Narrowband shortwave minima in spectra of backscattered light from the sea obtained from ocean color scanners as a remote indication of algal blooms

SummaryWe propose a new approach to indication of algal blooms. It stems from analysis of the multispectral satellite reflectance Rrs of areas where blooms were documented during recent decades. We found that spectra of algal blooms exhibit minima at wavelengths of channels of Moderate Resolution Imaging Spectroradiometer (MODIS) λ = 443 and λ = 488 nm (Baltic, Black, and Caspian seas), λ = 443 nm (Southwest Tropical Pacific (SWTP)), and λ = 443 nm and λ = 469 nm (Patagonian Continental Shelf (PCS)), attributable to absorption bands of chlorophyll a and accessory pigments. We quantified the minima using indices D1 = Rrs(443) − Rrs(412) and D2 = Rrs(488) − Rrs(469) and proved their diagnostic potential by comparing their distributions to that of Rrs(555). Linear dependence of D1 upon chlorophyll a was found from MODIS data for the bloom of Nodularia spumigena. Time dependences of D1 and D2 point to the latter as a probable remote forerunner of cyanobacterial blooms. In the PCS, D1 and D2 proved to be too simplistic owing to diversity of spectral shapes at λ < 550 nm. Cluster analysis revealed close linkage of the latter and local oceanological conditions. Our findings bear witness to the diagnostic potential of the indices by virtue of their direct relation to pigment absorption and because the broadband background reflectance changes reduce when calculating the indices as a difference of spectrally close reflectances. Further studies are needed to convert the indices to band-difference algorithms for retrieving the bio-optical characteristics of algal blooms.