Assessing Water Through Its Life: Bioassays as Indicators of Water Quality in Sewer Overflows

Abstract

The ecological status of surface water quality in the Netherlands, continues to fall short of the targets set by the Water Framework Directive (WFD). Monitoring coverage, both in terms of geographic scope and the variety of pollutants, remains a major challenge, especially in urban waters affected by diverse pollution sources. This study, conducted within the citizen science initiative Putting Water on the Map, evaluated bioassays using the microgreen algae, Chlamydomonas, reinhardtii and the freshwater zooplankton, Daphnia pulex, as practical and sensitive indicators of urban water quality. Bioassay trials assessed growth and behavioral endpoints across paired sewer overflow and reference sites. While no significant differences were detected between sites, likely due to the absence of sewer overflow events during sampling, both bioassays detected measurable impacts when compared to laboratory controls. D. pulex exhibited up to 20% declines in behavioral movement metrics upon exposure to urban water samples, followed by recovery when returned to control conditions, demonstrating both sensitivity and plasticity. Algal bioassays, while challenged by interference from non-target organisms, offered advantages in scalability due to low sample volume requirements and high-throughput capability. This study highlights critical trade-offs between ecological sensitivity and practical scalability. Zooplankton bioassays may offer deeper ecological insights, whereas algal bioassays provide a more feasible approach for widespread, citizen science-based monitoring. Bioassays are unlikely to replace traditional chemical monitoring but can serve as complementary tools that broaden spatial and temporal coverage and facilitate early detection of ecological stress. The success of community-integrated monitoring efforts hinges on the synergy of sensitive bioassays, event-driven sampling strategies, and motivated citizen participation. Future research should focus on refining bioassay protocols for field application, developing dynamic sampling frameworks responsive to pollution events, and exploring multi-trophic interactions to better understand food web impacts. By addressing current methodological and logistical limitations, bioassays can become more reliable, relevant, and scalable tools, critical for advancing urban water quality assessment and meeting regulatory goals under frameworks such as the WFD.