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 matter (SPM) have been investigated at a site in the southern North Sea characterised by a seasonally stratified water column and a muddy sand bed. Significant variability in SPM concentration, composition, size and settling velocity was observed over tidal, spring/neap and seasonal time scales. Tidal currents alone are not sufficient to resuspend the bulk of the sea-bed sediment. However, sea-bed erosion does occur when wind/wave action enhances the bed shear stress during winter storms. Also, during winter and summer aggregated organic-rich material with a modal settling velocity of 0.2 mm s-1, originating from the sediment/water interface or from pelagic organic detritus, is subject to spring-tide or wind/wave resuspension. Phytoplankton production in the surface mixed layer (SML) is nutrient limited during much of the summer, but there can be sufficient light penetration to allow production at or below the thermocline. So production in the bottom mixed layer (BML) can be either light or nutrient limited, and BML nutrient levels do not gradually recover during the summer. Aggregation of phytoplankton is an important process at this site, resulting in significant modification of settling behaviour and hence dynamics of SPM. Settling velocity and particle size measurements indicate that both diatom and dinoflagellate blooms form large aggregates towards the end of their growth phase, with modal settling velocities between 2 and 5 mm s-1. Nitszchia pseudoseriata aggregates were relatively stable, remaining as viable sites for primary production over several cycles of tidal resuspension and deposition. Moreover, they efficiently ‘scavenged’ fine inorganic particles from the water column as they settled out. Tidal resuspension of these aggregates results in much higher water-column residence times than would be calculated simply from settling rate and water depth. This allows a greater proportion of remineralisation to be performed in the water column, rather than within the sea bed.