Isolated turbidity maxima in shelf seas

Visible-band satellite pictures of the Irish Sea reveal the presence of isolated areas of enhanced turbidity which are geographically fixed and present all year, although they are more strongly marked in winter. The positions of these maxima coincide with areas of fast tidal currents: some of the dissipated tidal energy is used to raise fine sediments into suspension, producing the turbidity. However, there is no obvious source of fine sediment at the maxima, and without a source they would be expected to diffuse away, down the turbidity gradient. Their continued presence is therefore a puzzle, and it has proved difficult to reproduce these features in numerical models. Recent observations, presented here, suggest a possible mechanism by which isolated turbidity maxima may be maintained. These measurements show that the gradients of concentration on the sides of the turbidity maximum are in opposite senses for different particle sizes, and that there is a flux of fine particles out of the maximum and one of larger particles into it. A mechanism which can explain this observation, and which can also explain the continued presence of turbidity maxima isolated from a local source is as follows: the high turbulent energy levels at the centre of the patch tear flocculated particles apart. These fine particles then diffuse away down the gradient of fine particles and to areas of lower energy. Here they aggregate to form larger particles which diffuse back down the gradient of large particles towards the centre of the turbidity maximum, where they are torn up and the cycle continues. The source of material for the turbidity maxima is therefore larger flocculated particles in the surrounding water. This idea is tested quantitatively with an analytical solution to the steady-state diffusion equation incorporating aggregation and dis-aggregation of particles as simple functions of turbulent energy. The solution shows that isolated maxima of fine suspended particles can be maintained at regions of high turbulence without the necessity to invoke a local (non-sustainable) source. Within the maximum, strong vertical mixing lifts the slow settling fine particles to the surface to produce an isolated surface turbidity maximum as observed. We conclude that for models to successfully produce turbidity maxima in the presence of diffusion they should incorporate at least two particle size classes and aggregation and dis-aggregation of particles according to the local level of turbulence.