Suspended sediment characteristics, primarily size and concentration, were
measured, in conjunction with temperature, salinity and turbulent dissipation at two sites: one in the northern North Sea (59° 20' N, 1°E; 14/10/98 - 09/11/98) and one in the Clyde Sea (55° 21' N, 5° 2' W; 21 /05/00 - 22/05/00). An in-situ laser particle sizer (a LISST-100B) and a laboratory based time-of-transition particle sizer (a Galai) were used to obtain suspended particle size distributions. These instruments were evaluated against microscope and Coulter Counter measurements using spherical particles as well as directly comparing field results.
Both instruments performed well when measuring standardised particles in the
laboratory and distributions obtained from the marine environment if the coarse
part of the distribution was ignored. It was concluded that for mixed composition populations the Galai was likely to discard a proportion of the population if the optical properties varied greatly from the majority. For the LISST, it was concluded that the returned size distribution was representative of the population unless particles greater than 250 µm were present, when the coarse part of the distribution was "contaminated" by these particles.
At both study sites an apparent minimum in mass concentration (measured by a
transmissometer) and a maximum in mean grain size occurred within the
thermocline. The transmissometer results are found to be representative of the
concentration of the fine portion of the population rather than the whole population. Using a simple aggregation model it is shown that the mass concentration minimum measured in the North Sea was not real, however in the Clyde Sea it is, although exaggerated. It is concluded that enhanced aggregation in the thermocline causes increased settling velocities, compared to the source particles, resulting a negative mass flux. These results are the first direct evidence that turbulence controls the equilibrium floe diameter in the marine environment.