Invasion of an estuarine transition zone by Dreissena polymorpha veligers had no detectable effect on zooplankton community structure

Abstract

Large numbers of the filter-feeding larval stage of zebra mussels (Dreissena polymorpha) (veligers) have been advected into the estuarine transition zone (ETZ) of the St. Lawrence River. In 1994, they became the dominant member of the tidal freshwater zooplankton assemblage of the ETZ (and subsequently the true estuarine assemblage) during summer months. These changes in community structure have not, however, been reflected in changes in the abundance of preinvasion taxa. Abundance of potential planktonic competitors such as rotifers and the cladoceran Bosmina longirostris fluctuated between years but did not decline following the invasion. Only densities of the copepod Eurytemora affinis declined in 2000 and 2001, but this could not be definitively associated with the invasion. Synchronous variation among different zooplankton groups suggests that environmental forcing related to or correlated with temperature anomalies is the major cause of interannual variation in the abundance of zooplankton taxa. The negative correlation between veliger abundance and salinity indicates that the ETZ acts as a sink for the larval stages of the zebra mussel. Their precise fate remains uncertain, but our study suggests that this invasion has had little or no impact on this pelagic community in spite of the numerical dominance of veligers.Résumé : Un grand nombre de larves véligères à alimentation par filtration de la moule zébrée (Dreissena polymorpha) ont été entraînées par advection dans la zone de transition estuarienne (ZTE) du fleuve Saint-Laurent. En 1994, elles sont devenues l'élément dominant des peuplements intertidaux de zooplancton de la ZTE (pour former ensuite le véritable peuplement estuarien) durant les mois d'été. Cependant, ces changements de structure de communauté n'entraînent pas de changements d'abondance des taxons présents avant l'invasion. L'abondance des compétiteurs planctoniques potentiels, tels que les rotifères et le cladocère Bosmina longirostris, fluctue d'une année à l'autre, mais il n'y a pas eu de déclin après l'invasion. Seules les densités du copépode Eurytemora affinis ont diminué en 2000 et 2001, mais le déclin ne peut être clairement associé à l'invasion. L'existence de variations synchrones parmi les diffé-rents groupes du zooplancton laisse croire que le forçage environnemental associé aux anomalies de température, ou en corrélation avec elles, est la cause principale de la variation d'abondance des taxons du zooplancton. La corrélation né-gative entre l'abondance des véligères et la salinité indique que la ZTE agit comme un puits pour les stades larvaires de la moule zébrée. Leur sort exact reste incertain, mais notre étude laisse croire que cette invasion a eu peu ou pas d'impact sur la communauté pélagique, malgré la dominance numérique des véligères.[Traduit par la Rédaction] Winkler et al. 592

Trophic position of zebra mussel veligers and their use of dissolved organic carbon

“…Prey of fish larvae-Consistent with previous studies (Winkler et al 2005 and references therein), the isotopic data indicate that the zooplankton B. longirostris and E. affinis are important food sources for fish larvae. All the fish larvae analyzed in the present study (M. villosus, M. tomcod, and O. mordax) had very similar carbon and nitrogen isotopic signatures, suggesting their dependence on the same combination of prey items.…”

“…and cladocera (Bosmina longirostris) are the predominant zooplankton in the freshwater part of the estuary and in the south channel of the ETZ, shifting further into the ETZ to estuarine mysids (Neomysis americana and Mysis stenolepis), copepods (notably Eurytemora affinis) and the sand shrimp Crangon septemspinosa. The high primary production and zooplankton biomass sustain large stocks of fish larvae, with concentrations of 85 Osmerus mordax (rainbow smelt) and 153 Microgadus tomcod (Atlantic tomcod) per 100 m 3 (Winkler et al 2003 and references therein).…”

“…Some studies have argued for a dominant role of allochthonous carbon in sustaining food webs in rivers (McCallister et al 2004), whereas other authors have stressed the greater importance of autochthonous sources (Martineau et al 2004 and references therein). The St. Lawrence River ETZ has long been recognized as supporting high biological productivity and an important larval fish nursery, with large standing stocks of phytoplankton and zooplankton (Frenette et al 1995;Winkler et al 2003). In spite of the ecological importance of this ETZ, there has been no analysis of its overall food web structure.…”

“…These veligers are now recognized as the dominant zooplankton component, constituting between 52% and 90% of the total zooplankton counts in summer (Winkler et al 2005). The larvae are likely retained and concentrated in the ETZ by the same physical mechanisms that cause the high turbidity (Frenette et al 1995).…”

Dreissenid mussels (Dreissena polymorpha and Dreissena bugensis) reduce microzooplankton and macrozooplankton biomass in thermally stratified lakes

“…1). Although changes in macrozooplankton biomass have been documented in some systems, the community structure of macrozooplankton did not change in the St. Lawrence River after invasion (Winkler et al 2005). …”