Influencia de la luz y los nutrientes en la distribución vertical de grupos de fitoplancton marino en el máximo profundo de clorofila

Ecological traits of phytoplankton are being incorporated into models to better understand the dynamics of marine ecosystems and to predict their response to global change. We have compared the distribution of major phytoplankton groups in two different systems: in surface waters of the NW Mediterranean during key ecological periods, and in the DCM (deep chlorophyll maximum) formed in summer in the temperate NE Atlantic. This comparison disentangled the influence of light and nutrients on the relative position of diatoms, dinoflagellates, prymnesiophytes, pelagophytes, chlorophytes, Synechococcus and Prochlorococcus in these environments. Three clusters formed according to their affinity for nutrients: diatoms, chlorophytes and dinoflagellates as the most eutrophic groups; Synechococcus, pelagophytes and prymnesiophytes as mesotrophic groups; and Prochlorococcus as an oligotrophic group. In terms of irradiance, the phytoplankton groups did not cluster clearly. Comparing the nutrient and light preferences of the groups with their distribution in the DCM, dinoflagellates and chlorophytes appear as the most stressed, i.e. their position was most distant from their optimal light and nutrient conditions. Diatoms stayed in deeper than optimal irradiance layers, probably to meet their high nutrient requirements. On the opposite side, low nutrient requirements allowed Prochlorococcus to remain in the uppermost part of the DCM layer. The slight sub-optimal position of Synechococcus and prymnesiophytes with regard to their nutrient requirements suggests that their need for high irradiance plays a significant role in their location within the DCM. Finally, pelagophytes remained in deep layers without an apparent need for the high nutrient concentrations at those depths.Las características ecológicas del fitoplancton se están incorporando en modelos con el fin de comprender mejor la dinámica de los ecosistemas marinos y para predecir su respuesta al cambio global. En este trabajo, hemos comparado la distribución de los principales grupos del fitoplancton en dos sistemas diferentes: en las aguas superficiales del Mediterráneo noroccidental durante períodos ecológicos clave, y en el Máximo Profundo de Clorofila (MPC) que se forma en verano en el Atlántico NE templado. Esta comparación permitió diferenciar la influencia de la luz y los nutrientes en la posición relativa de diatomeas, dinoflagelados, primnesiofitas, pelagofitas, clorofitas, Synechococcus y Prochlorococcus en estos ambientes. Se pudieron diferenciar tres agrupaciones de acuerdo con su afinidad por los nutrientes: diatomeas, clorofitas y dinoflagelados como los grupos más eutróficos; Synechococcus, pelagofitas y primnesiofitas como grupos mesotróficos; y Prochlorococcus como el grupo más oligotrófico. En términos de irradiancia los grupos de fitoplancton no se agruparon de una manera clara. La comparación de las preferencias por nutrientes y luz con su distribución en el MPC permite distinguir que dinoflagelados y clorofitas aparecen como los más estresados en su posición en el MPC, es decir, su posición era la más distante de sus condiciones óptimas de irradiancia y nutrientes. Las diatomeas permanecieron por debajo de su irradiancia óptima probablemente para satisfacer sus altos requisitos de nutrientes, que se encuentran en las capas más profundas. En el lado opuesto, los bajos requerimientos de nutrientes de Prochlorococcus les permitieron permanecer en la parte más superior de la capa del MPC. La ligera posición subóptima de Synechococcus y primnesiofitas con respecto a sus requerimientos de nutrientes sugiere que su necesidad de condiciones relativamente altas de irradiancia juega un papel significativo en su ubicación dentro del MPC. Por último, las pelagofitas permanecieron en capas profundas sin que aparentemente necesitaran las altas concentraciones de nutrientes que se encuentran en esas profundidades

Novel interactions between phytoplankton and bacteria shape microbial seasonal dynamics in coastal ocean waters

Trophic interactions between marine phytoplankton and heterotrophic bacteria are at the base of the biogeochemical carbon cycling in the ocean. However, the specific interactions taking place between phytoplankton and bacterial taxa remain largely unexplored, particularly out of phytoplankton blooming events. Here, we applied network analysis to a 3.5-year time-series dataset to assess the specific associations between different phytoplankton and bacterial taxa along the seasonal scale, distinguishing between free-living and particle-attached bacteria. Using a newly developed network post-analysis technique we removed bacteria-phytoplankton correlations that were primarily driven by environmental parameters, to detect potential biotic interactions. Our results indicate that phytoplankton dynamics may be a strong driver of the inter-annual variability in bacterial community composition. We found the highest abundance of specific bacteria-phytoplankton associations in the particle-attached fraction, indicating a tighter bacteria-phytoplankton association than in the free-living fraction. In the particle-associated fraction we unveiled novel potential associations such as the one between Planctomycetes taxa and the diatom Leptocylindrus spp. Consistent correlations were also found between free-living bacterial taxa and different diatoms, including novel associations such as those between SAR11 with Naviculales diatom order, and between Actinobacteria and Cylindrotheca spp. We also confirmed previously known associations between Rhodobacteraceae and Thalassiosira spp. Our results expand our view on bacteria-phytoplankton associations, suggesting that taxa-specific interactions may largely impact the seasonal dynamics of heterotrophic bacterial communities

Major role of nutrient supply in the control of picophytoplankton community structure.

abstractThe 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.RADIALES (IEO

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

extended abstract del posterIn 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.RADIALES-20 (IEO), CHAOS (CTM 2012-30680), Malaspina-2010(CSD2008-00077

Climate Influence on Deep Sea Populations

Dynamics of biological processes on the deep-sea floor are traditionally thought to be controlled by vertical sinking of particles from the euphotic zone at a seasonal scale. However, little is known about the influence of lateral particle transport from continental margins to deep-sea ecosystems. To address this question, we report here how the formation of dense shelf waters and their subsequent downslope cascade, a climate induced phenomenon, affects the population of the deep-sea shrimp Aristeus antennatus. We found evidence that strong currents associated with intense cascading events correlates with the disappearance of this species from its fishing grounds, producing a temporary fishery collapse. Despite this initial negative effect, landings increase between 3 and 5 years after these major events, preceded by an increase of juveniles. The transport of particulate organic matter associated with cascading appears to enhance the recruitment of this deep-sea living resource, apparently mitigating the general trend of overexploitation. Because cascade of dense water from continental shelves is a global phenomenon, we anticipate that its influence on deep-sea ecosystems and fisheries worldwide should be larger than previously thought

Growth, grazing and carbon flux of high and low nucleic acid bacteria differ in surface and deep chlorophyll maximum layers in the NW Mediterranean Sea

9 pages, 6 figures, 2 tablesGrowth and grazing mortality of marine heterotrophic bacteria were measured in the summer of 2000 in coastal waters of the NW Mediterranean Sea. Serial-dilution experiments were performed with water from surface and deep chlorophyll maximum (DCM) layers. Bacterial abundances (mean ± SD) were very similar at the surface (7.2 ± 2.9 × 10^5 cells ml–1 and DCM (7.4 ± 1.1 × 10^5 cells ml–1). Intrinsic bacterial growth rates (mean ± SD) were 0.88 ± 0.43 d–1 in the surface layer and 0.71 ± 0.23 d–1 at the DCM. Grazing rates on bacteria (mean ± SD) were 0.75 ± 0.23 and 0.58 ± 0.29 d–1, in the surface and DCM layers, respectively. Nucleic acid content analysis by flow cytometry revealed different intrinsic growth rates and grazing pressure on bacteria of high (HNA) and low (LNA) content depending on their location in the water column. Generally, growth and grazing rates were balanced in both groups in both layers. At the surface, HNA bacteria revealed significantly higher intrinsic growth rates than LNA bacteria (1.18 ± 0.60 and 0.47 ± 0.28 d–1, respectively). Average growth rates at the DCM were higher for LNA (0.90 ± 0.46 d–1) than for HNA (0.36 ± 0.23 d–1), but not significantly. At the surface, grazing rates on HNA bacteria were also significantly higher than on LNA bacteria (1.02 ± 0.31 and 0.37 ± 0.19 d–1, respectively). At the DCM, the opposing tendency, though not statistically significant, was observed (0.26 ± 0.17 and 0.77 ± 0.50 d–1). Bacteria were responsible for a large portion of the C flux through the system. Bacterial C flux was funneled mostly by HNA bacteria at the surface (70%) and by LNA at the DCM (80%). HNA bacteria were the most active component of the bacterial community in the surface layer. However, we found that LNA bacteria were also active, particularly at the DCM, suggesting differences in structure and functioning of the corresponding microbial networks. The most significant result was the clear relation between depth and activity of each bacterial fraction.Field work was supported by research projects ARO2000 (MAR99-1202) and DILEX (MAR98-0855), and writing by EFLUBIO (REN2002-04151-C02-01), all funded by the Spanish Ministry of Education and Science. We thank the captain and crew of the RV 'García del Cid'.Peer reviewe