Speaker: Will Fairchild, ORISE Researcher at US Environmental Protection Agency (EPA)

Topic: Evaluating the suitability of common autonomous monitoring sensors for characterizing ocean acidification impacts in a biophysically dynamic estuarine setting

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 have large implications for the ability of monitoring programs to derive the full suite of marine carbonate system variables with acceptable accuracy. Additionally, there remain unresolved questions surrounding best instrument pairings to meet “weather” and “climate” goals of OA monitoring in coastal settings. 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 Sea-Bird SeapHOx; and a Sunburst SAMI-CO₂. We calculated carbonate system uncertainties for all possible pairings of sensor observations and alkalinity derived from an alkalinity-salinity relationship specific to our study area (Alk_sal = 56.1*Salinity+410, R²=0.98, n=197, SE = 54.5).  Monitoring data from the SAMI-CO₂ and SeapHOx were capable of meeting aragonite saturation state (Ω_a) “weather” goals when paired with Alk_sal, but no data product was capable of achieving “climate” goals for pH_T, partial pressure of CO₂ (pCO₂), or Ω_a.  Our analysis demonstrated how data quality objectives for meeting “weather” and ‘climate” goals were less stringent than commonly reported for ocean waters due to the different underlying carbonate chemistry of estuarine waters.  Observed pH_T from SeaFET and YSI sensors were in better agreement than commonly assumed (0.079 ± 0.073 units), but YSI pH_T uncertainties introduced the largest errors in carbonate system calculations.  Water quality and organismal threshold exceedance uncertainty was reduced by 78% when using SeaFET pH_T as an input when compared with YSI pH_T.  We suggest that for OA monitoring groups with limited funding, a high-quality pH_T or pCO₂ sensor paired with a local alkalinity-salinity relationship may be suitable for estuarine OA research.