A variable lacustrine environment in Lake Winnipeg, Manitoba: Evidence from modern thecamoebian distribution

Thecamoebians (testate protozoan) were examined in 18 surface sediment samp les from the North and South basins and the Narrows of Lake Winnipeg, Manit oba. Significantly higher numbers of thecamoebians and tintinnids in the No rth Basin compared to the Narrows and South Basin are attributed to the eff ects of urban development around the South Basin of Lake Winnipeg. Human po pulation growth in this area has led to increased nutrient concentration in runoff, causing eutrophication of the southern lake, which in turn allows for increased algal productivity. Cucurbitella tricuspis is found in large abundances in the South Basin, particularly close to the inlet of the Red a nd Assiniboine rivers. High abundances of this species have been attributed to eutrophic conditions, which this species appears to withstand more succ essfully than other species. Increases in domestic waste output, that have led to elevated heavy metal concentrations in lake bottom sediments of the South Basin, may have resulted in lower abundances of thecamoebians, furthe r reducing competition. Strong currents in the Narrows cause a slightly coarser substrate and sweep away food sources such as phytoplankton. This results in a lower faunal ab undance and slightly lower species richness of thecamoebians. Robust specie s such as the coarse-grained Difflugia viscidula and species which feed on bacteria such as Centropyxis aculeata show increased abundances. Modern thecamoebian assemblages are comparable to Late Holocene faunal asso ciations in terms of species composition. Individual species abundances, ho wever, have changed. For example, in the North Basin the Late Holocene domi nance of Difflugia manicata is replaced by various strains of Difflugia obl onga during recent times. A common species of the South Basin from Late Hol ocene to recent times is Difflugia globulus. It would appear that faunal di fferences between basins are the result of differences in algal food source s.