Derivation of the specific optical properties of suspended mineral particles and their contribution to the attenuation of solar irradiance in offshore waters by ocean color remote sensing

Two independently derived algorithms which had previously been validated against in situ data were applied to 8 years of MODIS observations of the Irish Sea to obtain (i) concentrations of lithogenic mineral particles (MSSl) in surface waters and (ii) the specific backscattering and absorption coefficients for these particles in the 488 nm waveband (the values obtained were a*_MSSl488=0.031 m² g^-1 and b_b*_MSSl488=0.010 m² g^-1). This information was used to calculate the mean attenuation coefficient for downward irradiance K-d(488) in the surface mixed layer, and the fraction of this coefficient that was attributable to suspended mineral particles. Mineral particles at relatively low concentrations (<5 g m^-3) were the major determinant of K-d(488) values throughout the region in winter, and in the central Irish Sea this influence persisted for much of the spring/autumn primary production period. In the north and south, however, marked short-term increases in K-d(488) due to phytoplankton blooms occurred during periods when MSSl values were relatively low. Seasonally averaged maps of the fractional contribution of MSSl to K-d(488) show strong links to vertical mixing, with sharp contrasts developing in spring at the boundaries between mixed and stratified waters. We conclude that the ocean color processing sequence presented here can reveal spatial and seasonal patterns in the dynamics of lithogenic mineral particles which have potentially valuable applications in ecosystem status assessment, environmental impact monitoring, and the tuning and validation of numerical models of shelf sea ecosystems.