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

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 lambda(max) from 412 to 555 nm and retrieved the estimates of lambda(max) (proxy for abundance of colored dissolved organic matter (CDOM)), chl_a (MODIS chlorophyll), R-rs (555) (turbidity proxy), and C-ALH (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) C-ALH was consistent with chlorophyll in the BMCZ waters samples, (3) positive linear correlation of R-rs (555) and C-ALH was characteristic of the TS waters at any lambda(max), (4) the MODIS chl_a was overestimated when lambda(max) > 488 nm, (5) localization and outlines of mesoscale structures agreed well in the fields of pairs R-rs(555) - C-ALH and lambda(max) - chl_a, but not in the C-ALH - chl_a pair. The NSM-based chlorophyll C-ALH outperformed the standard chl_a determinations in exactness because the C-ALH 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. (C) 2018 Institute of Oceanology of the Polish Academy of Sciences. Production and hosting by Elsevier Sp. z o.o.