Integrated environmental monitoring in the canals of Padova (Italy) shows the need for trade-off solutions in water resources management in complex historical cities

The sustainable management of urban watercourses is a complex task, having to balance between ecological requirements and multifaceted societal needs which generate multiple pressures on urban aquatic ecosystems, such as pollutant emissions and morpho-hydraulic alterations. Complexity peaks in historical river cities: here, aesthetic and cultural assets require preservation while ecological functions are crucial for biodiversity and to meet emerging socio-economic needs. Yet, current management often focuses on flood control and is inattentive towards water quality and ecological conditions, constraining urban ecosystem services provision. Building on a two-year integrated monitoring program of physical-chemical, microbiological, biological, and hydraulic indicators in the ancient canal network of Padova, we highlight the potential trade-offs related to multiple water management objectives in historical river cities. Results show the strong impact of combined sewer overflows on the ecological status of the canals within and downstream the city, where ammonium concentration and Escherichia coli increased by 1-2 orders of magnitude during several rainfall events. Macroinvertebrate monitoring showed a clearly reduced ecological status downstream of combined sewer outlets, with a shift towards families more resistant to organic pollution. Multivariate modelling identified dissolved oxygen scarcity as a key environmental factor driving these biotic shifts. High-resolution water level and dissolved oxygen measurements showed that river flow diversion out of the city for flood control exacerbates chemical and ecological quality impairment, particularly due to wet-weather discharges, with hypoxic-anoxic conditions following floodgate operation. As such, the current management conflicts with ecological, touristic, and socio-cultural needs, while our results suggest that it should prioritize middle-high flow whenever possible to limit urban emission impacts. By demonstrating how an integrated monitoring approach can unveil the trade-offs linked to multiple management goals in complex cities, we call for more holistic approaches to urban water resources management beyond flood control, accounting for water quality and multifunctionality.