Chlorophyll measured by autonomous pinniped oceanographers: Calibration and validation of in situ data collected by northern elephant seals
Abstract
Increased sampling of the ocean is imperative in today’s rapidly changing climate. In situ chlorophyll fluorescence data collected by northern elephant seals (Mirounga angustirostris) instrumented with oceanographic tags in the northeastern Pacific offer a supplement to other autonomous oceanographic samplers. I carried out a series of cross calibrations to evaluate the quality of these chlorophyll data. I calibrated the fluorometers in Conductivity-Temperature-Depth-Fluorescence tags (CTDF tags, Sea Mammal Research Unit) in the laboratory using extractions of mixed algal cultures representative of the North Pacific and further validated these calibrations in a controlled field setting. CTDF tags were deployed on five adult female northern elephant seals in 2014 at Año Nuevo State Park in California, USA for 2 to 8 month long offshore foraging trips reaching 3,116 to 4,476 km offshore. These deployments yielded 1394 temperature, salinity, and chlorophyll fluorescence casts of at least 180 m depth. The instrumented elephant seals documented subsurface chlorophyll maxima below the first optical depth in 80.7% of casts. Evidence of fluorescence quenching during periods of high irradiance was inconsistent and did not introduce a bias to our results. I compared the in situ chlorophyll data to satellite derived values. Overlapping satellite chlorophyll data were available for 5.9 - 23.5% of the in situ seal-collected data points using matchup criteria ranging from 1 to 8 days and 5 to 10 km. In situ chlorophyll fluorescence readings were higher than overlapping satellite ocean color chlorophyll data by a factor of 1.53 to 4.96. In light of these cross-calibration results, I strongly recommend system specific calibration procedures for fluorometers sampling from autonomous platforms and urge consideration of errors in the sole use of satellite-derived chlorophyll data for ground-truthing This in situ chlorophyll dataset measures an Essential Ocean Variable (EOV) at low cost and can be a valuable resource to the broader scientific community.