Reservoir water quality deterioration due to deforestation emphasizes the indirect effects of global change

Abstract

Deforestation is currently a widespread phenomenon and a growing environmentalconcern in the era of rapid climate change. In temperate regions, it is challenging toquantify the impacts of deforestation on the catchment dynamics and downstreamaquatic ecosystems such as reservoirs and disentangle these from direct climatechange impacts, let alone project future changes to inform management. Here, wetackled this issue by investigating a unique catchment-reservoir system with tworeservoirs in distinct trophic states (meso- and eutrophic), both of which drain into thelargest drinking water reservoir in Germany. Due to the prolonged droughts in 2015-2018, the catchment of the mesotrophic reservoir lost an unprecedented area of forest(exponential increase since 2015 and ca. 17.1% loss in 2020 alone). We coupledcatchment nutrient exports (HYPE) and reservoir ecosystem dynamics (GOTM-WET)models using a process-based modelling approach. The coupled model was validatedwith datasets spanning periods of rapid deforestation, which makes our futureprojections highly robust. Results show that in a short-term time scale (by 2035),increasing nutrient flux from the catchment due to vast deforestation (80% loss) canturn the mesotrophic reservoir into a eutrophic state as its counterpart. Our resultsemphasize the more prominent impacts of deforestation than the direct impact ofclimate warming in impairment of water quality and ecological services to downstreamaquatic ecosystems. Therefore, we propose to evaluate the impact of climate changeon temperate reservoirs by incorporating a time scale-dependent context, highlightingthe indirect impact of deforestation in the short-term scale. In the long-term scale (e.g.to 2100), a guiding hypothesis for future research may be that indirect effects (e.g., asmediated by catchment dynamics) are as important as the direct effects of climatewarming on aquatic ecosystems.