Zooplankton composition in a gravel pit lake invaded by the Ponto-Caspian mysid Hemimysis anomala G . O . Sars 1907

During a zooplankton monitoring program in the Rhine catchment (Southern Germany) during the 2006 sampling year, we recorded the mysid Hemimysis anomala in an oligotrophic gravel pit lake. The crustacean and rotifer abundance in the lake was low; maximum density was 28 and 145 ind. L, respectively. Crustaceans were dominated by copepods, which constituted 90100% of the zooplankton community. Cladocerans were represented only by Diaphanosoma brachyurum . Rotifers were dominated by Polyarthra spp. for most part of the year. We suggest that the reason for the lack of cladocerans (except Diaphanosmoma) could be competition and/or predation by H. anomala. That D. brachyurum could persist in the presence of Hemimysis may be a result of spatial segregation. Possibly, due to differences in thermal preferences, H. anomala avoids the warm epilimnion of the lake in summer while Diaphanosoma may prefer it.

The blood-red mysid Hemimysis anomala G.O. Sars 1907, which originates from the Ponto-Caspian region, i.e. the Black, Caspian and Azov Seas, has invaded various Central, Western and North European countries during the last decade.For example, the species was recorded in Austria and Switzerland (Wittmann et al. 1999;Wittmann 2007), the Netherlands (Faasse 1998), Belgium (Verslyche et al. 2000), Czech Republic (Horecky et al. 2005), France (Devin et al. 2005;Dumont 2006;Wittmann and Ariani 2009), Hungary (Borza 2008) and more recently even in the UK (Holdich et al. 2006;Stubbing et al. 2008) and Ireland (Minchin and Holmes 2008).The two invasion pathways of Hemimysis into European waters were: a) via the Baltic Sea and the Rhine delta and b) via the Danube Delta through the Main-Danube and the Rhine system (Audzijonyte et al. 2008).In Germany H. anomala was recorded for the first time in 1997 in the Rhine and Neckar (Schleuter et al. 1998).
Meanwhile the species has spread in several freshwaters dispersed over the whole country (Schleuter and Schleuter 1998;Eggers et al. 1999;Haesloop 2001;Rehage and Terlutter 2002;Müller et al. 2005;Tappenbeck 2006;Habermehl 2008).Hemimysis anomala is an euryhyaline species that exerts day-night migrations; it lives near the bottom during daytime and ascends into the pelagic zone at night (e.g.Ketelaars et al. 1999;Borcherding et al. 2006).Recent investigations of zooplankton composition in freshwater lakes and analyses of stomach contents revealed that large H. anomala feed on zooplankton (Ketelaars et al. 1999;Borcherding et al. 2006).
During a zooplankton monitoring program in gravel pit lakes in the Rhine catchment, we collected 3 specimens of H. anomala (total length: 3 mm; estimated abundance 13 ind.m -3 ) in September 2006 and 6 specimens (total length: 5-6 mm; estimated abundance 38 ind.m -3 ) in October 2006 in a lake near Rastatt (geographic position: 8°02.377'E;48°47.034'N).The lake is situated in a bow of the Rhine River, at a distance of only 100 m to the river shore; it has a surface area of 71 ha, a maximum depth of 41 m and a mean depth of 18 m.The lake is oligotrophic.Concentrations of total P were < 20 µg L -1 and concentrations of chlorophyll-a < 10 µg L -1 throughout the investigation year.Conductivity is between 414 and 454 µS cm -1 , Cl - concentrations are between 33 and 38 mg L -1 and NO 3 -N concentrations are between 0.3 and 0.5 mg L -1 .Maximum water temperature in the epilimnion was 27°C in July.Zooplankton was sampled by vertical hauls from the bottom to the surface with a 55 µm plankton net (opening diameter 10 cm, length 1 m) at the deepest place of the lake.Samples were immediately preserved in a 6-8 % formalin-sugar solution.In the laboratory the samples were diluted in a volume of 50-250 ml of water and several 10 ml aliquots (with at least 100 specimens of the dominant species) were counted.After the counts the entire sample was transferred to petri dishes and searched for larger zooplankton (Chaoborus, predatory cladocerans, mysids).Hemimysis anomala was identified by the shape of the telson and the appendage (scaphocerite) of the 2. antenna (cf.Bacescu 1954;Eggers et al. 1999;Pothoven et al. 2007).
Crustaceans in the lake were clearly dominated by copepods (Figure 1).Copepods constituted 90-100 % of the crustacean community (= 100 %) throughout the investigation year.The most frequent species in summer was Thermocyclops oithonoides G.O Sars, 1863, and, However, the proportion of Diaphanosoma never exceeded 10% of the crustacean community.The most common rotifer taxa were Polyarthra spp., which dominated for most part of the year.Overall densities of crustaceans and rotifers were <30 000 ind.m -3 and <150 000 ind.m -3 , respectively.
Before the interpretation of results one should keep in mind that the diameter of our plankton net was only 10 cm which suggests that the sampling method was insufficient to get quantitative data on H. anomala.The actual abundance of Hemimysis may be higher in the study lake than estimated in this study.Furthermore, the results presented here are simple field observations.Thus we cannot separate possible effects of physical/chemical conditions and phytoplankton on the abundance and composition of zooplankton from those due to mysids or fish.However, as cladocerans appear to be almost absent in the study lake (except for Diaphanosoma), this could possibly be as a result of competition and/or predation by Hemimysis.Generally, gravel pit lakes contain a much richer crustacean fauna with Daphnia, Bosmina and/or Ceriodaphnia species (e.g.Frenzel 1977;Maier and Buchholz 1996).Ketelaars et al. (1999) reported that cladocerans were severely impacted by H. anomala while copepods remained unaffected which points in the direction of our findings.In contrary, Borcherding et al. (2006) found mainly copepods in Hemimysis stomachs.Yet the findings of the latter authors may result from the fact that they analyzed stomachs from specimens in the cold season when copepods dominated the zooplankton.The reason why D. brachyurum could persist in the presence of H. anomala may originate from its preference for warm water (cf.Stich 2004).Water temperature in the epilimnion of the study lake rose to 27°C in summer.According to Ioffe (1973) and Pothoven et al. (2007), H. anomala avoids water temperatures > 20°C.The avoidance of upper water layers by H. anomala may result in spatial segregation between the mysid and Diaphanosoma thus permitted their co-existence.Polyarthra spp.may be able to build significant population densities due to their capability to flee from predators using their swimming appendages.In terms of abiotic factors, oxygen concentrations ≤4 mg L -1 seem to be lowest critical level for H. anomala (Wittmann 2007).In the study lake, oxygen concentration is high; in fact the lowest concentrations in the hypolimnion (5.3 mg L -1 in October) were above the minimum oxygen level for this species.Finally we emphasize, that our results are simply based on the observations in one year.We cannot exclude that the zooplankton composition in the study lake can be somewhat richer in other years.Whether predation or competition by H. anomala can be responsible for a sparse cladoceran fauna needs further, more detailed investigations.

Figure 1 .
Figure 1.Zooplankton abundance (ind.m -3 ) and composition in the gravel pit lake during the year 2006.Arrows indicate dates when Hemimysis anomala was caught