Inadequacy of size distributions of stream benthic diatoms for environmental monitoring

Benthic diatom assemblages respond to changes in water quality, and this response is evidenced by shifts in taxonomic composition. As a result, several taxon-based indices have been developed for monitoring purposes. Some authors have suggested that diatom body size might provide a simpler method for bioassessment than taxonomy-based approaches. Moreover, current knowledge of algal ecology suggests that the slopes and intercepts of density size distributions should vary with environmental characteristics. However, results from studies of the relationship between algal size and trophic variables including P have been mixed. Our objectives were to examine normalized density size distributions and richness size distributions of benthic diatoms in streams along a gradient of agricultural land use to determine whether the size distributions changed in relation to environmental variables. Benthic diatoms from 29 streams in eastern Canada were identified and average body size measurements were obtained for each taxon. Normalized density size distributions and richness size distributions were plotted, and slopes and intercepts were compared among sites using general linear models (GLMs). Despite taxonomic differences in the assemblages, slopes of the normalized density size distributions did not differ among the 29 sites, and distribution shape and intercepts for the richness size distributions did not differ among sites. The intercepts of the normalized density size distributions were significantly different among sites, and this variation was attributed in part to the effect of several environmental variables including N and P. However, this difference in intercepts represented a change in density across all size classes, rather than a size-specific change. Environmental variables did not explain a significant amount of the variance in the shapes of density or richness size distributions. These results suggest that the slopes of lotic benthic diatom size distributions do not differ in response to environmental variables including P. Thus, benthic diatom body size should not be used as a proxy for nutrient status in environmental monitoring.