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. Editor's summary: River organisms are the most important influence on the transport of carbon and other nutrients from land to sea. These ecosystems have been considered predominantly net heterotrophic, but Maavara et al. reveal that the rivers of the continental US also display a broad range from extreme heterotrophy to autotrophy. Increases in riverine gross primary productivity are particularly sensitive to drought conditions, and more than 70% of river metabolism occurs in the western US. —Jesse Smith [ABSTRACT FROM AUTHOR]