Productivity Studies on the Madison River, Yellowstone National Park1

Net photosynthesis and respiration in the Madison River, Yellowstone National Park, was calculated using pH measurements to estimate CO 2 changes. The upstream‐downstream method was used on five successive reaches. Primary production and macrophyte standing crops decreased downstream and the ratio of gross to net photosynthesis also became less. The downstream decline in productivity was correlated with successively lower concentrations of free CO 2 . The decline in free CO 2 was due to photosynthetic removal during daylight and outward diffusion of CO 2 during all hours of the day and night.