River metabolism in the contiguous United States: A West of extremes.

River metabolism is among the most uncertain fluxes in the global carbon cycle. We present estimates for gross primary productivity (GPP) and ecosystem respiration (ER) for more than 175,000 rivers across the contiguous United States (CONUS), including metabolic responses to extreme hydrological conditions. Our model predicts an annual GPP in CONUS rivers of 10.1 teragrams of carbon per year and an ER of 18.7 teragrams of carbon per year, which implies that net ecosystem productivity (NEP; where NEP = GPP - ER) is a small contributor to river carbon dioxide emissions. More than 70% of river metabolism occurs in the West, where regions of both extreme heterotrophy and autotrophy exist. Autotrophy is prominent across the West and is sensitive to drought, particularly in understudied biomes such as arid desert shrublands, which may indicate that global riverine uptake of carbon dioxide is higher than hypothesized.