Wastewater treatment effluent reduces the abundance and diversity of benthic bacterial communities in urban and suburban rivers

Abstract

In highly urbanized areas, wastewater treatment plant (WWTP) effluent can represent a significant component of freshwater ecosystems. As it is impossible for the composition of WWTP effluent to match the composition of the receiving system, the potential exists for effluent to significantly impact the chemical and biological characteristics of the receiving ecosystem. We assessed the impacts of WWTP effluent on the size, activity, and composition of benthic microbial communities by comparing two distinct field sites in the Chicago metropolitan region: a highly urbanized river receiving effluent from a large WWTP and a suburban river receiving effluent from a much smaller WWTP. At sites upstream of effluent input, the urban and suburban rivers differed significantly in chemical characteristics and in the composition of their sediment bacterial communities. Although effluent resulted in significant increases in inorganic nutrients in both rivers, surprisingly, it also resulted in significant decreases in the population size and diversity of sediment bacterial communities. Tag pyrosequencing of bacterial 16S rRNA genes revealed significant effects of effluent on sediment bacterial community composition in both rivers, including decreases in abundances of Deltaproteobacteria, Desulfococcus, Dechloromonas, and Chloroflexi sequences and increases in abundances of Nitrospirae and Sphingobacteriales sequences. The overall effect of the WWTP inputs was that the two rivers, which were distinct in chemical and biological properties upstream of the WWTPs, were almost indistinguishable downstream. These results suggest that WWTP effluent has the potential to reduce the natural variability that exists among river ecosystems and indicate that WWTP effluent may contribute to biotic homogenization.

Fig 1

Heterotrophic plate counts (A) and direct bacterial cell counts (B) for sediments collected from rivers within two land use types (urban and suburban) from locations upstream and downstream of WWTP effluent inputs. Each data point is the mean (n = 5) ± standard error. Two-way ANOVA for plate counts demonstrated significant effects of land use (P = 0.005) and effluent input (P = 0.000) but no significant interaction effect (P = 0.607). Two-way ANOVA for direct counts demonstrated no significant effect of land use (P = 0.091) but a significant effect of effluent input (P = 0.001) and no significant interaction effect (P = 0.243).

Fig 2

Community respiration normalized by surface area (A) and by bacterial cell numbers based on heterotrophic plate counts (B) for sediments collected from rivers within two land use types (urban and suburban) from locations upstream and downstream of WWTP effluent inputs. Each data point is the mean (n = 5) ± standard error. Two-way ANOVA for respiration normalized by surface area demonstrated no effect of land use (P = 0.572) or effluent input (P = 0.189) and no interaction effect (P = 0.176). Two-way ANOVA for respiration normalized by cell numbers demonstrated a significant effect of land use (P = 0.000) and effluent input (P = 0.000) and a significant interaction effect (P = 0.000).

Fig 3

NMDS ordination of 16S tag pyrosequencing data comparing community structures of sediment bacterial communities collected from rivers within two land use types (urban and suburban) from locations upstream and downstream of WWTP effluent inputs.

Fig 4

Phylotype analysis of 16S tag pyrosequencing data for sediment bacterial communities collected from rivers in two land use types (urban and suburban) from locations upstream and downstream of WWTP effluent inputs. The y axis represents the percentage of all sequences within a sample that were within the phylotype listed on the x axis. Each data point is the mean (n = 5) ± standard error. An asterisk indicates a significant effect of effluent input (P < 0.05).

Fig 5

Shannon diversity index (A) and Chao 1 richness estimator (B) based on 16S tag pyrosequencing data for sediment bacterial communities collected from rivers within two land use types (urban and suburban) from locations upstream and downstream of WWTP effluent inputs. Each data point is the mean (n = 5) ± standard error. Two-way ANOVA of Shannon data demonstrated no significant effect of land use (P = 0.328) but a significant effect of effluent input (P = 0.019) and no significant interaction effect (P = 0.936). Two-way ANOVA of Chao1 data demonstrated no significant effect of land use (P = 0.604) but a significant effect of effluent input (P = 0.000) and no significant interaction effect (P = 0.895).