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

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

Influence of the wind field on the radiance of a marine shallow: evidence from the Caspian Sea

The influence of the near-water wind field on the radiance of a marine shallowwas studied on the basis of daily SeaWiFS ocean colour scanner data andQuickScat scatterometer wind data collected from 1999 to 2004 in the southernCaspian Sea, where the deep basin borders a vast shallow west of the shore ofmeridional extent. It was found that radiance distributions, clustered bywind rhumbs, exhibited different long-term mean patterns for winds of opposingdirections: within the shallow's boundaries, the radiances were about twice ashigh for winds having an offshore component with reference to the onshore windconditions. The zonal profile of radiance across the shallow resembleda closed loop whose upper and lower branches corresponded to the offshore and onshore winds respectively. Theloop was the most pronounced at sites with 10-15 m of water for any wavelengthof light, including the red region. On the basis of specific features of the studyarea, we attributed this pattern to sunlight backscattered from bottom sedimentsresuspended by bottom compensation currents induced by the offshorewinds

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=488nm (Baltic, Black, and Caspian seas), λ=443nm (Southwest Tropical Pacific (SWTP)), and λ=443nm and λ=469nm (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<550nm. 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