Controls of riverine CO₂ over an annual cycle determined using direct, high temporal resolution <i>p</i>CO₂ measurements

Abstract

Autonomous CO 2 sensors were deployed in the Clark Fork River, Montana, USA, to characterize the partial pressure of CO 2 ( p CO 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 p CO 2 varied from a winter minimum of ∼100 μ atm to a fall maximum of ∼900 μ atm. The p CO 2 changed by as much as 460 μ 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 p CO 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 p CO 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 p CO 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 p CO 2 through cooling.