AGGREGATION AND RESUSPENSION OF SUSPENDED PARTICULATE MATTER AT A SEASONALLY STRATIFIED SITE IN THE SOUTHERN NORTH-SEA - PHYSICAL AND BIOLOGICAL-CONTROLS

The physical and biological characteristics of suspended particulate m atter (SPM) have been investigated at a site in the southern North Sea characterised by a seasonally stratified water column and a muddy san d bed. Significant variability in SPM concentration, composition, size and settling velocity was observed over tidal, spring/neap and season al time scales. Tidal currents alone are not sufficient to resuspend t he bulk of the sea-bed sediment. However, sea-bed erosion does occur w hen wind/wave action enhances the bed shear stress during winter storm s. Also, during winter and summer aggregated organic-rich material wit h a modal settling velocity of 0.2 mm s(-1), originating from the sedi ment/water interface or from pelagic organic detritus, is subject to s pring-tide or wind/wave resuspension. Phytoplankton production in the surface mixed layer (SML) is nutrient limited during much of the summe r, but there can be sufficient light penetration to allow production a t or below the thermocline. So production in the bottom mixed layer (B ML) can be either light or nutrient limited, and BML nutrient levels d o not gradually recover during the summer. Aggregation of phytoplankto n is an important process at this site, resulting in significant modif ication of settling behaviour and hence dynamics of SPM. Settling velo city and particle size measurements indicate that both diatom and dino flagellate blooms form large aggregates towards the end of their growt h phase, with modal settling velocities between 2 and 5 mm s(-1). Nits zchia pseudoseriata aggregates were relatively stable, remaining as vi able sites for primary production over several cycles of tidal resuspe nsion and deposition. Moreover, they efficiently 'scavenged' fine inor ganic particles from the water column as they settled out. Tidal resus pension of these aggregates results in much higher water-column reside nce times than would be calculated simply from settling rate and water depth. This allows a greater proportion of remineralisation to be per formed in the water column, rather than within the sea bed. (C) 1998 E lsevier Science Ltd. All rights reserved.