Biogeochemical sensor performance in the SOCCOM profiling float array

Abstract

The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program has begun deploying a large array of biogeochemical sensors on profiling floats in the Southern Ocean. As of February 2016, 86 floats have been deployed. Here the focus is on 56 floats with quality‐controlled and adjusted data that have been in the water at least 6 months. The floats carry oxygen, nitrate, pH, chlorophyll fluorescence, and optical backscatter sensors. The raw data generated by these sensors can suffer from inaccurate initial calibrations and from sensor drift over time. Procedures to correct the data are defined. The initial accuracy of the adjusted concentrations is assessed by comparing the corrected data to laboratory measurements made on samples collected by a hydrographic cast with a rosette sampler at the float deployment station. The long‐term accuracy of the corrected data is compared to the GLODAPv2 data set whenever a float made a profile within 20 km of a GLODAPv2 station. Based on these assessments, the fleet average oxygen data are accurate to 1 ± 1%, nitrate to within 0.5 ± 0.5 µmol kg−1, and pH to 0.005 ± 0.007, where the error limit is 1 standard deviation of the fleet data. The bio‐optical measurements of chlorophyll fluorescence and optical backscatter are used to estimate chlorophyll a and particulate organic carbon concentration. The particulate organic carbon concentrations inferred from optical backscatter appear accurate to with 35 mg C m−3 or 20%, whichever is larger. Factors affecting the accuracy of the estimated chlorophyll a concentrations are evaluated.

Hydrography and biogeochemistry dedicated to the Mediterranean BGC-Argo network during a cruise with RV <i>Tethys 2</i> in May 2015

“…However, sensors for biogeochemical properties, even with recent factory calibration, are subject to substantial systematic errors when deployed on BGCArgo floats, as reported by Bittig et al (2012) for oxygen measurements or by for nitrate measurements. As a consequence, even if a BGCArgo float is supposed to be completely autonomous after deployment, reference data for quality assessment of most of its sensors need to be collected by ship Johnson et al, 2017). Automatic quality controls are rapidly advancing for the Argo program , although most of the methods and protocols are still under assessment.…”

Oxygen Optode Sensors: Principle, Characterization, Calibration, and Application in the Ocean

“…This apparent correlation (compare Johnson et al, 2017) is avoided by using Equation 23 including the additional pO 2 -T-slope a, which, in practice, eliminates any gain m i temperature dependence observed after Equation (21) (Figure 9 and Table S3). Inclusion of a significantly reduces the range of apparent drifts from −0.9 to +1.4 % year −1 (after eq.…”

“…Inclusion of a significantly reduces the range of apparent drifts from −0.9 to +1.4 % year −1 (after eq. 21, see Johnson et al, 2017) to a more realistic −0.6 to +0.6 % year −1 (after Equation 23) by decoupling the oxygen slope time series from the temporal evolution in temperature. For this reason, we focus on the results following Equation (23).…”

Perspectives on Chemical Oceanography in the 21st century: Participants of the COME ABOARD Meeting examine aspects of the field in the context of 40 years of DISCO

“…Further, the public availability of these data will likely fuel independent physical, chemical, and biological oceanographic research that is not related to SOCCOM's mission or core scientific team, much like Argo has (Riser et al, 2016). Notably, SOC-COM (and Argo) data quality assurance (Johnson et al, 2017) hinges on high-quality ship observations for sensor calibration such that the inclusion of autonomous sensors augments rather than obviates shipbased oceanography. Nevertheless, our expanding abilities to obtain in situ data in real time are transforming Oceanography.…”