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<p>Biological invasion has become a major threat to economy, ecology, global biodiversity and ecosystem function of aquatic ecosystems. The main aim of the thesis was to study the effects of the zebra mussel <i>(Dreissena polymorpha)</i>, a versatile invasive species, on phytoplankton dynamics and ecosystem function of lakes. </p><p>In a first attempt, I compared the density of <i>Dreissena</i> and the physicochemical data of ecosystems that it invaded among North American and European lakes to identify important factors in its invasion success. Secondly, I investigated the impact of zebra mussels on phytoplankton community composition in a natural lake. Thirdly, I evaluated whether zebra mussel feeding behavior were affected by the presence of predatory waterborne cues. Finally, I examined the effect of <i>Dreissena</i> on seston stoichiometry.</p><p>A Generalized Additive Model revealed that a joint effect of surface area, mean depth, total phosphorus and calcium concentrations can explain the variability in <i>Dreissena</i> density. Selective grazing by zebra mussels varied in relation to seasonal phytoplankton dynamics. Risk cues released by predators affected both feeding rate and prey selection of the mussels and had cascading indirect effects on phytoplankton biomass and community structure. I found that the flux in nutrients caused by differences in zebra mussel consumption lead to a variation in phytoplankton nutrient limitation.</p><p>The flexibility of zebra mussel feeding behavior and variation in susceptibility among phytoplankton groups to mussel ingestion indicate that invading zebra mussels could alter phytoplankton community composition of lakes and have important ecosystem consequences. The results of this thesis contribute to the growing evidence that predators indirectly affect resource dynamics and food web structure through their non-lethal effects on consumers. The results suggested that zebra mussel can indirectly both reduce and increase the energy transfer efficiency from primary producers to upper trophic levels in the pelagic and benthic food webs, respectively. </p>

This research examined how an invasive macrofouling organism, the Eurasian zebra mussel (Dreissena polymorpha), alters benthic communities in a riverine ecosystem. Controlled field experiments using artificial substrates showed that macroinvertebrate abundance is significantly enhanced within Dreissena beds, and that the physical habitat provided by mussel shells has a greater impact on macroinvertebrate abundance than biological factors (e.g., food provided by mussel biodeposits). Data collected at field sites before and after the establishment of dense Dreissena colonies suggested that these colonies alter macroinvertebrate communities on hard substrates primarily by enhancing populations of deposit-feeding organisms (e.g., amphipod crustaceans) and predators (e.g., flatworms), and by displacing fauna that are poorly adapted to interstitial substrate (e.g., large gastropods, net-spinning caddisfly larvae). Freshwater sponges were the only organisms found to compete successfully with Dreissena for hard substrate; sponge overgrowth caused significant local mortality of zebra mussels at all sites where sponges were abundant. / The capacity for Dreissena to displace native freshwater mussels (Family Unionidae) in the St. Lawrence River was examined over a four year period by quadrat sampling at selected sites. Dreissena preferentially colonized unionid mussels in the river. Mean infestation loads (number of zebra mussels per unionid mussel) in the St. Lawrence were 10-100 times lower than in the Great Lakes,.but resulted in similar high mortality. Severe declines in unionid species richness and abundance occurred in areas of the river that supported dense Dreissena populations ($>$4,000 mussels/m$ sp2).$ Analysis of data from the Great Lakes-St. Lawrence River system suggests that mass mortality and extirpation of unionid populations typically occurs within 4-5 years following initial colonization of unionids by Dreissena, and that Dreissena infestation will increase the future tate of extinction of North American unionids by nearly 10-fold.

Since their accidental introduction into the Great Lakes in mid- to late-1980s, the freshwater zebra mussels, Dreissena polymorpha, have colonized most lakes and waterways across eastern North America. Their rapid spread is partly attributed to their ability to tenaciously attach to hard substrates via an adhesive apparatus called the byssus, resulting in serious environmental and economic impacts. A detailed ultrastructural study of the bysuss revealed a 10 nm adhesive layer at the attachment interface. Distributions of the main adhesive amino acid, 3,4-dihydroxyphenylalanine (DOPA), and its oxidizing (cross-linking) enzyme, catechol oxidase, were determined histochemically. It was found that, upon aging, DOPA levels remained high in the portion of the byssus closest to the interface, consistent with an adhesive role. In contrast, reduced levels of DOPA corresponded well with high levels of catechol oxidase in the load-bearing component of the byssus, presumably forming cross-links and increasing the cohesive strength.

Since their accidental introduction into the Great Lakes in mid- to late-1980s, the freshwater zebra mussels, Dreissena polymorpha, have colonized most lakes and waterways across eastern North America. Their rapid spread is partly attributed to their ability to tenaciously attach to hard substrates via an adhesive apparatus called the byssus, resulting in serious environmental and economic impacts. A detailed ultrastructural study of the bysuss revealed a 10 nm adhesive layer at the attachment interface. Distributions of the main adhesive amino acid, 3,4-dihydroxyphenylalanine (DOPA), and its oxidizing (cross-linking) enzyme, catechol oxidase, were determined histochemically. It was found that, upon aging, DOPA levels remained high in the portion of the byssus closest to the interface, consistent with an adhesive role. In contrast, reduced levels of DOPA corresponded well with high levels of catechol oxidase in the load-bearing component of the byssus, presumably forming cross-links and increasing the cohesive strength.