Narrowband shortwave minima of multispectral reflectance as indication of algal blooms associated with the mesoscale variability in the Brazil-Malvinas Confluence

Summary: We examine the narrowband shortwave minima (NSM) of multispectral reflectance as indication of mesoscale algal blooms. They are frequent in the Brazil-Malvinas confluence zone (BMCZ) where our testing site (TS) belongs. Its MODIS A images of December 2008 and 2014 were the source of initial data. Classification of reflectance spectra in these images revealed that the TS look from space was determined by the most populated cluster of pixels having the only NSM at 443 nm. We divided this cluster into sub-clusters by maximum wavelengths λmax from 412 to 555 nm and retrieved the estimates of λmax (proxy for abundance of colored dissolved organic matter (CDOM)), chl_a (MODIS chlorophyll), Rrs (555) (turbidity proxy), and CALH (NSM-based chlorophyll) on a pixel-by-pixel basis. This allowed us to demonstrate: (1) the NSM magnitude at 443 nm peaked in mesoscale structures, (2) CALH was consistent with chlorophyll in the BMCZ waters samples, (3) positive linear correlation of Rrs (555) and CALH was characteristic of the TS waters at any λmax, (4) the MODIS chl_a was overestimated when λmax > 488 nm, (5) localization and outlines of mesoscale structures agreed well in the fields of pairs Rrs(555) – CALH and λmax – chl_a, but not in the CALH – chl_a pair. The NSM-based chlorophyll CALH outperformed the standard chl_a determinations in exactness because the CALH is insensitive to CDOM. This is advantageous when studying the Case 1 waters of intensive mesoscale variability where chlorophyll co-exists with the CDOM from eddy-induced blooms. Keywords: Algae, Bloom, Pigments, Reflectance spectrum, MODIS, Brazil-Malvinas Confluenc