Evaluating the suitability of common autonomous monitoring sensors for characterizing ocean acidification impacts in a biophysically dynamic estuarine setting
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Current ocean acidification (OA) monitoring within coastal and estuarine environments presents a variety of logistical and technological hurdles. Understanding the intricacies of sensing technology and assessing measurement uncertainty has large implications for calculating the full suite of marine carbonate system variables. Additionally, there remain unresolved questions surrounding best instrument pairings to meet “weather” and “climate” goals of OA monitoring. In this study, we examined the performance of three commonly used biogeochemical sensors in a dynamic temperate Oregon estuary for coastal acidification monitoring: a YSI-EXO sonde; a Seabird SeapHOx; and a Sunburst SAMI-CO2. We assessed measurement uncertainty using a local alkalinity-salinity relationship (y=56.1x+410, R2=0.98, n=197) and analyzed all instrument pairings. We determined a SeapHOx-pHT uncertainty of 0.036 based on evaluation of CO2 in Seawater CRMs; YSI and SAMI-CO2 measurement uncertainties were not assessed and instead the manufacturer-recommended values was used. No instrument data product was capable of achieving “climate” goals for pH, partial pressure of CO2 (pCO2), or aragonite saturation state (Ωa) when paired with the local alkalinity-salinity curve. However, carbonate calculations based on data from the SAMI-CO2 and SeapHOx were capable of meeting Ωa “weather” goals. YSI-pHT uncertainties introduced the largest errors in carbonate system calculations. We suggest that for OA monitoring groups with limited funding, a SAMI-CO2 or SeapHOx paired with a local alkalinity-salinity relationship may be most suitable for estuarine OA research.