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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. 

To assess the distribution, frequency, and global extent of riverine hypoxia, we compiled 118 million paired dissolved oxygen (DO) and water temperature measurements from 125,158 unique locations in rivers in 93 countries and territories across the globe. The dataset also includes site characteristics derived from StreamCat, the National Hydrography and HydroAtlas datasets and proximal land cover derived from MODIS-based IGBP land cover types compiled using Google Earth Engine (GEE). 

Deoxygenation of aquatic ecosystems is a key feature of the Anthropocene. Studies are increasingly reporting low oxygen conditions in rivers and headwater streams even in the absence of high nutrient loads. We examined the frequency of river hypoxia (dissolved oxygen [DO] < 50% saturated in O₂) in the North Carolina Piedmont by examining monitoring records collected since the 1960s, and by collecting high‐resolution measurements of DO saturation along a 20 km segment of New Hope Creek. State records reported nearly 11,000 incidences of hypoxia from a total of ~ 140,000 measurements (7.8% over 55 yr). In contrast, our measurements in New Hope Creek suggest that assessing river hypoxia from point measurements is highly problematic. We propose new approaches for evaluating and comparing river oxygen regimes. In a detailed longitudinal survey of DO in May 2018, 31% of measurements over 20 km were hypoxic. Over a 3‐week period, 11 of our 12 sites throughout this segment experienced hypoxia 5%–96% of the time. Interannual comparisons for several long‐term monitoring sites document significant potential for hypoxia even in well‐aerated reaches during particularly warm, low flow periods. Oxygen regimes within this river vary between near continuous hypoxia to near continuous saturation and call into question the binary distinction between lotic and lentic oxygen dynamics with which we tend to categorize and model freshwater ecosystems.

 

Hypoxia in coastal waters and lakes is widely recognized as a detrimental environmental issue, yet we lack a comparable understanding of hypoxia in rivers. We investigated controls on hypoxia using 118 million paired observations of dissolved oxygen (DO) concentration and water temperature in over 125,000 locations in rivers from 93 countries. We found hypoxia (DO < 2 mg L⁻¹) in 12.6% of all river sites across 53 countries, but no consistent trend in prevalence since 1950. High‐frequency data reveal a 3‐h median duration of hypoxic events which are most likely to initiate at night. River attributes were better predictors of riverine hypoxia occurrence than watershed land cover, topography, and climate characteristics. Hypoxia was more likely to occur in warmer, smaller, and lower‐gradient rivers, particularly those draining urban or wetland land cover. Our findings suggest that riverine hypoxia and the resulting impacts on ecosystems may be more pervasive than previously assumed.