Urban hot-tubs: Local urbanization has profound effects on average and extreme temperatures in ponds

While urbanization-driven warming (urban heat island effect, UHI) has been extensively studied and demonstrated for air temperature, UHI effects on water temperature of ponds are unknown. We investigated (1) whether the UHI impacts man-made urban ponds and tested whether urban ponds have higher mean, maximum and minimum water temperatures and lower daily water temperature fluctuations than rural ponds, (2) whether this is related to time of the day (day versus night), season, and urbanization scale (3200 versus 50 m radius around the pond), and (3) whether the approximated length of growing season is prolonged in urban ponds. Temperature loggers were placed in 30 ponds in Northern Belgium, spanning a broad range of urbanization. We found strong evidence of urban-driven warming. Mainly local urbanization (50 m radius) drove temperature differences throughout the year and even more so in spring and summer, with mean summer temperatures being up to 3.04 °C higher in urban compared to rural ponds, and maximum summer temperatures on average up to 3.69 °C higher. Strikingly, daily temperature fluctuated around 2 °C more in locally urban ponds compared to rural ponds in summer. Length of the growing season estimates show prolongation with up to 45 days in locally urban compared to rural systems, mainly due to an earlier start. Generally, our results show that UHIs impact water temperature of ponds. This warming can have profound consequences for biota inhabiting these systems, and should therefore be considered in future urban planning to reduce deterioration of these habitats and improve their socio-ecological value.