Wolffia columbiana is a member of the family Lemnaceae and native to the Americas. In 2013, the first two occurrences of Wolffia columbiana in Europe were found in Germany and the Netherlands. Differentiation between W. columbiana and the rare native W. arrhiza is difficult and a scanning electron microscope (SEM) was used to confirm plant identification. A brief description of both populations of W. columbiana and some identifying characteristics are given. Due to their small size, Wolffia species are easily overlooked in the field and/or misidentified, and thus it seems likely, that even more unknown occurrences of alien Wolffia species might occur in Europe. The remaining known occurrences of putative W. arrhiza should be inspected to check their identity. We hypothesize, that alien W. columbiana might be sometimes overlooked or misidentified as the native W. arrhiza, and thus it may have a more widespread distribution within Europe than thought.

Populations in upstream versus downstream river locations can be exposed to vastly different environmental and ecological conditions and can thus harbor different genetic resources due to selection and neutral processes. An interesting question is how upstream–downstream directionality in rivers affects the evolution of immune response genes. We used next‐generation amplicon sequencing to identify eight alleles of the major histocompatibility complex (MHC) class II β exon 2 in the cyprinid longnose dace (Rhinichthys cataractae) from three rivers in Alberta, upstream and downstream of municipal and agricultural areas along contaminant gradients. We used these data to test for directional and balancing selection on the MHC. We also genotyped microsatellite loci to examine neutral population processes in this system. We found evidence for balancing selection on the MHC in the form of increased nonsynonymous variation relative to neutral expectations, and selection occurred at more amino acid residues upstream than downstream in two rivers. We found this pattern despite no population structure or isolation by distance, based on microsatellite data, at these sites. Overall, our results suggest that MHC evolution is driven by upstream–downstream directionality in fish inhabiting this system.

Ecological impacts of contaminants on population patterns in wild fish are impacted by many contaminants that readily enter aquatic systems. Responses to toxicants by individuals in lab studies generally do not predict population level consequences in natural systems. Trace levels of contaminants are present in all major rivers in southern Alberta, Canada, with concentrations higher downstream of anthropogenic inputs like agricultural land-use and inputs of municipal wastewater effluents. Longnose dace (Rhinichthys cataractae) were used as a sentinel species to study field-based population-level responses to contaminants. We hypothesized that biomarker activity, triggered by contaminant exposure, should increase downstream of anthropogenic inputs in two southern Alberta rivers, with corresponding relations between biomarker activity and sex ratios, after accounting for age structure. Liver detoxification (ethoxyresorufin-O-deethylase activity = EROD) measured at reference and exposed sites on each river differed significantly in only the Bow River system. Sex ratios varied more downstream of anthropogenic inputs than upstream, but the direction of sex ratio bias was inconsistent and temporally dynamic. Sex ratios correlated with liver detoxification in only the Bow River. Taken together, these results suggest that contaminants alter sex ratios in longnose dace, but that there is variation in anthropogenic stressors among rivers.

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