The role of advection and turbulent mixing in the vertical distribution of phytoplankton

The purpose of this study is to analyse the role of the flow field on the horizontal and vertical distributions of different phytoplankton populations thriving in the water column of a shallow coastal ecosystem. Two extreme flow conditions are illustrated. The first was a low energetic flow, under calm meteorological conditions and a stratified temperature of the water column. The second flow, coincident with the passage of a storm front, was more energetic resulting in increased mixing that homogenized the temperature in the whole water column. Although the mixing level homogenized the temperature of the water column in the high-energy period, it was not enough to homogenize the temperature in the low-energy period. In contrast, in both periods, the mixing level was enough to homogenize the vertical distribution of particles. A decrease in the concentration of particles from the calm period to the high-energy period was attributed to an advection event with warmer water of lower plankton concentration that resulted in a decrease of the total concentration of suspended particles in the water column. Data are used to test a model of plankton mixing proposed by Ruiz et al. (J. Plankton Res., 18 (1996) 1727).