Carbon fluxes through major phytoplankton groups under contrasting environmental conditions

Resumen

The distribution of phytoplankton in the ocean remains relatively constant despite its active growth. This is because growth is accompanied by an active grazing that consumes most of the phytoplankton production and channels it towards higher trophic levels. Phytoplankton constitutes the base of the pelagic trophic web in the ocean and its production sets an upper limit for the yield of the ecosystem. Among the different macromolecules (ATP, lipids, pigments, carbon) used as the biomass unit, carbon is the desired currency for productivity models. However, only pigments are specific to phytoplankton and the chosen tool to measure phytoplankton productivity rates. A further advantage of pigment-based approaches is that they allow discrimination among major phytoplankton groups providing further insights into community functioning compared to bulk measurements. The major objective of this thesis was to assess the carbon fluxes of main phytoplankton groups growing under contrasting environmental conditions, aiming to link the phytoplankton community structure with growth and grazing rates and associated carbon fluxes. We have combined pigment and carbon-based approaches in order to quantify carbon production and consumption rates of major phytoplankton groups. Dilution experiments analyzed with High Pressure Liquid Chromatography (HPLC) were used to obtain group-specific phytoplankton growth and grazing rates, while parallel 14C-incubations yielded the carbon incorporation rates. Combination of both techniques provided the C:Chla ratio of phytoplankton allowing the transformation of pigment-based fluxes into carbon equivalents. Since phytoplankton community structure is largely determined by physical conditions, we have surveyed different physico-chemical conditions associated to either temporal or spatial gradients including the contrasting conditions observed between the spring bloom and post-bloom of the open northwestern Mediterranean, the trophic gradient from the productive waters of the Mauritanian coastal upwelling to the unproductive waters of the Atlantic subtropical gyre and the seasonally driven variability in an oligotrophic coastal site. In all cases we have adopted a pigment based approach to describe phytoplankton structure, growth and grazing dynamics and associated carbon fluxes. Different dynamics were observed among major phytoplankton groups: Diatoms, dominant in nutrient rich conditions, tended to grow faster than the rest of the groups and Prymnesiophyceae slowest, while Synechococcus grew fast in the summer oligotrophic period in the coastal site but slow in the oligotrophic subtropical gyre. In addition, we found that the grazing impact on phytoplankton production was not significantly different in nutrient rich and poor environments, supporting the importance of microbial food web even in blooms. Finally, we have critically evaluated the dilution technique applied throughout the thesis. Firstly, we have provided a method to quantify photoacclimation during experimental incubations and to correct estimated rates if necessary. Application of this method to natural samples have revealed the large impact that photoacclimation have on estimated rates and the need to consider it, especially when rates are estimated using a pigment-based approach. Secondly, we have analyzed the influence of the mathematical model assumed in the dilution technique in the correlation between growth and grazing rates frequently reported. We have shown that this correlation is influenced by the error in the measurement of the state variable (i.e. chla), and the range of the estimated growth and grazing rates. Therefore, these factors should be considered before interpreting the ecological relation between growth and grazing obtained in dilution experiments.