Long‐term decline in carbon dioxide supersaturation in rivers across the contiguous United States

The partial pressure CO 2 ( p CO 2 ) in streams and rivers reflects both internal carbon dynamics and external biogeochemical processes in terrestrial ecosystems. CO 2 is typically supersaturated in streams because rates of organic matter decomposition exceed photosynthetic uptake, and because of inflow of CO 2 supersaturated ground waters. Using the U.S. Geological Survey's NASQAN database, we calculated p CO 2 and dissolved inorganic carbon (DIC) concentration in 417 streams and rivers distributed across the contiguous United States over a 22‐year period. In addition to demonstrating widespread supersaturation and distinct regional variation in p CO 2 across the U.S., our analysis revealed that p CO 2 significantly declined 78.4 ppmv y −1 from 1973 to 1994. This decline is not attributable to changes in terrestrial weathering or in‐stream processes, but instead suggests large‐scale declines in terrestrial CO 2 production and import into aquatic ecosystems during this 22‐year period.