Combined sewer overflows alter zooplankton communities in an urban river

Combined sewer systems, common worldwide, transport both ‘stormwater’ and ‘wastewater’ within a single network. Many such systems include overflows, called Combined Sewer Overflows (CSOs), to prevent sewer pipes and treatment plants from being overwhelmed during heavy rainfall. In Chicago, IL, USA pumping stations actively discharge combined stormwater and wastewater into the Chicago River during extreme rain events. In 2023 12.3 billion litres of untreated stormwater and wastewater was discharged into the Chicago River, with 9.1 billion litres entering Bubbly Creek. Zooplankton populations were notably impacted: Chydoridae were eliminated, while Moinidae densities exceeded 300 individuals L −1 . In contrast, zooplankton communities at other locations remained relatively unaffected. We suggest that sewage-derived nutrients fuelled a phytoplankton bloom, while low oxygen levels suppressed predators, allowing Moinidae populations to surge. This study provides in situ evidence that CSOs can significantly alter zooplankton communities in receiving water bodies.