Influence of bio-optical parameter variability on the reflectance peak position in the red band of algal bloom waters

•Relationships of peak position in red bands vs. Chl are specific for algal species.•Chlorophyll fluorescence inhibits the shift of the red peak to longer wavelengths.•Increasing aph* in the red bands causes a sharper shift in the red peak position.•We can use Rrs(708nm)/Rrs(665nm) instead of Chl for the practical discrimination.

On the basis of field measurements, the quantitatively different relationships of peak position in the red band of the remote sensing reflectance vs. Chl concentration are found in the bloom waters of the diatom Skeletonema costatum and the dinoflagellate Prorocentrum donghaiense in coastal areas of the East China Sea. Model simulations of remote sensing reflectance, Rrs, accounting for the influence of variations in the bio-optical parameters such as chlorophyll fluorescence quantum efficiency, Φ, and specific absorption coefficient, aph⁎, are carried out to analyze the characteristics of this spectral peak. The strong effect of fluorescence on the magnitude of Rrs results in the inhibition of the shift of the peak to longer wavelengths, increasing Φ enhances this effect. Increasing aph⁎, specifically in the red-wavelength band, causes a sharper shift in the red peak position by decreasing the effect of the fluorescence. The dominant parameter governing the slope of the shift is aph⁎. The analysis indicates that the higher aph⁎ of S. costatum in the red region is primarily responsible for the much higher slope of the peak shift than for that of P. donghaiense. We show that the relationship between the peak position and Chl concentration may be useful for discriminating S. costatum blooms from those due to P. donghaiense, although information about chlorophyll fluorescence quantum efficiency should be included. Finally, we show that using the band ratio Rrs(708 nm)/Rrs(665 nm) instead of Chl in the relationship with peak position can be useful for the practical identification of S. costatum blooms from hyperspectral measurements of remote sensing reflectance.