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[1] Autonomous CO 2 sensors were deployed in the Clark Fork River, Montana, USA, to characterize the partial pressure of CO 2 ( pCO 2 ) during an annual cycle. A total of 23,941 measurements were made spanning the period [2002][2003][2004][2005][2006]. These data were compiled into a composite data set covering ∼309 days, giving an unprecedented yearlong view of the carbon cycle dynamics of a riverine system. Seasonal pCO 2 varied from a winter minimum of ∼100 matm to a fall maximum of ∼900 matm. The pCO 2 changed by as much as 460 matm during a diel period, much larger than the range of the seasonal mean, in contrast to most other aquatic ecosystems where seasonal variability dominates. The diel pCO 2 amplitude was primarily controlled by the net ecosystem production (NEP) throughout the year, although heating/cooling and air-water exchange significantly altered the diel pCO 2 (and pH) magnitude. Although infrequent, rain events contributed ∼21% to the cumulative short-term changes in inorganic carbon through CO 2 -enriched runoff. The seasonal cycle was controlled by temperature, NEP, and discharge. The Clark Fork River maintained pCO 2 levels that were supersaturated with respect to the atmosphere for the majority of the year. River-to-atmosphere CO 2 gas exchange was estimated to be between 4.7 and 7.1 mol C m −2 yr −1. The loss of CO 2 to the atmosphere arises from net heterotrophy that averaged 13.8 mmol m −2 d −1 . The time series also captured important episodic events including macrophyte sloughing that led to a pulse of respiration that represented 7% of the annual CO 2 gas efflux and cloudy periods that occurred every 7-18 days that dramatically decreased the pCO 2 through cooling.Citation: Lynch, J. K., C. M. Beatty, M. P. Seidel, L. J. Jungst, and M. D. DeGrandpre (2010), Controls of riverine CO 2 over an annual cycle determined using direct, high temporal resolution pCO 2 measurements,

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