uglich and Gorki Reservoirs this species has spread over large areas and is now replacing aboriginal species. Recently, several attempts have been made to intentionally introduce the tropical species Pistia stratioides and Eichhornia crassipes. Howev- er, it is unlikely these introductions will be successful despite reports of P. stratioides in 1989 in one water body in the Volga delta and in 1991 in waters around the city of Astrakhan (Barmin & Kuzmina 1993). Since the early 1970s exotic zooplankton species have been reported in the Upper Volga River including Brachionus sp. (typical of polluted waters), Lecane bulla, Hexartra mira, Keratella tropica, Moina sp. etc. Dramatic increases in the number of Moina micrura was noted in Ivankovo Reservoir during the summer of 1973 when temperatures were well above average and water levels were low. Increased abundance of the rotiferan Brachionus was also noted in the Rybinsk Reservoir during hot, low water years (1972, 1973, 1989), when the impact of anthropogenic pollution became especially pronounced.

6 Intentional introductions

Between 1948 (when intentional introductions were started) and 1963 (when introductions drastically increased), 66 species of benthic macroinvertebrates have been intentionally introduced to waters of the former USSR including the Volga basin (Karpevich 1975). The Caspian mysids Paramysis lacustris, P. intermedia, P. ullskyi and P. baeri were introduced extensively into Kuybyshev, Volgograd and Ry- binsk Reservoirs. In Rybinsk Reservoir the mysids did not survive the first winter after introduction and further introductions were stopped. The shrimp Palaemon modestus, native to the Far East, was intentionally introduced into the Rybinsk Reservoir but following two failed attempted introductions, one in 1957 and the other in 1959 no additional attempts were made (The Volga and its Life 1978). Between 1958-1965 more than 15.7 million mysids were introduced into the Kuybyshev Reservoir and more than 14.4 million were introduced into the Volgograd Reservoir (Ioffe 1968). None of the introduced mysid species became dominant among higher crustaceans in either Kuybyshev or Volgograd Reservoir. Cur- rently, the amphipods Dikerogammarus haemobaphes and Pontogamma-rus obesus dominate reservoir benthic communities. In addition to mysids, in 1960 two polychaete species, Hypania invalida and Hypaniola kowalewskyi, in total 15,400 specimens were introduced to Volgograd Reservoir from the Don delta (Ioffe 1968). The latter species was never found again. It should be noted that there were no intentional introductions of H. invalida into other Volga reservoirs. This species appeared in the Lower Volga basin benthos in 1968 (S. V. Danilova pers. comm.). In 1977 H. invalida was found for the first time in two reservoirs (Saratov and Kuybyshev) in densities up to 1,000 ind m-2 at 25-30 m depth (Dzuban & Slobodchikov 1980). At that time, in Saratov Reservoir its numbers were low (20-90 ind m-2), suggesting a recent appearance in the waterbody. However, by 1979 the number of Hypania had increased to 1,200-1,400 ind m-2, reaching a level typical for the silty sediments of the deep-water zone of Kuybyshev Reservoir. In 1993, Hypania was found in the Moskva River (Lvova et al. 1996) and in 1994 in the Uglich Reservoir (Scherbina et al. 1997). Thus, between 1971-1994 H. invalida invaded a major part of the Volga from the Volgograd to the Ivankovo Reservoir and, via the canal, appeared in the Moskva River. Individual body weight and fecundity of this species increased significantly along its northward invasion pathway (Scherbina 2001). The dwelling tubes are placed either between shells in zebra mussel clusters or above the bottom surface. This makes the worm readily available for many benthivorous fish. In 1955-1956 70,000 specimens of the bivalve mollusc Monodacna colorata, naturally inhabiting the Caspian and Black Seas were introduced into Veselovsky Reservoir in the Don basin. In 1965-1970, 1.6 million specimens were released into Kuybyshev Reservoir, where it is now established. In 1967 M. colorata was found in the Saratov Reservoir. In 1962-1965, the Baikalian gammarids Microropus possolskyi and Gmelinoides fasciatus were intentionally introduced in the Gorki Reservoir. G. fasciatus naturalized successfully and inhabits many waterbodies in north-western Russia. In 1986 it was found for the first time in the Rybinsk Reservoir and in 1994 in the Beloye Lake (Scherbina et al. 1997). In its natural range this species prefers shallow (2-3 m) waters overgrown by vegetation, where it lives at depths of 6-10 m in Dreissena-dominated reservoir communities (The Kuybyshev reservoir 1983). The greatest abundance and biomass of G. fasciatus (54,000 ind m-2 and 160 g m-2, respectively) are known from Lake Ladoga (Panov 1996). Since first appearing in Upper Volga River reservoirs, G. fasciatus has become a primary prey species for young perch, ruffe, silver bream, roach and other benthivorous fish. To date, none of the intentionally introduced mysid species have established in the Volga River reservoirs, including Gorki. The Baikalian gammarid G. fasciatus and the Caspian polychaete H. invalida have spread considerably through the Volga basin since the mid-1980s. The amphipod G. fasciatus expanded its range due to high mobility as an adult, while the settled worm (lacking a mobile larval stage) expanded its range due to bottom trawling in zebra mussel clusters (Scherbina 2001). Intentional introduction of G. fasciatus to Karelian lakes in the 1970s resulted in "self-introduction" to Lake Ladoga in the 1990s and has since been reported from the Neva river estuary during 1997 (Alimov et al. 1998). Recently, G. fasciatus has invaded all ecosystems in the northern part of the Ponto-Volga-Baltic corridor including the Kuybyshev Reservoir downstream. Although more than 20 species of fish were intentionally introduced in the Volga basin, a majority of these introductions were not successful (Kudersky 2001). Ecological conditions in the Volga-Baltic corridor were favourable only for species such as the common carp Cyprinus carpio, eel Anguilla anguilla and grass carp Ctenopharyngodon idella. In addition, some species introduced accidentally have also succeeded including Amur sleeper Perccottus glenii, Pseudorasbora parva and the guppy Poecilia reticulata. After the intentional introduction in 1965 to the Volga delta, the grass carp invaded upstream rivers and as early as the late 1960s it had successfully naturalized in the Kuybyshev Reservoir. In the 1960s, A. anguilla was intentionally introduced in Lake Seliger and since the 1970s it has been found along the entire Volga River and desalinated parts of the Caspian Sea. Historically, there have been several attempts to intentionally introduce carp in the Middle and Upper Volga River reservoirs but until the 1990s these attempts were unsuccessful. During the 1990s, carp have formed sustainable populations in Gorki and Rybinsk Reservoirs where they exhibit both increased abundance and expanded ranges in the Upper Volga (Slynko et al. 2001). P. glenii was introduced accidentally in small water bodies; and has expanded its range in the Middle and Lower Volga basins (Evlanov et al. 2000) including Ivankovo Reservoir. The invasion of this species as well as of the guppy is facilitated by the abundance of thermal effluents in the Volga basin and continuing uncontrolled introduction of game fish and unwanted aquarium species. Similar reasons for the sleeper expansion are given for the Leningrad region (Alimov et al. 1998). A fast expansion of P. parva became evident in the Don River basin, including Tsimlyansk Reservoir during the 1990s.

7 Conclusions

The major factor determining species invasions along the Volga River is shipping activities along this continuous deep-water system connecting the Ponto-Caspian region and northwest Russia. Intended introductions and accidental releases, as well as secondary natural and human-mediated dispersal of introduced organisms, also contribute significantly to xenodiversity along the Ponto-Caspian - Volga- Baltic Waterway. Reconstruction of paleolimnological conditions of the Khvalyn and Novo-Caspian transgressions, have changed riverine conditions to limnetic ones. The disappearance of natural hydro- logical and chemical barriers by the transformation of the Volga River into a cascade of reservoirs has facilitated the establishment of invasive species along the entire corridor. As a result the "two-way" trans-European Volga-Baltic invasion corridor became a new habitat for both species of northern origins and those of Ponto-Caspian origins. Since the late 1970s, the influence of global warming and thermal pollution from power plants, as well as eutrophication, increased along the Volga River and facilitated the establishment of thermophilic and mesosaprobic species. The expected intensification of shipping traffic along the United Deep Waterway System and the development of this transportation route into the Persian region, will create an invasion corridor for predominantly euryhaline and stenolimnophilic organisms from Atlantic and Indo-Pacific coastal regions via this new transcontinental route. Continued global warming and the recent Caspian Sea transgression itself, suggest further acceleration of species invasions along the Ponto-Volga-Caspian corridor. It is false to say that every species that could have been introduced would be in our waters by now. The timeline of introductions does not suggest a saturation level. The chance of a species to become introduced, established and then to become a serious problem for the environment or economy at the recipient coast is small. But, one single introduced species may be able to cause severe ecological change and economic damage, and this might be the next species about to arrive.