Remote sensing of temporal and spatial patterns of suspended particle size in the Irish Sea in relation to the Kolmogorov microscale

Suspended sediments form an integral part of shelf sea systems, determining light penetration for primary production through turbidity and dispersion of pollutants by adsorption and settling of particles. The settling speed of suspended particles depends on their size and on turbulence. Here a method of determining particle size via remote sensing measurements of ocean colour and brightness has been applied to a set of monthly satellite images of the Irish Sea covering a full year (2006). The suspended sediment concentration was calculated from the ratio between green (555 nm) and red (665 nm) wavelengths in MODIS imagery. Empirical formulae were employed to convert suspended sediment concentrations and irradiance reflectance in the red part of the spectrum into specific scattering by mineral particles and floc size. A geographical pattern was evident in all images with shallow areas with fast currents having high year-average suspended sediment concentrations (7.6 mg l−1), high specific scattering (0.225 m2 g−1) and thus small particle sizes (143 μm). The reverse is true for deeper areas with slower currents, e.g. the Gyre southwest of the Isle of Man where turbidity levels are lower (3.3 mg l−1), specific scattering is lower (0.081 m2 g−1) and thus particle sizes are larger (595 μm) on average over a year. Temporal signals are also seen over the year in these parameters with minimum seasonal amplitudes (a factor 3.5) in the Turbidity Maximum and maximum seasonal amplitudes twice as large (a factor 7) in the Gyre. In the Gyre heating overcomes mixing in summer and stratification occurs allowing suspended sediments to settle out and flocs to grow large. The size of aggregated flocs is theoretically proportional to the Kolmogorov scale. This scale was calculated using depth, current, and wind speed data and compared to the size of flocculated particles. The proportionality changes through the year, indicating the influence of biological processes in summer in promoting larger flocs.