3.5. Environmental Factors
Effects on Trace Metal Concentrations. For all metals except
arsenic, a significant and strong correlation between metal
concentrations and TOC concentrations was found (P-values<
0.008) (Table S5 in the SI). TOC levels, as would be expected, showed a
strong negative correlation (R=-0.69, P-value=0.028) with the sand
percentage in bed sediment samples.
Effects on Microbial Communities. Correlation analysis showed
no significant correlation among bed water quality parameters and alpha
diversity indices. However, the results did show significant and
negative correlations (R> 0.73, P-values< 0.038)
among trace metal concentrations and the Simpson index. For the
Shannon-Wiener index, all metals except arsenic and cadmium showed
significant correlations. Copper, chromium, nickel, and lead
concentrations at station S1 were identified as influential points.
The groups in the weighted Unifrac (Figure 4B) showed some similarities
to the PCA plot developed using field water quality parameters, and
grain size distributions as shown in Figure 9C. The PCA1 shown in the
x-axis of Figure 4C accounted for 42.85% of variation in the dataset
and was dominated by the level of sand, clay, silt, and TOC. The
sampling stations were grouped into S1, S10, S2/S5/S6/S11, and
S3/S4/S7/S8/S9. The similarities in groups between the weighted Unifrac
and environmental PCAs (see Figure 9) and the fact that the x-axis in
environmental PCA is describing sediment distribution may suggest that
unlike rare bacteria, the distribution of common bacteria in samples
could be a function of sediment composition in the system.
Figure 10 shows the results of the CCA for both scenarios described in
Section 2.6, one with all environmental factors except metals and the
other one with reduced size environmental factors including metals. For
both scenarios, the results were very similar to the PCoA plot of the
weighted Unifrac (Figure 4B). Extremophile phylum (rare) could be found
on the right side of the CCA figures (Figure 5) with the closest station
being S1. Among environmental factors, metals and TOC had the closest
distance to S1, indicating the potential effect of both organic and
inorganic pollutants on the microbial communities at S1. Station S7
through S11 all located in the side bays and Galveston Bay (see Figure
1B for locations) showed the shortest distance to salinity, DO, and pH
while stations S1 through S5 located in the western and deeper parts of
the system showed more proximity to total depth and sediment grain size
variables. The permutation test for both scenarios showed P-values
greater than 0.05 meaning failing to reject the null hypothesis that the
observed correlation among the environmental factors and microbial
diversities was due to chance. Overall, the lack of a significant
correlation among metal concentrations and the field water parameters at
the bed layer and among environmental factors and microbial diversities
could be an indication that the system has not reached equilibrium in
terms of chemical and microbial properties and partitioning among
phases.