Paul J. Bischoff

Zebra mussels (Dreissena polymorpha) are bivalve mollusks that have invaded and altered the ecology of many North American lakes and rivers. To identify possible ecological effects of zebra mussels on naked amoebae, this study compared abundances of sediment and water column naked amoebae in shallow water zones of four lakes with and four lakes without zebra mussels. Additional data was collected on the density of macroflagellates. Although no statistically significant difference in naked amoebae density was found, higher ratios of sediment to water column naked amoebae abundances in zebra mussel lakes were observed due to increased naked amoebae abundances in the sediments. However, we did not observe a concomitant decrease in water column naked amoebae abundances. Flagellate abundances revealed no significant differences between the two lake types. Taken together, the data show that naked amoebae and flagellates thrive in shallow water zones of zebra mussel lakes and that the filter feeding activities of zebra mussels and reported reduced water column protists abundances may be offset by the flocculation of protists from the rich zebra mussel colonies.

Among the most prolific invasive plant species posing threats to the ecological balance of ecosystems in North America and Europe is Japanese knotweed (Fallopia japonica). In order to identify the potential role of protists in the invasive capacity of Japanese knotweed the winter abundances of naked amoebae in soils from three roadside thickets of Japanese knotweed were compared to nearby non-invaded sites. Japanese knotweed soils had higher abundances (t = 5.43, df, 16, p < 0.001) of amoebae than comparison sites. This is one of the first studies to document higher abundances of soil naked amoebae associated with an invasive plant, and may indicate more generally that below-ground abundances of amoebae can promote soil fertility and support the successful adaptation and expansion of some invasive plant species. Moreover, analysis of encysted vs. trophic forms of naked amoebae in the winter soil, provided evidence of freezeresistant, amoeba resting cells in soil samples from the natural environment in support of prior findings that were based solely on laboratory experimental evidence. Overall, high densities of naked amoebae in the winter soils of Japanese knotweed, some as resting cells capable of forming rapidly activated trophic stages, likely increase soil fertility and strengthen the species’ invasive capacity.