THE RELATIONSHIP BETWEEN CONCENTRATIONS OF SUSPENDED PARTICULATE MATERIAL AND TIDAL PROCESSES IN THE IRISH SEA

The time-dependence of suspended particulate material (SPM) and its control by tidal currents was investigated in the Irish Sea from April to October in 1987. Recording transmissometers and current meters were deployed in mixed water (h=80 m) in an area where strong reflectance in visible band imagery indicated high SPM concentrations. A seasonal signal was observed in the data with a reduction in beam attenuation (c) from late May to September. During spring and autumn a semi-diurnal variation in beam attenuation was interpreted as due to the advection of a west-east gradient in SPM. There was also evidence of local resuspension since c was positively correlated with current speed. Both processes contributed to generally higher values of c being attained during spring tides than at neap tides. There was no evidence of the influence of windstirring on SPM concentrations even when windspeeds exceeded 12.5 m s(-1). A model to simulate the tidal control of SPM concentrations by resuspension and tidal excursion showed good agreement with the data during spring tides. It was less successful for the neap periods when the concentrations of SPM remained low.