Patterson, R.T., MacKinnon, K. D., Scott, D. B. and Medioli, F. S., 1985, Arcellaceans (“Thecamoebians”) in small lakes of New Brunswick and Nova Scotia: modern distribution and Holocene stratigraphic changes: Journal of Foraminiferal Research, 15 (2): 114-137.
Sediment-water interface samples from five lakes in New Brunswick and three lakes in Nova Scotia were quantitatively examined for both living and total populations of Arcellaceans. Two major assemblages (I and II) were recognized, both characterized by three main species: Difflugia oblonga, Lagenodifflugia vas, and Pontigulasia compressa. Assemblage I is divided into five sub-assemblages characterized by occurrences of minor species. Assemblage II basically has only three species with low abundances. The assemblage variations within Assemblage I are difficult to link with specific variables since we know relatively little about individual species. However, individual occurrences of two species can be linked to specific conditions: Difflugia bidens relates to increased sediment input and Difflugia tricuspis occurs with high concentrations of floating algae. The division between Assemblage I and II appears to be a function of seasonal temperatures. The high diversity Assemblage I occurs where summer temperatures reach values greater than 18°C (that is, above the thermocline) while Assemblage II occurs below the thermocline where temperatures do not exceed 6°C. Oxygen values in both assemblages are the same and, as in Lake Erie, low oxygen values do not appear to be a limiting factor. To complement the surface studies, cores from six of these lakes were also examined. Most lakes were formerly marine basins when sea level was much higher and the marine freshwater transition demonstrated the sharpest assemblage changes. Centropyxis aculeata usually dominated the transition sections. Arcellacean assemblages illustrated few changes once freshwater conditions were established in New Brunswick even though climatic changes are known in the same interval. However, assemblages in the Nova Scotia lakes changed sharply below the surface with D. tricuspis becoming much more common; this indicates more floating algae in these lakes in pre-modern times. Most of the species encountered here have been illustrated by us previously; however, three species found here were not illustrated in our Lake Erie study-Lesquereusia spiralis, Difflugia urceolata elongata, and Difflugia urens, n. sp.-and are fully illustrated here together with all the other species.