Stream microbial communities and ecosystem functioning show complex responses to multiple stressors in wastewater.

Francis J Burdon,Katja Räsänen,Christian Stamm,Yaohui Bai,Jukka Jokela,Simon Mangold,Marta Reyes,Manu Tamminen,Rik I L Eggen,Scott D Tiegs,Philipp Staudacher,Heinz Singer,Adriano Joss

doi:10.1111/gcb.15302

Abstract

Multiple anthropogenic drivers are changing ecosystems globally, with a disproportionate and intensifying impact on freshwater habitats. A major impact of urbanization are inputs from wastewater treatment plants (WWTPs). Initially designed to reduce eutrophication and improve water quality, WWTPs increasingly release a multitude of micropollutants (MPs; i.e., synthetic chemicals) and microbes (including antibiotic‐resistant bacteria) to receiving environments. This pollution may have pervasive impacts on biodiversity and ecosystem services. Viewed through multiple lenses of macroecological and ecotoxicological theory, we combined field, flume, and laboratory experiments to determine the effects of wastewater (WW) on microbial communities and organic‐matter processing using a standardized decomposition assay. First, we conducted a mensurative experiment sampling 60 locations above and below WWTP discharges in 20 Swiss streams. Microbial respiration and decomposition rates were positively influenced by WW inputs via warming and nutrient enrichment, but with a notable exception: WW decreased the activation energy of decomposition, indicating a “slowing” of this fundamental ecosystem process in response to temperature. Second, next‐generation sequencing indicated that microbial community structure below WWTPs was altered, with significant compositional turnover, reduced richness, and evidence of negative MP influences. Third, a series of flume experiments confirmed that although diluted WW generally has positive influences on microbial‐mediated processes, the negative effects of MPs are “masked” by nutrient enrichment. Finally, transplant experiments suggested that WW‐borne microbes enhance decomposition rates. Taken together, our results affirm the multiple stressor paradigm by showing that different aspects of WW (warming, nutrients, microbes, and MPs) jointly influence ecosystem functioning in complex ways. Increased respiration rates below WWTPs potentially generate ecosystem “disservices” via greater carbon evasion from streams and rivers. However, toxic MP effects may fundamentally alter ecological scaling relationships, indicating the need for a rapprochement between ecotoxicological and macroecological perspectives.

Highlights

Ecosystems are increasingly impacted by multiple anthropogenic pressures at a global scale, with a disproportionate and intensifying effect on freshwaters (Harrison et al, 2018; Reid et al, 2019)
We used a wide range of approaches to study the effects of WW and associated stressors on a fundamental stream ecosystem process using the cotton-strip assays (CSA)
Our study shows that WW-born MPs have the potential to harm ecosystem processes as indicated by the CSA, but these impacts may be “masked” by nutrients, warming, and stress-tolerant microbes

Summary

Introduction

Ecosystems are increasingly impacted by multiple anthropogenic pressures at a global scale, with a disproportionate and intensifying effect on freshwaters (Harrison et al, 2018; Reid et al, 2019). Designed to mitigate anthropogenic eutrophication (Vaughan & Ormerod, 2012), municipal wastewater treatment plants (WWTPs) are human-made aquatic ecosystems that continue to influence receiving environments as urban populations grow and developing countries increase their use (Burdon et al, 2016; Schwarzenbach, Egli, Hofstetter, von Gunten, & Wehrli, 2010). These impacts include hydrological changes, altered thermal dynamics, and inputs of chemicals (e.g., nutrients) and microbes (Hamdhani, Eppehimer, & Bogan, 2020; Stamm et al, 2016)

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Discussion

Conclusion