The 2008 North Atlantic Spring Bloom Experiment: Radiometric Measurements and Inversions

As part of the 2008 North Atlantic Spring Bloom Experiment (NAB08), hyperspectral radiometric and bio- optical measurements were taken from two Lagrangian Floats every 40-200 s. Measured quantities included downwelling irradiance Ed(λ,z) and upwelling radiance Lu(λ,z) from 320-950 nm as well as oxygen, chlorophyll fluorescence and two inherent optical properties (IOPs), optical backscattering bb(470 nm) and beam attenuation c(662 nm). In addition to Lagrangian drift mode sampling of mixed layers from 5-20 m during more stratified conditions and 20-150 m during pre-bloom conditions and storms, daily high resolution profiles with radiometric and bio-optical samples approx. every 2 m to 250 m were also performed within an hour of solar noon. Calibration profiles of hyperspectral radiometric and bio- optical properties (including attenuation a(λ,z) and c(λ,z) at nine wavelengths) were performed within sight of the floats during a 21-day R/V Knorr cruise. Combined with measurements of oxygen and chlorophyll fluorescence, we have a detailed bio-optical record of both the mixed layer and the entire euphotic zone during the 51 days of the floats' deployment spanning pre-bloom, chain diatom- dominated bloom, and diverse diatom / dinoflagellate / ciliate "post-peak bloom" conditions. We are developing computational inversions of the hyperspectral radiometric measurements, using IOPs as constraints and other bio-optical measurements as checks, to directly diagnose phytoplankton absorption aφ(λ,z) and from this the evolution of the phytoplankton community during the bloom. The hyperspectral resolution allows inquiry into spectral control of the bloom. The high resolution spatio-temporal sampling allows multiple realizations of data that increase the quality of inversions, while the mixed layer excursions allow us to compare inversion of direct radiometric measurements versus inversion of derived apparent optical properties of diffuse attenuation Kd(λ,z) = (d ln Ed(λ,z)/dz) and radiance reflectance RL(λ,z) = Lu(λ,z)/ Ed(λ,z). Our hope is that this technique could provide detailed measurements of phytoplankton community evolution and productivity from autonomous platforms.