Fluorescence contribution to reflectance spectra for a variety of coastal waters

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Improved remote sensing retrievals of the chlorophyll fluorescence component in coastal water reflectance can significantly help environmental impact assessments. While retrieval of chlorophyll fluorescence from satellite observations of open ocean reflectance using Fluorescence Line Height (FLH) algorithms is now routine, it is much more complicated in coastal waters where the fluorescence overlaps with a NIR elastic scattering peak arising from the combination of photosynthetic pigment and particulate scattering and absorption, and rapidly increasing water absorption. To examine retrieval accuracies attainable in coastal waters by MODIS and other FLH algorithms, we compared the results of extensive numerical simulations with those of our field measurements in the Chesapeake Bay. The relationship between the contribution of fluorescence in the reflectance spectra and [Chl] and other water constituents was analyzed by simulations of more than 1000 reflectances using the HYDROLIGHT radiative transfer program. For these, IOP were related to parameterized microphysical models, following the same procedures used to generate the IOCCG dataset, but with higher (1 nm) spectral resolution, and wider range of parameters including chlorophyll specific absorption more typical of coastal waters. Results of simulations and field measurements show that the variability of retrieved fluorescence can be attributed largely to its attenuation in the water by algae, CDOM and mineral particles, and much less to the variation of the fluorescence quantum yield. Our systematic parametric study of fluorescence as a function of the other water components is then used to define the range of water parameters where fluorescence contributes significantly to the NIR peak reflectance, and where it is almost undetectable.