Spatiotemporal variation of microeukaryotic biodiversity and biotic condition associated with disinfectants after the COVID-19 lockdown in Wuhan, China

Intensive application of chlorine-based disinfectants driven by the COVID-19 pandemic was suspected to be detrimental to receiving water ecosystems, but with little field evidence. We characterized the occurrences of typical disinfectants and microeukaryotic communities in surface waters associated with three wastewater treatment plants (WWTP)-river systems one year after the lockdown in Wuhan, China. Trihalomethanes (THMs) declined from summer 2020 (0.01 ∼ 1.82 μg/L) to summer 2021 (0.01 ∼ 0.95 μg/L), whereas quaternary ammonium salts (QAs) increased from summer 2020 (0.53 ∼ 6.35 ng/L) to summer 2021 (8.49 ∼ 191 ng/L). Biodiversity monitoring with environmental DNA (eDNA) revealed significant temporal variation in microeukaryotic community composition. The monitored disinfectants were correlated with some eukaryotic communities as demonstrated by redundancy analysis. For example, QAs were positively related to Cryptophyta relative abundance, but negatively related to Rotifera relative abundance. A microeukaryote-based multimetric index indicated ecological impairment near the Han River WWTP outlet in 2020. Our findings indicate the influence of heavily used disinfectants on river microeukaryotic communities, and the usefulness of assessing mid-term ecological risks from disinfectants in the post COVID-19 era.