Assessing the potential impacts of freshwater overconsumption beyond regional carrying capacity on riverine fish species richness

Consuming freshwater beyond regional carrying capacity—the maximum volume of water that can be sustainably used by human activities—creates ecological water deficits that pose critical threats to aquatic biodiversity and ecosystem integrity. Quantifying the impacts of such deficits is therefore essential for guiding sustainable water use and assessing environmental impacts along global value chains. These impacts are commonly assessed using Life Cycle Impact Assessment (LCIA) frameworks that develop spatially explicit characterization factors for freshwater ecosystems; however, most existing approaches treat water use as an undifferentiated pressure, without distinguishing sustainable freshwater consumption from overconsumption that leads to ecological water deficits. Here, we assess the global impacts of freshwater overconsumption and associated ecological water deficits on freshwater fish species richness using WaterGAP 2.2e data for approximately 11,000 watersheds worldwide. Regional carrying capacity was quantified as available freshwater resources minus environmental water requirements needed to sustain aquatic ecosystems, with ecological water deficits identified where human consumption exceeded this capacity (i.e., freshwater overconsumption). Biodiversity responses were evaluated using a global model linking freshwater fish species richness to river discharge and other covariates (elevation, basin area, and climate zone). Based on this model, two species–discharge relationships (SDRs) were derived to distinguish watersheds experiencing ecological water deficits from those operating within sustainable limits. These SDRs were then used to develop characterization factors quantifying the impacts of human freshwater consumption on freshwater fish biodiversity. Our analysis revealed higher characterization factors in watersheds affected by ecological water deficits, indicating stronger biodiversity impacts under freshwater overconsumption conditions. Explicitly accounting for ecological water deficits in LCIA water-impact assessment can enhance the ecological relevance and accuracy of global characterization factors for freshwater systems and aquatic ecosystems. This, in turn, can support more targeted freshwater management strategies aligned with biodiversity conservation goals.