Seasonal and tidal cycles of suspended particulates in the Irish Sea
In spite of the widely perceived importance of suspended particulate material (SPM) , its distribution in the shelf seas and the processes controlling its variation are little known. This thesis reports an exploratory study of the spatial and time dependant variability of SPM in an area of the northern Irish Sea. SPM was determined both directly by gravimetric methods and via measurements of beam attenuation (c). Spatial distributions were determined from grid surveys using a profiling transmissometer. In addition a six month record of beam attenuation and current velocity was obtained from a site off the north coast of Anglesey. A clear spatial pattern in the surface distribution of c was observed which was similar to the distribution of h/ta, suggesting that concentrations of SPM are determined by the availability of TKE from tidal stirring. A strong seasonal cycle of c was observed in mixed water, with values decreasing in June, July and August which suggested a reduction in the supply of SPM during summer. In stratified water, high concentrations of SPM remained confined to the dense layer below the thermocline. The seasonal cycle was observed in the time series from the mooring, but in addition there was a marked response to tidal currents in the spring and in autumn. Close analysis of the record in April and May-showed signals at M% and M4frequencies. These variations in c were attributed to a local response to tidal currents causing erosion of the sea-bed and to the oscillation of a horizontal gradient in c in the region. A regression model was found to explain 35% of the variance in data from a5 week time series. 70% of the variance was explained for four day time series, near spring tides. The relationship between c and tidal flows was more marked at spring tides than at neaps.