RESUSPENSION AND ADVECTION PROCESSES AFFECTING SUSPENDED PARTICULATE MATTER CONCENTRATIONS IN THE CENTRAL ENGLISH-CHANNEL

Suspended particulate matter (SPM) measurements obtained along a cross -section in the central English Channel (Wight-Cotentin transect) indi cate that the area may be differentiated into: (1) an English coastal zone, associated with the highest concentrations; (2) a French coastal zone, with intermediate concentrations; and (3) the offshore waters o f the Channel, characterised by a very low suspended-sediment load. Th e SPM particle-size distribution was modal close to the English coast (main mode 10-12 mu m); the remainder of the area was characterised by flat SPM distributions. Examination of the diatom communities in the SPM suggest:; that material resuspended in the intertidal zone and the estuarine environments was advected towards the offshore waters of th e English Channel. Considerable variations in SPM concentrations occur red during a tidal cycle: maximum concentrations were sometimes up to 3 times higher than the minimum concentrations, Empirical orthogonal f unction (EOF) analysis of the SPM concentration time series indicates that, although the bottom waters were more turbid than the surficial w aters, this was not likely to be the result of in situ sediment resusp ension. Instead, the observed variations appear to be controlled mainl y by advective mechanisms. The limited resuspension was probably cause d by: (1) the limited availability of fine-grained material within the bottom sediments, and (2) 'bed-armouring' processes which protect the finer-grained fractions of the seabed material from erosion and entra inment within the overlying flow during the less energetic stages of t he tide.