Environmental effects on constructed wetland microbial diversity and function in the context of wastewater management

Considering temporal and spatial change in biodiversity-ecosystem function (BEF) relationships is critical to predict and manage ecosystem services, especially in human mediated and impacted ecosystems. We propose that species responses to seasonal change and spatial distributions can act as a laboratory to reveal diversity-function relationships with management implications. This study investigates the relationship between bacterial diversity and ammonia removal function in a wastewater secondary treatment constructed wetland system. We took 8 samples across a system of 6 interconnected ponds, from August 2019 to February 2020, at the Arcata Wastewater Treatment Facility (AWTF), in Coastal Humboldt County (California, USA). We used 16S rRNA gene amplicon sequencing to measure bacterial diversity and composition, and an ammonia electrode probe to measure NH4 at the influx and efflux positions of each pond. We found a significant negative relationship between ASV richness and ammonia removal, suggesting that nitrifying and denitrifying bacteria are poor competitors, known a negative selection effect. Bacterial richness effect on ammonia removal was strongest, followed by direct effects of season on richness and location on function, based on structural equation modeling. We identify taxa associated with function that may influence management strategies, including Planktophila, Legionella, Sulfurimonadaceae and Sporichtyaceae that thrive in ponds located after chlorination steps. This result challenges the traditional wastewater treatment reactor paradigm to reveal negative BEF relationships that appear stronger than environmental influences. By expanding our views of BEF relationships, we can further unravel how community diversity and composition influence ecosystem processes in natural and humanized systems. IMPORTANCEThis study sheds new light on how biodiversity impacts ecosystem functions in human-made environments, specifically wastewater treatment systems. By examining bacterial diversity and ammonia removal efficiency across interconnected ponds, we challenge the conventional assumption that more species always lead to better ecosystem performance. The surprising finding that higher bacterial diversity can reduce ammonia removal efficiency (due to competition among key bacteria) offers fresh insights into how microbial communities work. This understanding is critical for improving wastewater treatment and designing systems that maximize efficiency. Moreover, identifying specific bacteria linked to ammonia removal provides practical information for better managing and enhancing treatment processes. By broadening how we think about the relationship between biodiversity and ecosystem function, this study offers valuable tools for both scientists and environmental managers working to balance human impact with ecosystem health.