Effects of the Marangoni stress due to plankton slicks on the plankton population dynamics

Marine phytoplankton are known to produce surface-active materials as part of their metabolism. The surface tension gradient due to the presence of plankton-produced surfactants leads to a surface shear stress, commonly known as Marangoni stress. In this work we try to gain some understanding on these complex phenomena while, at the same time, obtaining some information on the order of magnitude of the surface stress and the resulting velocity in the bulk flow. As the direct modelling of the surfactants is extremely difficult due to the complexity (and uncertainty) of their chemical composition and to the absence of information on the rate of production by marine phytoplankton, we have formulated a model of their effects based on the data of surface activity of seawater samples as they are presented by ?uti? et al. (1981) [?uti?, V., ?osovi?, B., Mar?enko, E., Bihari, N., 1981. Surfactant production by marine phytoplankton. Mar. Chem. 10, 505-520] and ?uti? and Legovic (1990) [?uti?, V., Legovic, T., 1990. Relationship between phytoplankton blooms and dissolved organic matter in the Northern Adriatic. In: Final reports on research projects dealing with eutrophication and plankton blooms (activity h). Mediterranean Action Plan (MAP) Technical Reports Series No. 37. United Nations Environment Program (UNEP), Athens]. Using satellite data and a chlorophyll concentration profile reported in the literature we predict a Marangoni stress whose maximum order of magnitude is 10- 6 N m- 2. We also present the results of a numerical test, where we evaluate the Marangoni stress caused by horizontal variations of the plankton distribution due to the converging flows in a thermal bar, obtaining a Marangoni stress of order of magnitude 10- 5 N m- 2 and a corresponding velocity in the bulk that is at most equal to 10- 4 m s- 1, with values of film pressure within the range of the data available in the literature.