2.6 Effect of Environmental Factors on Metals and Microbial Communities
Visual analysis and correlation analysis in SPSS ®were undertaken to determine the correlation, if any, between the concentrations of different metals, microbial richness and diversity, bed water field water quality parameters, grain size and TOC levels. Moreover, a Principal Component Analysis (PCA) using environmental factors including water quality parameters, total water depth, grain size distribution, and TOC was performed to reduce the number of variables and observe any possible clustering behaviors based on measured environmental factors. To investigate the effect of environmental factors on microbial diversity, the results of UniFrac PCoA were compared with the developed environmental PCA to determine if there were supporting similarities between the two approaches.
In addition to the assumed linear response in PCA, and to consider unimodal species response to environmental factors, a canonical correspondence analysis (CCA) with log-transformation was performed using the environmental factors and microbial data at the phylum level. To consider the effect of trace metals, a separate CCA was conducted; only samples with measured metal data were considered due to the approach within the statistical software in treating missing data points (e.g., using the column average). Due to the limited number of samples for metals and multitude of environmental factors, the sum of all six measured metal concentrations was used as a single variable representing the metal effect, and the clay plus silt percentage was used to represent the sediment grain size effect. The Monte Carlo permutation test (N= 5,000) was performed to test if the effect of environmental variables on the observed abundance of different phylum is significant. In other words, using such high number of N increases the likelihood of capturing random correlations. The CCA analysis and Monte Carlo permutation test were both performed in the PAST Package (Hammeret al. , 2001).
3. Results and Discussion