Abstract
Abstract. The commercially available Sea-Bird SeaFET™ provides an accessible way for a broad community of researchers to study ocean acidification and obtain robust measurements of seawater pH via the use of an in situ autonomous sensor. There are pitfalls, however, that have been detailed in previous best practices for sensor care, deployment, and data handling. Here, we took advantage of two distinctly different coastal settings to evaluate the Sea-Bird SeaFET™ and examine the multitude of scenarios in which problems may arise confounding the accuracy of measured pH. High-resolution temporal measurements of pH were obtained during 3- to 5-month field deployments in three separate locations (two in south-central Alaska, USA, and one in British Columbia, Canada) spanning a broad range of nearshore temperature and salinity conditions. Both the internal and external electrodes onboard the SeaFET™ were evaluated against robust benchtop measurements for accuracy using the factory calibration, an in situ single-point calibration, or an in situ multi-point calibration. In addition, two sensors deployed in parallel in Kasitsna Bay, Alaska, USA, were compared for inter-sensor variability in order to quantify other factors contributing to the sensor's intrinsic inaccuracies. Based on our results, the multi-point calibration method provided the highest accuracy (< 0.025 difference in pH) of pH when compared against benchtop measurements. Spectral analysis of time series data showed that during spring in Alaskan waters, a range of tidal frequencies dominated pH variability, while seasonal oceanographic conditions were the dominant driver in Canadian waters. Further, it is suggested that spectral analysis performed on initial deployments may be able to act as an a posteriori method to better identify appropriate calibration regimes. Based on this evaluation, we provide a comprehensive assessment of the potential sources of uncertainty associated with accuracy and precision of the SeaFET™ electrodes.
Highlights
The intrusion of excess anthropogenic CO2 into the global oceans – referred to as ocean acidification (OA) – induces a series of geochemical reactions that increases seawater hydrogen ion concentration [H+] while concomitantly reducing the ocean’s overall buffering capacity by reducing the carbonate concentration [CO23−] (Caldeira and Wickett, 2003; Orr et al, 2005)
In the face of rapidly changing coastal conditions, tracking and quantifying the progression of OA requires precise and accurate measurements of carbonate chemistry over long periods of time; these can be achieved by appropriately constraining the carbonate system by measuring at least two of the system’s parameters: total dissolved inorganic carbon (TCO2), total alkalinity (TA), pH, and the partial pressure of CO2
Finalized pHt values from the first test tank deployment produced two different values, of which each was dependent on whether the calibration coefficient from the header file or the disc file was selected, the result was a difference of ∼ 0.0011 units for both the internal and external electrodes
Summary
The intrusion of excess anthropogenic CO2 into the global oceans – referred to as ocean acidification (OA) – induces a series of geochemical reactions that increases seawater hydrogen ion concentration [H+] (lowering pH) while concomitantly reducing the ocean’s overall buffering capacity by reducing the carbonate concentration [CO23−] (Caldeira and Wickett, 2003; Orr et al, 2005). Open-ocean acidification of surface waters is predominately a function of equilibration with atmospheric pCO2, increasing on yearly and decadal timescales as anthropogenic sources of CO2 production continue (Hofmann et al, 2011; Orr et al, 2005). Miller et al.: Considerations for the broader oceanographic community nity metabolism and organization, tidal cycles, upwelling, and groundwater input (Duarte et al, 2013; Sunda and Cai, 2012; Waldbusser and Salisbury, 2014), all of which can act in conjunction with increasing atmospheric CO2, leading to more frequent, intense, and longer-lasting acidification events (Hales et al, 2016; Harris et al, 2013). Despite the marked increase in OA research over the past decade (Riebesell and Gattuso, 2015; Rudd, 2017), nearshore monitoring efforts – in estuarine waters – have been slow to ramp up; efforts are beginning to intensify as technological advancements are made (Feely et al, 2010, 2016; Hales et al, 2016; Harris et al, 2013; Newton et al, 2012; Waldbusser and Salisbury, 2014; Chan et al, 2017)
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