Genrik S. Karabashev

The inconstancy of the composition of phytopigments during intensive mesoscale mixing of the photic zone of the Tasman Sea was investigated from MODIS images of its surface. For this, each pixel of the image is assigned a WRM (wavelength of reflectance minimum) index equal to the sum of the wavelengths of the minima in the reflectance spectrum of the water surface within the boundaries of a pixel on the ground. The WRM is acceptable as an indicator of the inconstancy of the composition of phytopigments in the water column, since the spectra of light absorption by phytopigments in an aqueous medium change following the species affiliation of phytoplankton, even though light attenuation by water as a solvent and its admixtures of a different nature are inferior to light absorption by phytopigments in spectral selectivity. A comparative analysis of the WRM index distributions and characteristics of Tasman Sea water showed that the increased mesoscale variability of open ocean waters is accompanied by an increase in phytopigment content to a level where pigment minima appear in the spectra of backscattered solar radiation at 400–550 nm, distinguishable by multispectral ocean color scanners. This effect is not considered by common algorithms for chlorophyll determination using data from multispectral ocean color scanners (band-ratio algorithms) and is probably one of the reasons for the known tendency of such algorithms to overestimate chlorophyll concentration with respect to its actual content even in water areas free of the influence of external sources of optically significant admixtures in the water column.

A method is proposed for visualizing the inconstancy of the phytopigment composition in the water layer of the formation of backscattered solar radiation recorded by the MODIS multispectral ocean color scanner. The method relies on the fact that each pixel of the MODIS raster image of the investigated water area is assigned a wavelength reflectance minimum (WRM) index. The latter is equal to the sum of the wavelengths of minimums in the spectrum of a pixel or index 100 if they are absent. Such minimums are possible only in the presence of phytoplankton as the only carrier of plant pigments in the open sea. Pixel attributes of a MODIS raster image of the surface of water, like pixel coordinates and standard medium definitions, serve as WRM estimates. This makes it possible to visualize the variability of manifestations of the spectral diversity of phytoplankton against the background of the variability of the given medium properties. The effectiveness of the method is tested using the example of MODIS images of a spiral-eddy structure in the Caspian Sea and mesoscale variability of the bio-optical characteristics of waters over the continental slope in the Southwest Atlantic. The spectral indexing of the pixels of MODIS images of the water surface makes it possible to reveal some forms of variability of phytopigments of the aquatic environment that are inaccessible to the standard methods of ocean-color data processing.

The wavelength λ_max of the maximum of spectral reflectance Rrs is examined as a tool for remote indication of water exchange in straits, coastal zones, and other transition areas. Estimates of λ_max were calculated from data of MODIS-Aqua sensor by means of spline interpolation of Rrs at 469, 488, 531, 547, and 555 nm for pixels of images of the strait of Skagerrak and the North Caspian Sea as testing areas of active water exchange. The distributions of Rrs(555) and color index Q = Rrs(555)/Rrs(488), found from the same data, were used to compare the behavior of λ_max with the variability of habitual characteristics. We demonstrate that the new indicator λ_max, being a characteristic of chromaticity of natural waters, is preferable in areas where water exchange is accompanied by redistribution of colored dissolved organic matter of natural origin. The estimates of λ_max may be useful as a remote indicator of salinity deficit in river mouth regions.

A method for estimating the wavelength of the maximum of the reflectance spectrum of a water surface is proposed. It is based on spline interpolation of reflectances at 469, 488, 531, 547, 555 nm that were recorded with the MODIS-Aqua satellite sensor. The approach was tested using reflectance spectra that were measured with a floating spectral radiometer; it was applied to the set of MODIS images of the Black Sea from 2003 to 2011. We found that the wavelength of the reflectance spectrum peak of water surface shifts towards shorter wavelengths from April to September in the open Black Sea. This phenomenon is attributable to seasonal variations of the composition and contents of optically significant admixtures that are associated with the annual cycle of the vital activities of the phytoplankton.

Based on the archived data of the ocean color scanner MODIS-Aqua for 2003–2011, we constructed the long-term monthly mean distributions of the reflectance Rrs of the Black Sea for April, May, June, and September in order to visualize the contributions of seasonal factors to the long-term variability of the basin’s images. In April, the Rim Current’s branch west of 34° E is visualized by higher Rrs in different regions of the visible spectrum due to the transport of suspended matter caused by intensification of the Rim Current during the winter-spring period. During the June coccolithophore “blooms,” the long-term Rrs level is 2–3 times higher when compared to the previous and subsequent months. This excess is particularly considerable outside the shelf and coastal areas. The open sea Rrs distribution in June features horizontal inhomogeneity. The seasonal trend of the Rrs spectra on the Black Sea NW shelf is explicitly related to the annual cycle of the continental runoff effects.

We used the archive data of the ocean color scanners (OCS) SeaWiFS and MODIS to reveal the impact of their design and operational characteristics on the estimates of the sea surface’s reflectance as a spatial variable in an extremely changeable small aquatic area as compared to the OCS’s swath. Accounting for these characteristics is particularly necessary when studying the long-term changes in such areas to avoid the distortions of the trends and distributions due to biased data.

Influence of the wind field on the radiance of a marine shallow: evidence from the Caspian Sea* doi:10.5697/oc.54-4.655 OCEANOLOGIA, 54 (4), 2012. pp. 655–673. ©C Copyright b

The authors propose to treat the structure of the sea surface images from the ocean color scanners (OCS) as a complex integrated indication of the state of the marine environment. Supposedly, such an approach will make current the information on the water environment processes hidden in the spectral-radiance patterns of arbitrary shapes. They are typical of inland seas, straits, and other transition areas but attract no attention due to recognition difficulties. The authors give preliminary estimates of the repeatability of mesoscale structures found from data of the OCSs SeaWiFS and MODIS for the period of 1998–2007 in the oceanologically diverse Caspian Sea regions. The authors’ approach is shown to be useful for studying the long-term variability of marine aquatic areas from the satellite data archives.

Based on the scatterometer QuickScat wind data and the water-leaving radiances from the ocean color scanner SeaWiFS data for 2000–2004, we composed a pair of mean annual images of a testing site in the South Caspian Sea under conditions of west-bound and east-bound winds corresponding to the offset and onset state of the shallows in the east of the testing site. It was established that both the “offset” and “onset” radiances grow shorewards, but the former becomes about twice as large as the latter in the middle of the shallows with 10–15 m of water, while their difference tends to zero at the seaward and coastal boundaries of the shallow. These and other findings are hypothetically attributable to the inclination of the sea floor of the shallows, due to which the surfacing of bottom sediments resuspended by the drift current takes less time under the “offset” wind conditions than under the “onset” ones. The study’s results indicate that the effect of the bottom sediments resuspension upon the structure of the images of the marine shallows is universal in character and needs to be taken into account to improve the bio-optical algorithms for estimating the admixtures in the thickness of the shallows from the satellite data.