Long-Term Stability of a Moored Optical Oxygen Sensor in an Estuary

Oxygen is a key parameter for understanding biological production, deep water mass transport, biogeochemical cycling, and other oceanographic processes. Improvements and assessments of the long-term stability of oxygen sensors contribute to a better understanding of oxygen data quality. For moored platforms, increased sensor stability can also lead to fewer field servicing visits. In this study, the stability of the Sea-Bird SBE 63 optical dissolved oxygen (ODO) sensor is evaluated over a two-year period. A SBE37SMP-CTD-ODO instrument with Sea-Bird’s standard anti-foulant protection was deployed at a high-fouling site in an estuary with the goal of assessing undisturbed sensor drift (without any cleaning or servicing). Periodic field site visits were completed every 1 – 3 months using side-by-side comparisons with a calibrated SBE 43 reference sensor, replicate water sample collection for Winkler titration, and vertical CTD profiles to assess water column structure. After 1.5 years, the moored ODO instrument drifted low of correct by 1.5% compared to Winkler samples and post calibration results. Dry storage drift tests completed at Sea-Bird indicate ODO shelf drift can be on the order of -1.0 to -1.3% per year. Post calibration results after two years agreed well with field test comparisons. Overall, the ODO sensor showed stability within the initial accuracy specification (+/- 2%) and negligible effects from biofouling. Further studies in different environments will continue to provide more information on sensor performance in the field.