Effects of Mesoscale Stirring on Phytopigment Determinations in the Photic Water Layer from Multispectral Ocean Color Data (The Case of the Tasman Sea)

Abstract

The inconstancy of phytopigment composition during intensive mesoscale mixing of the Tasman Sea photic layer was investigated using MODIS images of its surface. To do this, each pixel of such an image is assigned a WRM 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 (Spectral Indexing of Pixels, or SIP approach). WRM is acceptable as an indicator of phytopigment composition variability in the water column, since the attenuation of light by water as a solvent and by its admixtures of other nature is inferior to light absorption by phytopigments in spectral selectivity, while the composition of phytopigments in the aquatic environment depends on the species composition of local phytoplankton. A co-analysis of WRM distributions and characteristics of Tasman Sea waters showed that with increased mesoscale variability in open ocean waters, phytopigment content in the near-surface layer reaches levels at which minimums of pigment origin at 400–550 nm, discernible by multispectral ocean color scanners, occur in the backscattered solar radiation spectrum. This phenomenon is ignored by common algorithms for chlorophyll determination based on the data of multispectral ocean color scanners (band-ratio algorithms) and, apparently, is one of the reasons for the known tendency of such algorithms to overestimate chlorophyll concentration relative to its real content in the water column. The conclusion is applicable to any ocean basins if they, like the Tasman Sea, are not affected by external sources of optically significant admixtures in water.