In 1994, zebra mussels Dreissena polymorpha invaded the St. Lawrence estuarine transition zone (ETZ), a productive large river ecosystem. Their larval veligers have become the dominant zooplankton. Their seasonal dominance of the zooplankton community caused much concern about their potential impacts on the ETZ food web. There is a paucity of information on the zebra mussel larval veliger stage, particularly concerning their distribution, the carbon sources they use, their impacts on food webs and their capacity to use dissolved organic carbon (DOC) as adults have been shown to do. The objectives of this research were to determine: 1) their impacts on the structure of the pelagic microbial community through their dominance and grazing pressure; 2) the role the ETZ plays in the context of a large river system; 3) the environmental factors controlling the distribution of veligers across the ETZ (freshwater to saltwater); 4) the carbon sources assimilated by the veligers in situ; 5) the trophic role they play; and 6) the veligers’ capacity to directly assimilate DOC and use this carbon source in the ETZ.
The microbial variations observed in the microbial community structure cannot be ascribed to the veliger invasion. The ETZ appears productive enough to support these dominant invaders and despite minor changes in the biovolume size spectrum, the microbial community remains dominated by autotrophs, in the picoplankton and nanoplankton size classes, and the cell concentrations of microbial components have remained similar to pre-invasion values. Grazing by the abundant zooplankton, salinity, and hydrodynamic retention are the environmental factors controlling the structure and composition of the microbial community from freshwater to saltwater across the transition zones. According to the traditional view of rivers as longitudinal continua, the ETZ inserts itself as a discontinuous segment within the St. Lawrence River corridor.
The sharp gradients in the environmental variables incur abrupt changes in the structure of the planktonic community. The veligers’ longitudinal distribution was limited by salinity, with maximum decreases in concentration at 2 ‰. A sharp decline in prey availability at > 2 ‰ may be a secondary stressor for the veligers, in addition to the direct effects of salinity. Their vertical distribution was homogeneous throughout the water column, even in the presence of a pycnocline. Redundancy analysis revealed that veliger concentrations were positively correlated with temperature and turbidity and negatively correlated with salinity and total phosphorus. Veligers were also positively correlated with chlorophyll a and picophytoplankton concentrations, suggesting little effect on their phytoplankton prey. The veligers appear to have no severe negative impacts on the plankton community and are restricted to favorable conditions (high seston quality) for their survival in the upstream, low salinity region of the ETZ.
The trophic structure of the ETZ food web was evaluated by stable isotope analysis as well as the trophic role played by the veligers. Isotopic results 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 DOC or both, and freshwater algae incubated in situ. To investigate the possibility of DOC uptake by the veligers, veligers were incubated with 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.