Factors controlling the community structure of picoplankton in contrasting marine environments

The effect of inorganic nutrients on planktonic assemblages has traditionally relied on concentrations rather than estimates of nutrient supply. We combined a novel dataset of hydrographic properties, turbulent mixing, nutrient concentration, and picoplankton community composition with the aims of (i) quantifying the role of temperature, light, and nitrate fluxes as factors controlling the distribution of autotrophic and heterotrophic picoplankton subgroups, as determined by flow cytometry, and (ii) describing the ecological niches of the various components of the picoplankton community. Data were collected at 97 stations in the Atlantic Ocean, including tropical and subtropical open-ocean waters, the northwestern Mediterranean Sea, and the Galician coastal upwelling system of the northwest Iberian Peninsula. A generalized additive model (GAM) approach was used to predict depth-integrated biomass of each picoplankton subgroup based on three niche predictors: sea surface temperature, averaged daily surface irradiance, and the transport of nitrate into the euphotic zone, through both diffusion and advection. In addition, niche overlap among different picoplankton subgroups was computed using nonparametric kernel density functions. Temperature and nitrate supply were more relevant than light in predicting the biomass of most picoplankton subgroups, except for Prochlorococcus and low-nucleic-acid (LNA) prokaryotes, for which irradiance also played a significant role. Nitrate supply was the only factor that allowed the distinction among the ecological niches of all autotrophic and heterotrophic picoplankton subgroups. Prochlorococcus and LNA prokaryotes were more abundant in warmer waters ( \u3e 20°C) where the nitrate fluxes were low, whereas Synechococcus and high-nucleic-acid (HNA) prokaryotes prevailed mainly in cooler environments characterized by intermediate or high levels of nitrate supply. Finally, the niche of picoeukaryotes was defined by low temperatures and high nitrate supply. These results support the key role of nitrate supply, as it not only promotes the growth of large phytoplankton, but it also controls the structure of marine picoplankton communities

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

Fixación de nitróxeno e fluxo difusivo no NO da Península Ibérica

Molecular nitrogen (N2) is the most abundant form of nitrogen, however only a limited number of organisms can use this reservoir through a process named nitrogen fixation. Therefore, nitrogen is the main limiting nutrient in both marine and terrestrial ecosystems. Nitrogen fixation was initially considered a minor source in the ocean, mainly attributed to organisms of the genus Trichodesmium that inhabit regions where surface temperature is above 20°C, and the stability of the water column is high. The discovery of other groups of marine diazotrophs has evidenced that the range of environments where nitrogen fixation may be relevant is more extensive than it was originally thought. Recent studies demonstrate the activity of nitrogen-fixing organisms in relatively rich nitrogen regions, as for example the Canadian Arctic, the English Channel, the Mekong River plume in the China Sea, the equatorial Atlantic, and the NE coast of the United States. In addition, a study carried out in Cabo Silleiro in summer 2009 described nitrogen fixation rates similar in magnitude to the ones reported for subtropical regions. In this project we propose to determine the seasonal variability in nitrogen fixation rates, and also its biogeochemical relevance as a source for new nitrogen into the shelf of the Galician upwelling system. We propose a multidisciplinary approach that combines experiments of nitrogen fixation rates and microstructure turbulence observations. Although, nowadays biological nitrogen fixation probably represents a minor input of nitrogen into the system, its quantification is crucial to understand the functioning of the global nitrogen cycle. At the local scale, under an ocean threatened by global change, this study will provide a basis to determine changes in the relative importance of the different mechanisms controlling the input of new nitrogen into the system, a process that determines the productivity of fisheries in this region.Aunque el nitrógeno molecular es la forma más abundante de nitrógeno, sólo un reducido número de organismos es capaz de utilizar este reservorio mediante el proceso de fijación de nitrógeno. Constituye el principal nutriente limitante de la productividad de los ecosistemas marinos y terrestres. La fijación de nitrógeno se consideró inicialmente una fuente de entrada menor en el océano, atribuida principalmente a organismos del género Trichodesmium, que habitan regiones tropicales y subtropicales. El descubrimiento de otros grupos de diazótrofos marinos evidenció que el rango de ambientes en los cuales la fijación de nitrógeno puede ser relevante es más amplio de lo que se pensaba. Trabajos recientes evidencian la actividad de organismos fijadores de nitrógeno en regiones relativamente ricas en nitrógeno, como son el Ártico Canadiense, el canal de la Mancha, el Atlántico ecuatorial, o la costa NE de Estados Unidos. Además, un estudio realizado en Cabo Silleiro en verano de 2009 describió tasas de fijación de nitrógeno de magnitud similar a la región subtropical. En esta propuesta planteamos determinar la variabilidad estacional en las tasas de fijación de nitrógeno, así como su relevancia biogeoquímica como fuente de entrada de nitrógeno nuevo en la plataforma del sistema de afloramiento gallego. Para ello proponemos una aproximación con un elevado grado de multidisciplinaridad que combina experimentos de tasas de fijación de nitrógeno y observaciones de turbulencia de microestructura. A pesar de que, en la actualidad, la fijación biológica de nitrógeno probablemente representa una entrada menor de nitrógeno en este sistema, su cuantificación es crucial para entender el funcionamiento del ciclo del nitrógeno a escala global. Este estudio servirá de base para determinar cambios en la importancia relativa de los diferentes mecanismos que controlan la entrada de nitrógeno nuevo en el ecosistema, proceso que determina la productividad de esta región.Aínda que o nitróxeno molecular é a forma máis abundante de nitróxeno, só un número reducido de organismos é capaz de utilizar este reservorio mediante o proceso da fixación de nitróxeno. Constitúe, por tanto, o principal nutriente limitante da productividade dos ecosistemas tanto mariños coma terrestres. A fixación de nitróxeno considerouse inicialmente unha fonte de entrada menor no océano, atribuída principalmente a organismos do xénero Trichodesmium, que habitan rexións con temperaturas superficiais >20ºC e condicións de elevada estabilidade da columna de auga. O descubrimento doutros grupos de diazótrofos mariños evidencia que os ambientes nos que a fixación de nitróxeno pode ser relevante é máis amplio do que se pensou nun principio. Traballos recentes evidencian a actividade de organismos fixadores de nitróxeno en rexións relativamente ricas en nitróxeno, como o Ártico Canadiense, o Canal da Mancha, o Atlántico ecuatorial, ou a costa NE dos EEUU. Ademais, un estudo realizado en Cabo Silleiro no verán de 2009 describiu tasas de fixación de nitróxeno de magnitude similar á rexión subtropical. Nesta proposta propoñemos determinar a variabilidade estacional nas tasas de fixación de nitróxeno, así como a súa relevancia bioxeoquímica coma fonte de entrada de nitróxeno novo na plataforma do sistema de afloramento galego. Para iso propoñemos unha aproximación cun elevado grao de multidisciplinaridade que combina experimentos de tasas de fixación de nitróxeno e observacións de turbulencia de microestrutura. A pesar de que, na actualidade, a fixación biolóxica de nitróxeno probablemente representa unha entrada menor de nitróxeno neste sistema, a súa cuantificación é crucial para entender o funcionamento do ciclo do nitróxeno a escala global. Este estudo servirá de base para determinar cambios na importancia relativa dos diferentes mecanismos que controlan a entrada de nitróxeno novo no ecosistema, proceso que en última instancia determina a produtividade desta rexión.Xunta de Galicia | Ref. EM2013/021Instituto Español de Oceanografía | Ref. Proyecto RADIALESMinisterio de Educación, Cultura y Deporte de España | Ref. FPU13/01674Ministerio de Educación, Cultura y Deporte de España | Ref. EST16/0014