Are diatoms better reflectors of stream quality than water chemistry at regional scales?

Background/Question/Methods

Diatoms are one organism group used in stream quality assessment in addition to water chemistry analyses. Typically nutrient concentrations in streams vary notably, but diatoms respond to changes with a short delay, integrating the variation in water chemistry. Therefore, diatoms typically do not react to small and short-term changes in water quality, but better reflect the significant and long-lasting changes. As there is regional variation in water chemistry, diatom communities of boreal streams also incorporate a strong spatial component at regional scales. What is not properly known yet, is whether the variation in diatom communities between regions is larger than the variation in environmental conditions. Diatoms were sampled at 40 stream sites located in four regions in different parts of Finland. The samples were collected at each site once in the summer 2010. A total of 500 frustules per sample were identified if possible and counted using phase contrast light microscopy. The statistical significance of differences between the community compositions and between environmental conditions at different regions was tested using Multiresponse Permutation Procedure (MRPP). Then, we studied the relationship between diatom species and environmental and geographical variables of different sampling locations with Canonical Correspondence Analysis (CCA). We also used Mantel test to study the relationship between community dissimilarity and environmental and geographical distance.

Results/Conclusions

Our results suggest that the four study regions differed in their diatom species composition more than in their environmental conditions. Concentration of N and P and water conductivity were the most important factors affecting the diatom occurrence. Of physical factors, current velocity seemed also to be significant. However, according to Mantel tests geographical distance appeared to be a minor factor for diatom distribution at this study scale. These results imply that species composition reflect the differences between regions better than measured environmental variables. However, the pure effect of geographical location did not affect the diatom communities. The finding that diatoms are efficient indicators of stream quality at regional scales is also beneficial for stream quality assessment programs using snapshot sampling.