THE DISTRIBUTION OF FINE SUSPENDED SEDIMENTS IN THE SURFACE WATERS OFTHE 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 reflec tance of the Irish Sea pulses on seasonal and spring-neap tidal time s cales. The seasonal signal has been analysed by fitting a harmonic cur ve to a geographical grid of points. Maximum reflectance throughout th e 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 t han 2.4%. Seasonal amplitudes vary from 0.2% to 0.9%. A smaller, but s till significant, cycle of reflectance occurs with the fortnightly spr ings-neaps cycle, with maximum reflectance occurring at spring tides. Both these cycles are in phase with the known variation of suspended s ediments in the Irish Sea. Direct measurements, in situ arid in the la boratory, of the inherent optical properties of sediments in the Irish Sea have been used to construct an algorithm relating AVHRR visible b and reflectance R-A to sediment concentration. The reflectance is most sensitive to changes in mineral suspended solids (mss) with phytoplan kton pigments having a secondary effect and yellow substance hardly an y at all. The algorithm is a saturating curve: initially R-A increases in proportion to mss, but at high mss concentrations R-A 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 wint er 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 suspende d sediment concentration and tidal stirring is explored using a simple energy model. Equating the potential energy of the sediment in suspen sion to the turbulent kinetic energy available from the tide suggests a linear relationship between the concentration of mss and the tidal p ower density. It is found that there is a significant relationship bet ween these quantities in the water depth range 40 to 80 m.