Suspended particulate matter fluxes through the Straits of Dover, English Channel: observations and modelling

Suspended Particulate Matter (SPM) concentrations at various levels within the water column, together with salinity and temperature, were measured usi ng water samples collected from six stations across the Straits of Dover. T he sampling programme covered a 16-month period, undertaken during 23 cruis es. On the basis of the spatial variability in the concentrations, the wate r bodies are divided by several boundaries, controlled by tidal and wind co nditions. Within the water column, SPM concentrations were higher near the sea bed than in the surface waters. Throughout the cross-section, maximum c oncentrations occurred adjacent to the coastlines. Temporal variability in the SPM concentration exists on daily and seasonal scales within the coasta l waters (4.2 to 74.5 mg L-1): resuspension processes, in response to semi- diurnal tidal cycles (with a period of around 12.4 h) and spring-neap cycle s (with a period of 15 days) make significant contributions. Distinctive se asonal/annual concentration changes have also been observed. In the offshor e waters, such variability is much less significant (0.9 to 6.0 mg L-1). In the summer the English Coastal Zone is associated with relatively high SPM concentrations: the Central Zone has a low and stable SPM concentration be tween these zones, there is a Transitional Zone, where there is a rapid res ponse of SPM concentration to wind forcing. Finally, the French Coastal Zon e is characterized by variable (sometimes high) SPM concentrations. Because of the zonation, SPM fluxes within the Dover Strait are controlled by diff erent transport mechanisms. Within the Central Zone, the flux can be repres ented by the product of mean water discharges and SPM concentrations. Howev er, within the coastal zones fluctuations in SPM concentrations on various time-scales must be considered. In order to calculate the maximum and minim um SPM fluxes, 10 cells were divided in the strait. A simple modelling calc ulation has been proposed for this complex area. The effect of spring-neap tidal cycles and seasonal changes can contribute significantly to the overa ll flux, which is of the order of 20 x 10(6) t.yr(-1) (through the Dover St rait, towards the North Sea). Such an estimate is higher than most obtained previously. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.