SUMMARY1. Accumulation of organic material by the zebra mussel Dreissena polymorpha is assumed to be the source of a biodeposition-based food web. However, only little is known about the importance of the biodeposited material as a food source and its contribution to increased abundances of macroinvertebrates in the presence of D. polymorpha. 2. Feeding, assimilation and growth of the amphipods Gammarus roeselii and Dikerogammarus villosus on food sources directly and indirectly associated with D. polymorpha (biodeposited material and chironomids) and on conditioned alder leaves were measured. The stoichiometry of carbon, nitrogen and phosphorus of the diets was measured as an important determining factor of food quality. 3. Chironomids had the highest nitrogen and phosphorus contents, alder leaves were depleted in nitrogen and phosphorus, and the stoichiometry of biodeposited material was intermediate. 4. Both amphipod species had highest feeding rates and assimilation efficiencies on chironomids. Gammarus roeselii fed more on biodeposited material than on alder leaves, but assimilation efficiencies were similar; D. villosus also had similar feeding rates and assimilation efficiencies on the two diets. 5. Both amphipod species had highest growth rates on chironomids and lowest growth rates on alder leaves. Both grew at intermediate rates on biodeposited material of D. polymorpha. The growth rates of the amphipod species were related to food stoichiometry. Overall, the invasive D. villosus grew faster than the indigenous G. roeselii. 6. Food resources directly and indirectly associated with D. polymorpha are potential diets for amphipods, providing further evidence for a D. polymorpha biodeposition-based food web.
SUMMARY1. River ecosystems are threatened by multiple stressors, including habitat degradation, pollution and invasive species. However, freshwater ecologists have largely disregarded the contribution of toxicants to stress in rivers, whereas ecotoxicologists have primarily examined toxicant effects in artificial systems. As a result, there is a paucity of information on the co-occurrence of organic toxicants with other stressors and on the relative importance of toxicants for overall ecological risk in rivers. 2. We used monitoring data for German rivers to analyse the individual and joint occurrence of four stressors: habitat degradation, invasive species, nutrient pollution and organic toxicants. All stressors were examined for ecological risks in terms of whether they exceeded low-and high-risk thresholds derived from published studies and regulatory thresholds. 3. Nutrients and habitat degradation exceeded low and high risk thresholds at c. 85% of the sites and invasive species and organic toxicants at c. 50% of the sites. At least one stressor exceeded thresholds at all sites for which data on all four stressors were available. Toxicity showed weak positive correlations with nutrients and habitat degradation (0.2 < Spearman's q < 0.34, 0.009 < P < 0.08). The risks of ecological effects arising from habitat degradation and invasive species were higher in lowland rivers, particularly for invasive species. 4. Our assessment shows that organic toxicants contribute notably to risks of ecological effects in rivers, to a similar extent as invasive species, although habitat degradation and nutrients are the dominant stressors. Exposure to multiple stressors is the typical situation prevailing in rivers. Consequently, mitigation measures focusing on individual stressors may not be effective at reducing ecological risks. This suggests that integrating concepts and data from freshwater ecology and ecotoxicology is essential to meet the challenge of managing multiple stressors in river ecosystems.
Invasive species can cause indirect effects on native biota by modifying parasite-host interactions and disease occurrence in native species. This study investigated the role of the invasive Pacific oyster (Crassostrea gigas) in potential spillover (co-introduced parasites infect native hosts) and spillback (native or established parasites infect invasive hosts and re-infect native hosts) scenarios of recently introduced (Mytilicola orientalis) and previously established (Mytilicola intestinalis) marine parasitic copepods in two regions in northern Europe, the Dutch Delta and the Wadden Sea. By examining 3416 individuals of 11 potential host species from sympatric host populations, we found that the recently introduced parasite M. orientalis does not only infect its principal host, the invasive Pacific oyster (prevalence at infected sites 2-43 %, mean intensity 4.1 ± 0.6 SE), but also native blue mussels (Mytilus edulis; 3-63 %, 2.1 ± 0.2), common cockles (Cerastoderma edule; 2-13 %, 1.2 ± 0.3) and Baltic tellins (Macoma balthica; 6-7 %, 1.0 ± 0), confirming a spillover effect. Spillback effects were not observed as the previously established M. intestinalis was exclusively found in blue mussels (prevalence at infected locations 3-72 %, mean intensity 2.4 ± 0.3 SE). The high frequency of M. orientalis spillover, in particular to native mussels, suggests that Pacific oysters may cause strong parasite-mediated indirect impacts on native bivalve populations.
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