Nutrient supply does play a role on the structure of marine picophytoplankton communities

Conference communicationThe Margalef´s mandala (1978) is a simplified bottom-up control model that explains how mixing and nutrient concentration determine the composition of marine phytoplankton communities. Due to the difficulties of measuring turbulence in the field, previous attempts to verify this model have applied different proxies for nutrient supply, and very often used interchangeably the terms mixing and stratification. Moreover, because the mandala was conceived before the discovery of smaller phytoplankton groups (picoplankton <2 µm), it describes only the succession of vegetative phases of microplankton. In order to test the applicability of the classical mandala to picoplankton groups, we used a multidisciplinary approach including specifically designed field observations supported by remote sensing, database analyses, and modeling and laboratory chemostat experiments. Simultaneous estimates of nitrate diffusive fluxes, derived from microturbulence observations, and picoplankton abundance collected in more than 200 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 sand picocyanobacteria as a function of changing nutrient supply. Our results indicate that nutrient supply controls the distribution of picoplankton functional groups in the ocean, further supporting the model proposed by Margalef.Spanish Governmen

Biological N2 fixation in the upwelling region off NW Iberia: magnitude, relevance and main players

Conference communicationThe classical paradigm about marine N2 fixation establishes that this process is mainly constrained to oligotrophic tropical and subtropical regions, and attributed mostly to the filamentous cyanobacterium Trichodesmium. However, the development of molecular techniques led to the discovery of a larger variety of marine diazotrophs, which extends the range of environments where N2 fixation may be relevant. Between February 2014 and December 2015 we carried out 10 cruises in the upwelling ecosystem off NW Iberia in order to: 1) quantify the magnitude of N2 fixation, 2) investigate its biogeochemical role as a mechanism of new nitrogen supply, and 3) identify and quantify the main diazotrophs in the region under contrasting hydrographic regimes. Our results indicate that the magnitude of N2 fixation in this region is comparable to the lower-end of rates described for subtropical regions. The comparison with nitrate diffusive fluxes reveals the minor role of N2 fixation (<5%), as a mechanism of new nitrogen supply into the productive euphotic layer. All the N2 fixation activity was detected in the smaller-sized (<10μm) fraction. Results obtained through phylogenetic analyses by Illumina® (NGS technology) show that the diazotrophic community was dominated by UCYN-A (the most abundant sublineage UCYN-A2), bacteria from Class γ-Proteobacteria and from Phylum Firmicutes. However, the diazotrophic community composition exhibited an important seasonal variability, being the highest diversity associated with autumn and winter mixing and downwelling conditions. Overall, our findings support the emerging view that mesotrophic regions should be considered in global budgets of marine N2 fixation.Xunta de Galicia (EM2013/021

ROLE OF INTERNAL WAVES ON MIXING, NUTRIENT SUPPLY AND PHYTOPLANKTON COMPOSITION DURING SPRING AND NEAP TIDES IN THE RÍA DE VIGO (NW IBERIAN PENINSULA)

The effect of internal wave activity on nutrient supply to the euphotic layer and its potential impact on phytoplankton composition and size-structure remain unexplored in the shelf edge off NW Iberian Peninsula. In August 2013, continuous measurements (yo-yo) of dissipation rates of turbulent kinetic energy, obtained with a microstructure profiler, were carried out during spring and neap tides, covering a complete tidal cycle (25 hours). Every hour the yo-yo was interrupted to collect samples for inorganic nutrient concentration. Microturbulence and nutrient concentration data were combined to compute the nutrient input into the euphotic zone driven by vertical diffusivity. Size-fractionated chlorophyll-a and pigment composition samples were taken for the determination of phytoplankton community structure. As a result of increased mixing levels, nitrate transport to the euphotic layer during spring tides was approximately 4-fold higher (34.6 mmol m-2 d-1) than during neap tides (9.4 mmol m-2 d-1). However, the phytoplankton community was always dominated by large (>20 µm) chain-forming diatoms and seemed to be mainly controlled by the upwelling conditions predominating in late August