Modeling the impact of surface water abstractions and climate on the ecological quality and recreational potential of a stratified lake.

Lakes fulfill essential functions for people and nature, but at the same time are being heavily impacted by human pressures and the climate in this era. In this study we modeled the impact of three surface water abstraction rates in the epilimnion for drinking water purposes in combination with climate change on the ecological quality and the recreational potential of a quarry lake in The Netherlands. Our results show that the highest surface water abstraction rate (1.36 m/s) - related to the estimated abstraction rate for 2040 - would substantially change the ecological quality of the lake. Specifically, this surface water abstraction rate caused a threefold extension of the period of hypoxia in the hypolimnion and a reduction of the phytoplankton summer bloom as compared to the baseline. Simultaneously, the reduction of cyanobacteria in the lake due to the surface water abstraction would improve the lake's suitability for recreation, as the risk of harmful blooms becomes negligible. The two lower surface water abstraction rates showed results in between the no-abstraction and the high abstraction rate results. When considering only the changes in the climate between 2010 and 2050, we found that the lake's ecological quality would decrease, mainly due to prolonged hypoxic conditions. However, the recreational potential remained relatively unchanged due to climate change. Overall, our findings indicate that surface water abstraction, in particular at the highest rate, would have a more profound impact on the lake than the projected climate change. Our study highlights the importance of including local water management strategies, such as surface water abstractions, in future water quality simulations.