The distribution of fine suspended sediments in the surface waters of the Irish Sea and its relation to tidal stirring
A time series of 165 Advanced Very High Resolution Radiometer (AVHRR) visible band images covering the years 1982-1988 shows that the reflectance of the Irish Sea pulses on seasonal and spring-neap tidal time scales. The seasonal signal has been analysed by fitting a harmonic curve to a geographical grid of points. Maximum reflectance throughout the whole Irish Sea occurs in January or early February. The annual mean reflectance varies spatially by a factor of three from 0.8% to more than 2.4%. Seasonal amplitudes vary from 0.2% to 0.9%. A smaller, but still significant, cycle of reflectance occurs with the fortnightly springs-neaps cycle, with maximum reflectance occurring at spring tides. Both these cycles are in phase with the known variation of suspended sediments in the Irish Sea. Direct measurements, in situ and in the laboratory, of the inherent optical properties of sediments in the Irish Sea have been used to construct an algorithm relating AVHRR visible band reflectance RA to sediment concentration. The reflectance is most sensitive to changes in mineral suspended solids (mss) with phytoplankton pigments having a secondary effect and yellow substance hardly any at all. The algorithm is a saturating curve: initially RA increases in proportion to mss, but at high mss concentrations RA tends to an asymptotic value of 3.5%. The algorithm has been used to produce mean winter and summer maps of mss in the Irish Sea. Concentrations in winter are greater than those in summer (by a factor of 2.7 for the Irish Sea as a whole), but the spatial pattern is similar. Highest sediment concentrations occur in the shallow eastern Irish Sea and also in the regions of strongest tidal currents. The relationship between suspended sediment concentration and tidal stirring is explored using a simple energy model. Equating the potential energy of the sediment in suspension to the turbulent kinetic energy available from the tide suggests a linear relationship between the concentration of mss and the tidal power density. It is found that there is a significant relationship between these quantities in the water depth range 40 to 80m.