Settling regimes for non-motile particles in stratified waters

Abstract

Simple models have been used to demonstrate that the differing settling dynamics of non-motile particles within the ocean surface mixed layer and the stratified thermocline can generate quite complex vertical profiles of concentration. This is found even when all particles have identical specific settling speeds, and both with and without cell reproduction. The range of profile features is further enriched by particle re-entrainment through mixed layer deepening and particle aggregation. The critical non-dimensional parameters in determining the general form of profiles are G=μh/V and F=(∂h/∂t)/V , where V is the specific settling speed, h is the mixed layer depth and μ is the mean reproduction rate in the surface layer. The role of particle interactions can be also characterised by non-dimensional parameters if simple expressions for aggregation and disaggregation rates are assumed. However, in this case the parameter values change as the particle size distribution evolves with time. Profile features generated by the models include concentration maxima and minima, as well as step profiles. Subsurface maxima can be generated even in the absence of reproduction and are likely to be ubiquitous transient features for particles and cells within the appropriate Peclet number regime. Marine particles that usually fall within this regime include diatom aggregates and most forms of marine snow.