Combined processing and mutual interpretation of radiometry and fluorimetry from autonomous profiling Bio-Argo floats: Chlorophyll a retrieval

Abstract

Eight autonomous profiling floats equipped with miniaturized radiometers and fluorimeters have collected data in Pacific, Atlantic, and Mediterranean offshore zones. They measured in particular 0–400 m vertical profiles of the downward irradiance at three wavelengths (412, 490, and 555 nm) and of the chlorophyll a fluorescence. Such autonomous sensors collect radiometric data regardless of sky conditions and collect essentially uncalibrated fluorescence data. Usual processing and calibration techniques are no longer usable in such remote conditions and have to be adapted. The proposition here is an interwoven processing by which missing parts of irradiance profiles (due to intermittent cloud occurrence) are interpolated by accounting for possible changes in optical properties (detected by the fluorescence signal) and by which the attenuation coefficient for downward irradiance, used as proxy for [Chl a] (the chlorophyll a concentration), allows the fluorescence signal to be calibrated in absolute units (mg m-3).This method is successfully applied to about 600 irradiance and fluorescence profiles. Validation of the results in terms of [Chl a] is made by matchup with satellite (MODIS-A) chlorophyll (24.3% RMSE, N = 358). Validation of the method is obtained by applying it on similar field data acquired from ships, which, in addition to irradiance and fluorescence profiles, include the [Chl a] HPLC determination, used for final verification.