Increasing oxygen consumption in global inland waters in the Anthropocene

The concentration of oxygen in aquatic environments influences redox reactions of chemicals, nutrient biogeochemistry, water quality, biological activities, and ecosystem health. While hypoxia and declining oxygen concentrations in marine environments have been widely reported, oxygen in global inland-water systems and its spatiotemporal changes with the changes in climate, hydrology and human activities remain unknown. To unravel the changing global inland-water oxygen cycle and driving mechanisms, here we quantify the global inland-water oxygen production, consumption, exchange with the atmosphere and transport along the aquatic continuum during 1900-2010 using the spatially-explicit, integrated assessment model IMAGE-DGNM including the mechanistic in-stream biogeochemistry module (DISC). The model keeps track of oxygen and nutrient supply from the land, and describes their coupled transformations and transport from upstream through various waterbodies to downstream. During 1900-2010, global inland-water oxygen production and consumption rapidly increased by over a factor of six and three, respectively, while river oxygen export to oceans stayed around 0.4 Pg yr^-1. Despite the increasing ratio of oxygen production to consumption, inland waters overall act as an increasing sink of oxygen in the atmosphere during 1900-2010. Globally, low-order streams contribute the most to the freshwater oxygen sink, followed by lakes and recently important reservoirs, while high-order rivers overall act as an oxygen source to the atmosphere.