The Influence of Density Stratification on Particle Settling, Dispersion and Population Growth

Density stratification is common throughout much of our atmosphere, oceans and inland waters. It can have a strong influence on the sinking and flotation of a wide range of particular matter, including organisms whose life cycle incorporates a water or airborne phase. Ecologically significant examples range from the dispersion of pollen, larvae and plankton, to the dispersion and deposition of sediment bound organic solids and heavy metals. While the biological effects of settling have been studied in turbulent environments, the influence of stratification has not previously been quantified. Here we develop a theory to predict the time dependent particle concentrations throughout a fluid characterised by non-uniform stratification. Factors such as dispersion, deposition and growth of particle populations have been incorporated into the model. It is shown that stratification can significantly enhance settling rates and thereby influence the survival and dispersion of colonising populations. The theory also provides simple physical explanations for commonly observed features of particle distributions, such as subsurface maxima and the onset and decay of algal blooms.