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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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Impurities in indicator salts can significantly bias spectrophotometric pH determinations. In this work, two purified sulfonephthalein indicators, meta-cresol purple (mCP) and phenol red (PR), were tested for analysis of freshwater pH on the free hydrogen ion concentration scale. These two purified indicators were characterized for the first time under low ionic strength conditions, providing their molar absorption coefficients and dissociation constants along with their temperature dependence from 8 8C to 30 8C. At 25 8C, the infinite dilution constants (pK I o ) were determined to be 8.6606 and 8.0642 for mCP and PR, respectively. The accuracy and precision of the method, evaluated with a variety of buffers with known pH, were found to be 10.0014 pH units and 60.0022 pH units, respectively (n 5 30). The pH values of different freshwater samples were also determined using both indicators. The mCP and PR results were all within 6 0.01 pH units of each other with three out of seven pH differences within 6 0.001 pH units, indicating the high consistency between these two indicator methods. The work presented here is the first parallel comparison with two purified indicators used to determine pH of the same freshwater samples.

Understanding the physical and biogeochemical processes that control CO2 and dissolved oxygen (DO) dynamics in the Arctic Ocean (AO) is crucial for predicting future air‐sea CO2 fluxes and ocean acidification. Past studies have primarily been conducted on the AO continental shelves during low‐ice periods and we lack information on gas dynamics in the deep AO basins where ice typically inhibits contact with the atmosphere. To study these gas dynamics, in situ time‐series data have been collected in the Canada Basin during late summer to autumn of 2012. Partial pressure of CO2 (pCO2), DO concentration, temperature, salinity, and chlorophyll‐a fluorescence (Chl‐a) were measured in the upper ocean in a range of sea ice states by two drifting instrument systems. Although the two systems were on average only 222 km apart, they experienced considerably different ice cover and external forcings during the 40–50 day periods when data were collected. The pCO2 levels at both locations were well below atmospheric saturation whereas DO was almost always slightly supersaturated. Modeling results suggest that air‐sea gas exchange, net community production (NCP), and horizontal gradients were the main sources of pCO2 and DO variability in the sparsely ice‐covered AO. In areas more densely covered by sea ice, horizontal gradients were the dominant source of variability, with no significant NCP in the surface mixed layer. If the AO reaches equilibrium with atmospheric CO2 as ice cover continues to decrease, aragonite saturation will drop from a present mean of 1.00 ± 0.02 to 0.86 ± 0.01.