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

We evaluated by stable isotope analysis the trophic structure of an estuarine transition zone (ETZ) food web and the role of an invasive species, the veliger stage of the zebra mussel Dreissena polymorpha. In the St. Lawrence ETZ, where zebra mussel veligers are now the dominant zooplankton in summer, d13C ranged from 231.2% (seston) to 216.1% (adult fish) and d15N ranged from 2.6% to 17.4%. Isotopic analysis of samples indicated that the overall food web was largely supported by autochthonous phytoplankton rather than by allochthonous terrestrial carbon. Large differences among the isotopic signals of veligers, cladocerans, and copepods suggested the use of different proportions of food items, and the isotopic values of fish larvae indicated no significant assimilation of veligers. The d13C signature of the veligers was in a range consistent with feeding on free-living bacteria and dissolved organic carbon (DOC) or both, and freshwater algae incubated in situ. To investigate the possibility of DOC uptake by the veligers, we incubated veligers on 14C-labelled algal lysates. There was rapid uptake of DOC and incorporation into biomass, equivalent to 6% of the soft tissue dry weight per hour. Zebra mussel veligers are likely using autochthonous DOC as an alternate food source, and they occupy an exotic trophic position in which there is little direct interaction with other major components of the ETZ food web. Estuarine transition zones (ETZs), where riverine waters first mix with seawater, are typically rich in both allochthonous (terrestrial) and autochthonous (in situ photosynthetic) organic matter. They also contain abundant consumer populations, but little attention has been given to the sources and pathways of carbon flow. 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. Like many waterways in North America and elsewhere, the St. Lawrence River has recently been invaded by the zebra mussel, Dreissena polymorpha, and since July 1994 large concentrations of zebra mussel larvae (veligers) have been registered in the river and ETZ. 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). There is now an extensive literature on the effects of adult zebra mussels in aquatic ecosystems; however, the ecological role of their abundant veliger stage has rarely been addressed. Our previous distributional analysis of veligers and microbial food web components in the St. Lawrence ETZ indicated that there was little effect of this invasion on the 1 Corresponding author ( [email protected]). Acknowledgments We thank Hugues Boulanger, Andy Casper, Marie-Audrey Livernoche, Marie-Noël M’Boutchou, Didier M’Radamy, Carl Martin, Leira Retamal, Sébastien Roy, Geneviève Trudel, and Gesche Winkler for field and laboratory assistance; Martine Savard and Anna Smirnoff for their expert advice and assistance with mass spectrometry at the Commission Géologique du Canada, Québec; Nicholas S. Fisher for helpful advice on protocols; Judy Acreman at University of Toronto Culture Collection; Lise Rancourt and Sébastien Duval at the Institut National de la Recherche Scientifique; Paul Middlestead at the G.G. Hatch Laboratory; and Ladd Johnson for his leadership of the NSERC strategic research program on the St. Lawrence estuary. This work was funded by strategic and discovery research grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) and a postgraduate NSERC fellowship to C.B. This is a contribution to the programs of Québec-Océan, Centre d’Études Nordiques, Groupe de Recherche sur les Écosystèmes Aquatiques (GRÉA) and the Canada Research Chair in Aquatic Ecosystem Studies. Limnol. Oceanogr., 51(3), 2006, 1473–1484 E 2006, by the American Society of Limnology and Oceanography, Inc.