Factors Affecting the Spatial Distribution of Diatoms on the Surface Sediments of Three Precambrian Shield Lakes

Spatial heterogeneity in species composition of surface sediment diatom assemblages was examined in three Precambrian Shield lakes of different water chemistry and basin morphometry. The study revealed that one sample cannot adequately represent the "average" diatom flora of a lake. The results of cluster analysis, multivariate analysis of variance, and analysis of covariance indicated that the lakes exhibited different patterns of spatial heterogeneity in their sedimentary diatom assemblages. In the morphometrically simple Plastic Lake the amount of variability was substantially lower than in the other lakes. In the morphometrically more complex Blue Chalk Lake and Chub Lake there was considerably more variability; however, very little of the variability could be attributed to depth or basin shape. The variability in surface diatom assemblages was markedly higher in Chub Lake, probably as a result of the influx of diatoms from the wetlands surrounding the lake. Despite a significant amount of heterogeneity in sediment diatom composition within each lake, between-lake differences were sufficiently high to suggest that comparisons between lakes based on sedimentary diatom remains are probably valid if sampling sites are selected following prescribed criteria.