Controls of riverine CO₂ over an annual cycle determined using direct, high temporal resolution pCO₂ measurements

Autonomous CO₂ sensors were deployed in the Clark Fork River, Montana, USA, to characterize the partial pressure of CO₂ (pCO ₂) during an annual cycle. A total of 23,941 measurements were made spanning the period 2002-2006. 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 pCO₂ varied from a winter minimum of ∼100 atm to a fall maximum of ∼900 atm. The pCO ₂ changed by as much as 460 atm 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 pCO₂ amplitude was primarily controlled by the net ecosystem production (NEP) throughout the year, although heating/cooling and air-water exchange significantly altered the diel pCO₂ (and pH) magnitude. Although infrequent, rain events contributed ∼21% to the cumulative short-term changes in inorganic carbon through CO₂-enriched runoff. The seasonal cycle was controlled by temperature, NEP, and discharge. The Clark Fork River maintained pCO₂ levels that were supersaturated with respect to the atmosphere for the majority of the year. River-to-atmosphere CO₂ gas exchange was estimated to be between 4.7 and 7.1 mol C m^-2 yr ^-1. The loss of CO₂ 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 pCO₂ through cooling.