Satellite remote sensing of the geographical distribution of suspended particle size in an energetic shelf sea

Abstract

A method is described for estimating near surface suspended particle size over whole shelf regions using visible band satellite data. The technique can be applied to the mainly mineral, flocculated particles commonly found in tidally mixed shelf seas and estuaries. It is based on estimating light scattering per unit concentration ( b ∗) from simultaneous measurements of water colour (expressed as a reflectance ratio) and brightness (expressed as the absolute value of reflectance at a specific wavelength), both of which can be measured from space. A test of the method, using in situ data, produces predictions of b ∗ which are in good agreement ( R 2 = 0.79) with direct measurements. An empirical relationship has been established between b ∗ and median particle size by volume, D V, using in situ measurements of particle size measured with a laser diffraction (LISST) instrument. This, together with the algorithm for b ∗ has been applied to two SeaWiFs images of the Irish Sea, one in winter, the other towards the end of summer. The maps show a decrease in particle size in regions of most intense tidal energy, in support of theories concerning the maintenance of turbidity maxima in the absence of a source of particles at these locations. The satellite observations of particle size are used to test the hypothesis that the maximum size is controlled by turbulence through the Kolmogorov microscale. A positive, statistically significant, correlation is found between median particle size and the turbulent microscale in both summer and winter. However, for a given turbulence level, particles are larger in summer than in winter, providing evidence for the importance of biological binding. These results mean that local knowledge of turbulence can be used to improve our estimation of suspended sediment load, underwater light attenuation and hence primary productivity from visible band satellite images of shelf seas.