Control of tHe structure of marine phytoplAnkton cOmmunities by turbulence and nutrient supply dynamicS (CHAOS)

Abstract

In order to investigate the role of turbulence mixing on structuring marine phytoplankton communities, the CHAOS project included a multidisciplinary approach involving specifically designed field observations supported by remote sensing, database analyses, and modeling and laboratory chemostat experiments. Field observations carried out in the outer part of Ría de Vigo in summer 2013 showed that, as a result of increased mixing levels, nitrate diffusive input into the euphotic layer was approximately 4-fold higher during spring tides. This nitrate supply could contribute to explain the continuous dominance of large-sized phytoplankton during the upwelling favorable season. Simultaneous estimates of nitrate diffusive fluxes, derived from microturbulence observations, and picoplankton abundance collected in more than 100 stations, spanning widely different hydrographic regimes, showed that the contribution of eukaryotes to picoautotrophic biomass increases with nutrient supply, whereas that of picocyanobacteria shows the opposite trend. These findings were supported by laboratory and modeling chemostat experiments that reproduced the competitive dynamics between picoeukaryote and picocyanobacteria as a function of changing nutrient supply. The results derived from this project confirm that turbulence and mixing control the availability of light and nutrients, which in turn determine the structure of marine phytoplankton communities.