Control of the structure of marine picoplankton communities by turbulence and nutrient supply dynamics

Resumo

Oligotrophic regions are characterized by a shortage of nutrients in surface waters, with nitrogen being the main limiting nutrient in most tropical and subtropical regions of the open ocean, as well as in temperate and polar seas during periods of seasonal stratification. Since some of the biological production in the photic layer is exported to the deep ocean (export), the maintenance of biological production will depend on the input of nutrients into the system. Mechanisms contributing to new production include biological nitrogen fixation, atmospheric deposition, and diffusive and advective vertical and horizontal transport of organic and inorganic forms of nitrogen. Calculation of vertical diffusive transport requires estimation of diffusivity (Kz). The methodological difficulties in obtaining Kz estimates led to the use of constant Kz values, and empirical parameterizations of vertical diffusivity. However, the commercialization of microstructure turbulence profilers has facilitated the obtaining of microstructure turbulence observations, which revealed an important Kz variability in the upper layer. Alternatively, the concentration of dissolved inorganic nutrients in the photic layer has been used as an estimator of nutrient availability to planktonic communities. However, under steady-state conditions, such as subtropical gyres, where nutrient supply by diffusion into the euphotic zone is slow, there may be no relationship between nutrient concentration in the photic layer and nutrient supply. The picoplankton refers to the fraction of plankton smaller than 2 µm and consists of Synechococcus, picoeukaryotes, Prochlorococcus and heterotrophic bacteria. The latter can be divided into bacteria with high (HNA) or low (LNA) nucleic acid content. Photosynthetic picoplankton generally dominate biomass and primary production in tropical and subtropical oligotrophic regions, while their contribution is less in nutrient-rich coastal regions. In marine ecosystems, a major source of environmental heterogeneity lies in the temporal fluctuation of nutrient supplies, which controls the diversity of the phytoplankton community. Under steady state conditions, the minimum level of resources that can sustain a population determines competition. Experimental studies and numerical models of competition support this theoretical basis for large phytoplankton. While numerous studies have investigated the effect of nutrient supply dynamics on interspecific competition of large phytoplankton species, their effect on the groups that make up phytoplankton has received much less attention. The main hypothesis of this thesis is that the dynamics of nutrient supply controls the composition of marine picoplankton communities. To achieve this goal, a multidisciplinary approach will be used, combining field observations made during 17 oceanographic campaigns in the Atlantic, Pacific and Indian tropical and subtropical oceans, the Northwest Mediterranean Sea, the Galician coastal upwelling ecosystem and the Antarctic Peninsula with laboratory experiments and ecological modeling of competitive interactions.