Distribution and abundance of Ponto-Caspian amphipods in the Belarusian section of the Dnieper River

During a survey of the Belarusian part of the Dnieper River conducted on 19-22 July 2006, we revealed five invasive Ponto-Caspian amphipod species in its benthos community: Chaetogammarus ischnus, Chelicorophium curvispinum, Dikerogammarus haemobaphes, Dikerogammarus villosus and Pontogammarus robustoides. All of them except C. curvispinum are reported in Belarus for the first time. The number of alien amphipod species and their abundance were found to decrease gradually in the upper sections of the river.


Introduction
The Republic of Belarus contains a considerable part of the so called central invasion corridor (Bij de Vaate et al. 2002), i.e. the Dnieper River with its large tributaries Pripyat, Berezina and Sozh (Figure 1).This means that the waterways of this country have and are likely to still play an important role in the process of Ponto-Caspian aquatic species spread to Central and Western Europe, in particular to the neighboring territories of Poland and Lithuania.However, until now Belarus remained a 'white spot' on European distribution maps of these species.The reasons for this situation are diverse, but firstly they include the absence of a national strategy in the field of biological invasions, poor financing of corresponding scientific studies and a critical lack of taxonomical specialists in major groups of aquatic invertebrates (First national report. 1998).As a result, so far only the most annoying, high impact invasive species were revealed and investigated satisfactorily, for example, the zebra mussel, Dreissena polymorpha (Pallas, 1771) (Karatayev 1983, Burlakova 1998;Karatayev et al. 2000Karatayev et al. , 2002;;Mastitsky 2004).Information on other species is highly dispersed among different sources (both literature and verbal), most of which are not readily accessible, especially for non-Russian scientists.At the same time, easy access to such information could become crucial for further understanding of both vectors and routes of aquatic invasions throughout Europe.To address this problem, since 2005 we are developing an online database 'Aquatic invaders of Belarus', which is to be launched in the early 2007.This work consists of two main parts: i) classification of all available literature on invaders and ii) execution of field surveys aimed at both verifying old records and detecting new species and their localities.The present paper is directly related to development of the database and is based on benthic sampling conducted from 19-22 July 2006 during an expedition along the Belarusian section of the Dnieper River (720 km, i.e. about 32% of the total length).We report some preliminary results of this survey, namely data on species composition, abundance and distribution of invasive crustaceans from the order Amphipoda.

Material and methods
The samples were collected near the bank from a depth of 0.3-0.8m at five stations distributed approximately uniformly along the river (Figure 1).Description of general physical and biotic conditions at each station is given in Annex 1.
Quantitative samples with three replicates were taken at stations D1 and D3 with an Eckmantype sampler (capture area 104 cm 2 ) installed on a 1.5 m long pole.Qualitative collections of macroinvertebrates were performed with a hand net (mesh size 1 mm) and via visual examination of stones, snags and other hard substrates (30minute effort by two people) at two latter stations and at all other visited sites.Macroinvertebrate samples were preserved in 4% formaldehyde for subsequent taxonomical identification.Alien amphipods were identified according to Konopacka (2004).

Results and discussion
We revealed five amphipod species originating from the Ponto-Caspian  1930).Later, C. curvispinum was found also in the Dnieper River (Vladimirova et al. 1965) and Berezina River (Tischikov and Tischikov 1999).
C. ischnus was never recorded in Belarus before our study; however, its record in the Polish River Vistula in 1928 (reviewed in Jazdzewski and Konopacka 2002;Grabowski et al. 2006Grabowski et al. , 2007) ) indicates that it is likely as old an invader as C. curvispinum.The absence of previous findings of C. ischnus in Belarusian waters may be explained by its typically low abundance in invasive amphipod communities (Haas et al. 2002;Jazdzewski and Konopacka 2002;Grabowski et al. 2007, see also Figure 1).(Konopacka 2004;Grabowski et al. 2007) indicates that it has also begun to spread through the 'central corridor'.It is not clear when and how Belarus was invaded by the remaining species, Pontogammarus robustoides, but a hypothesis can be made that it came in the second part of the 20th century because the species was not previously recorded in any part of that country.There are two possible ways that P. robustoides arrived in Belarus.Since its native range includes the lower courses of the Dnieper River, P. robustoides might have colonised at least part of the Europe through the Dnieper-Bug Canal, and results of our study are consistent with this assumption.On the other hand, this species was repeatedly intentionally introduced into reservoirs and lakes in Lithuania (reviewed in Arbačiauskas 2005).In several of these waterbodies P. robustoides has established stable populations that could be the secondary sources for invasion across European countries, including Belarus.Similar route of P. robustoides invasion from Lithuania into Poland is suggested by Grabowski et al. (2007).
Both the distribution and abundance of invasive amphipods in the Dnieper River have demonstrated a clear pattern.The maximum number of species was observed at the downstream station D1 (Figure 1).In terms of the density and biomass, the most abundant species sampled there were C. curvispinum and D. villosus (Annex 2).A similar dominance of these two species in invasive amphipod communities has been reported in several other European rivers, e.g.Meuse (Josens et al. 2005), Oder (reviewed in Jazdzewski and Konopacka 2002), and Rhine (Bij de Vaate et al. 2002;Bernauer and Jansen 2006;Van Riel et al. 2006b).High densities of C. curvispinum and D. villosus may be accompanied with numerous ecological consequences.For example, C. curvispinum is known to build mud tubes on stony substrates, often resulting in three dimensional changes in bottom relief (so called 'corophiid grounds') (Lubyanov et al. 1967;Haas et al. 2002).This, in turn, creates inter-tube spaces particularly suitable for the development of communities composed of gammarids (including D. haemobaphes and D. villosus), oligochaetes, leeches, molluscs, and chironomids (Lubyanov et al. 1967).Mud tubes of C. curvispinum, however, may negatively affect another litho-phylic species, Dreissena polymorpha (zebra mussel), by inhibiting successful larval settlement (Van den Brink et al. 1991, Van der Velde et al. 1998, Haas et al. 2002).Indeed, we observed very low densities of D. polymorpha at station D1 (authors, unpublished), even though the sandy-stony bottom there (Annex 1) presented a very suitable substrate for this mussel.Chelicorophium curvispinum is an active filter feeder, which especially benefits from high concentrations of phytoplankton in large rivers (Van der Velde et al. 1998).At the same time, it can serve as a prey item for several other invasive species, e.g.fish (Grabowska and Grabowski 2005) and gammarids (Van Riel et al. 2006a), thus participating in 'invasional meltdown' (Simberloff and Von Holle 1999).
Dikerogammarus villosus, which had the highest biomass at station D1 (Annex 2), is known to be one of the most aggressive amphipod invaders.Many researchers have observed rapid partial to complete displacement of native and naturalised invasive amphipods and isopods after arrival of this species (Haas et al. 2002;Van der Velde et al. 2000;Josens et al. 2005;MacNeil and Platvoet 2005;Bernauer and Jansen 2006;Van Riel et al. 2006a,b).Such distinctive invasiveness of D. villosus may be related to several ecological traits, including its: i) broad salinity and temperature tolerances (Muskó 1992; Bij de Vaate and Klink 1995), ii) top predator position in the food webs (Van Riel 2006a), iii) success in intra-guild predation (Dick and Platvoet 2000;Haas et al. 2002;MacNeil and Platvoet 2005), iv) ability to colonise a wide range of substrata (Dedju 1967;Devin et al. 2003) and to exploit available food resources more efficiently than native gammarids (Josens et al. 2005;Platvoet et al. 2006).As a consequence, the speed of active D. villosus upstream range extension may reach up to 40 km/year, i.e. ca. 100 m/day (Josens et al. 2005).
Despite the presence of suitable substrates in upper sections of the Dnieper River (Annex 1), the number of species and their abundance gradually declined, with no amphipods found at points D4 and D5 (Annex 2, Figure 1).The reasons for such distribution are not clear, especially taking into account that C. curvispinum and C. ishnus, which are the oldest amphipod invaders in Belarus (see above), have had enough time to reach upper sections of the river.According to Josens et al. (2005), upstream migration speed of C. curvispinum may be as high as 17 km/year.However, we predict that pattern of distribution of Ponto-Caspian amphipods in the Dnieper River within Belarus will change in the near future because of range extension of recently arrived D. villosus and D. haemobaphes.Already now, they are the most upstream distributed alien amphipods (Figure 1, Annex 2), which can be related to their pronounced invasiveness (Haas et al. 2002;Van der Velde et al. 2000;Bij de Vaate et al. 2002;Jazdzewski and Konopacka 2002;Josens et al. 2005;MacNeil and Platvoet 2005;Bernauer and Jansen 2006;Van Riel et al. 2006a,b).

Figure 1 .
Figure 1.Belarusian section of the Dnieper River and its main tributaries (modified from Karatayev et al. 2000).D1-D5 are stations sampled in this study.Pie-charts characterise composition of alien gammarid communities as % total number of individuals sampled.
Dikerogammarus villosus (Sowinsky, 1894) and Pontogammarus robustoides (G.O.Sars, 1894) (Figure 2, Annex 2).Most of them are likely to arrive in Belarusian waters through the shipping canal between the Pripyat River, a tributary of the Dnieper River (Black Sea basin), and the Zapadnyi Bug River (Baltic Sea basin) (reviewed in Bij de Vaate et al. 2002; Jazdzewski and Konopacka 2002), which was constructed about 200 years ago and is still navigable within the territory of Belarus.However, these species did not arrive synchronously.For example, C. curvispinum may be considered as the oldest crustacean invader in Belarus because it was first recorded in the Pripyat River in 1914 (Wolski region: Chaeto-gammarus ischnus (Stebbing, 1899), Chelicorophium curvispinum (G.O.Sars, 1895), Dikerogammarus haemobaphes (Eichwald, 1841), Vaate et al. 2002)losus was found for the first time outside its native range only in 1992, i.e. in the upper reaches of the Danube River, and two years later was discovered in the lower reaches of the Rhine River (reviewed in Bij deVaate et al. 2002).
(Konopacka 1998)aemobaphes and D. villosus are the most recent newcomers.This statement is validated by the fact that D. haemobaphes was detected only very recently (1997) in the protractedly monitored Polish River Vistula(Konopacka 1998).