LAUSR

Significance This paper provides a comprehensive analysis of the annual patterns of ecosystem productivity and respiration for more than 200 rivers, comparing the magnitude and phenology of river metabolic regimes with annual estimates from more than 150 terrestrial ecosystems. Although mean annual temperature and mean annual precipitation explain much of the variation in terrestrial productivity and are used to define biomes, for rivers the most important controls are annual light availability and flow stability. Attention to these gradients will substantially improve our ability to scale and model river ecosystem dynamics and may fundamentally change the way rivers are studied. Mean annual temperature and mean annual precipitation drive much of the variation in productivity across Earth's terrestrial ecosystems but do not explain variation in gross primary productivity (GPP) or ecosystem respiration (ER) in flowing waters. We document substantial variation in the magnitude and seasonality of GPP and ER across 222 US rivers. In contrast to their terrestrial counterparts, most river ecosystems respire far more carbon than they fix and have less pronounced and consistent seasonality in their metabolic rates. We find that variation in annual solar energy inputs and stability of flows are the primary drivers of GPP and ER across rivers. A classification schema based on these drivers advances river science and informs management.