Deriving in situ phytoplankton absorption for bio-optical productivity models in turbid waters

As part of Hyperspectral Coupled Ocean Dynamics Experiment, a high‐resolution hydrographic and bio‐optical data set was collected from two cabled profilers at the Long‐Term Ecosystem Observatory (LEO). Upwelling‐ and downwelling‐favorable winds and a buoyant plume from the Hudson River induced large changes in hydrographic and optical structure of the water column. An absorption inversion model estimated the relative abundance of phytoplankton, colored dissolved organic matter (CDOM) and detritus, as well as the spectral exponential slopes of CDOM and detritus from in situ WET Labs nine‐wavelength absorption/attenuation meter (ac‐9) absorption data. Derived optical weights were proportional to the parameter concentrations and allowed for their absorptions to be calculated. Spectrally weighted phytoplankton absorption was estimated using modeled spectral irradiances and the phytoplankton absorption spectra inverted from an ac‐9. Derived mean spectral absorption of phytoplankton was used in a bio‐optical model estimating photosynthetic rates. Measured radiocarbon uptake productivity rates extrapolated with water mass analysis and the bio‐optical modeled results agreed within 20%. This approach is impacted by variability in the maximum quantum yield (ϕmax) and the irradiance light‐saturation parameter (Ek(PAR)). An analysis of available data shows that ϕmax variability is relatively constrained in temperate waters. The variability of Ek(PAR) is greater in temperate waters, but based on a sensitivity analysis, has an overall smaller impact on water‐column‐integrated productivity rates because of the exponential decay of light. This inversion approach illustrates the utility of bio‐optical models in turbid coastal waters given the measurements of the bulk inherent optical properties.