Abstract
Abstract. High-frequency continuous measurements of the partial pressure of CO2 (pCO2) are crucial for constraining the spatiotemporal dynamics of CO2 emissions from inland water systems. However, direct measurements of pCO2 are scarce, and no systematic comparisons have been conducted on the suitability of the widely used measurement systems for continuous underway or long-term deployment in various field conditions. We compared spray- and marble-type equilibrators and a membrane-enclosed CO2 sensor to assess their suitability for continuous long-term or underway pCO2 measurements in an urbanized river system in Korea. Both equilibrators had a shorter response time compared with the membrane-enclosed sensor, and could capture large spatial variations of pCO2 during a transect study along a highly urbanized river reach. The membrane-enclosed sensor based on passive equilibration provided comparable underway measurements along the river sections where pCO2 varied within the sensor detection range. When deployed in a eutrophic river site, the membrane-enclosed sensor was able to detect large diel variations in pCO2. However, biofouling on the membrane could reduce the accuracy of the measurement during long deployments exceeding several days. The overall results suggest that the fast response of the equilibrator systems facilitates capturing large spatial variations in pCO2 during short underway measurements. However, the attendant technical challenges of these systems, such as clogging and desiccant maintenance, have to be addressed carefully to enable their long-term deployment. The membrane-enclosed sensor would be suitable as an alternative tool for long-term continuous measurements if membrane biofouling could be overcome by appropriate antifouling measures such as copper mesh coverings.
Highlights
Recent synthesis efforts have highlighted the importance of carbon dioxide (CO2) emissions from inland waters in the global carbon cycle (Cole et al, 2007; Battin et al, 2009; Butman and Raymond, 2011; Raymond et al, 2013; Borges et al, 2015)
Compared with the relatively narrow pressure of CO2 (pCO2) ranges in the oceans (∼ 100–700 μatm) (Valsala and Maksyutov, 2010), the pCO2 in inland waters ranges from values < 100 to > 10 000 μatm (Abril et al, 2015)
The excellent agreement found between the compared methods is consistent with the results of other studies that have demonstrated the accuracy of the equilibrators (Abril et al, 2006, 2015; Santos et al, 2012), or the membrane-enclosed sensor (Johnson et al, 2010), these previous comparisons were conducted separately for each equilibration system
Summary
Recent synthesis efforts have highlighted the importance of carbon dioxide (CO2) emissions from inland waters in the global carbon cycle (Cole et al, 2007; Battin et al, 2009; Butman and Raymond, 2011; Raymond et al, 2013; Borges et al, 2015). Various methods have been employed over the years to measure the partial pressure of CO2 (pCO2) in a wide range of aquatic systems (Takahashi, 1961; Keeling et al, 1965; Park et al, 1969; Smethie et al, 1985; Kling et al, 1992). The range of temporal and spatial variations in freshwater pCO2 is much wider than those in the oceans because of the substantial variations in environmental conditions, complex C transformation, and emission processes, as well as anthropogenic disturbances (Cole et al, 2007).
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