A novel, low-cost instrument capable of measuring surface water PCO₂ was designed for use in dynamic, shallow-water environments. The instrument was tested in the Yaquina River Estuary, a macrotidal estuary known to experience a wide range of conditions ranging from dominance by the coastal ocean during summer upwelling to substantial freshwater discharge events resulting from winter storms. This instrument depends on gas equilibration by the diffusion of CO₂ molecules through a microporous, hydrophobic membrane between the aqueous environment and an enclosed gaseous volume, and subsequent quantification of the concentration of CO₂ molecules in the equilibrated air via Non-Dispersive Infra-Red (NDIR) absorbance technology. The field-testing occurred between January and December 2013, collecting over 200 discrete samples and 30 hours of in situ data. The data collected by this instrument was compared to discrete samples analyzed in the laboratory and found to have an absolute average deviation, or imprecision, of 7%. Preliminary area-weighted average air-sea CO₂ flux estimations for the Yaquina River Estuary (3 mol C m⁻² y⁻¹) show the same order of magnitude as other estuarine studies where comparable PCO₂ measurement techniques were used, but significantly lower than studies where PCO₂ was not directly measured. The discrete sampling program executed in combination with the instrument development process allowed a closer look at the seasonality of this ecosystem. This study discusses the evidence of both physical and biogeochemical processes occurring in the study area.
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