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Spectral indexation of pixels of MODIS sea surface images for detecting inconstancy of phytopigment composition in water

Karabashev Genrik S.

Oceanologia

2021

No. 63 (4)

482--496

This paper presents the first results of a new way of using MODIS (Moderate Resolution Imaging Spectroradiometer) sensor data to visualize phytopigment inconstancy in the near-surface layer of water basins. Other sensors of this class alike, the MODIS spectral resolution is too low to reproduce the minimums of reflectance Rrs caused by phytopigments in water. However, MODIS is remarkable for the presence of a channel at 469 nm combined with channels at 412, 443, 488, 531, 547, and 555 nm. This makes it possible to distinguish the spectral limits of preferential light absorption by chlorophyll a (412-469 nm) and by accessory pigments (469-555 nm). These capabilities were realized thanks to spectral pixel indexation (SPI) of MODIS images of the sea surface. The SPI boils down to the fact that a user determines the presence of pigment minima in spectra of every image pixel, finds the sum of the wavelengths of these minima as a WRM code and assigns it to the image pixel as one of its attributes. WRM code = 100 is assigned to pixels free of the minima. Such indexation makes it possible to examine the inconstancy of phytopigments on the background of aquatic environment variability. Application of SPI approach to MODIS images of the Gulf of Mexico and the Baltic Sea made it possible to reveal new patterns of phytopigment dynamics during HABs events.

Thin chlorophyll layer concomitant of the thermohaline intrusion in the confluence of the Gulf Stream and Labrador Current (a case study)

Karabashev Genrik S.

Oceanologia

2019

No. 61 (4)

527--533

Based on the depth-cycling data of a fluorometer probe in the northwestern Atlantic ocean, this paper considers features of the chlorophyll fluorescence layer against a background of concurrent variability of seawater temperature and fluorescence of CDOM (Colored Dissolved Organic Matter). The vertical distributions of chlorophyll fluorescence complied with the criteria of a thin chlorophyll layer at the site where changes of temperature and CDOM fluorescence were indicative of the intrusion of Labrador waters into adjacent warmer waters below the upper mixed layer. As may be supposed, the thin chlorophyll layer was due to the gyrotactic trapping of phytoplankton cells induced by water shear between the upper mixed layer and the intrusion.