Springtime zooplankton size structure over the continental shelf of the Bay of BiscaY

Linking lower and higher trophic levels requires special focus on the essential role played by mid-trophic levels, i.e., the zooplankton. One of the most relevant pieces of information regarding zooplankton in terms of flux of energy lies in its size structure. In this study, an extensive data set of size measurements is presented, covering parts of the western European continental shelf and slope, from the Galician coast to the Ushant front, during the springs from 2005 to 2012. Zooplankton size spectra were estimated using measurements carried out in situ with the Laser Optical Plankton Counter (LOPC) and with an image analysis of WP2 net samples (200 μm mesh size) performed following the ZooScan methodology. The LOPC counts and sizes particles within 100–2000 μm of spherical equivalent diameter (ESD), whereas the WP2/ZooScan allows for counting, sizing and identification of zooplankton from ~ 400 μm ESD. The difference between the LOPC (all particles) and the WP2/ZooScan (zooplankton only) was assumed to provide the size distribution of non-living particles, whose descriptors were related to a set of explanatory variables (including physical, biological and geographic descriptors). A statistical correction based on these explanatory variables was further applied to the LOPC size distribution in order to remove the non-living particles part, and therefore estimate the size distribution of zooplankton. This extensive data set provides relevant information about the zooplankton size distribution variability, productivity and trophic transfer efficiency in the pelagic ecosystem of the Bay of Biscay at a regional and interannual scale

In situ imaging reveals the biomass of giant protists in the global ocean

International audiencePlanktonic organisms play crucial roles in oceanic food webs and global biogeochemical cycles1, 2. Most of our knowledge about the ecological impact of large zooplankton stems from research on abundant and robust crustaceans, and in particular copepods3, 4. A number of the other organisms that comprise planktonic communities are fragile, and therefore hard to sample and quantify, meaning that their abundances and effects on oceanic ecosystems are poorly understood. Here, using data from a worldwide in situ imaging survey of plankton larger than 600 μm, we show that a substantial part of the biomass of this size fraction consists of giant protists belonging to the Rhizaria, a super-group of mostly fragile unicellular marine organisms that includes the taxa Phaeodaria and Radiolaria (for example, orders Collodaria and Acantharia). Globally, we estimate that rhizarians in the top 200 m of world oceans represent a standing stock of 0.089 Pg carbon, equivalent to 5.2% of the total oceanic biota carbon reservoir5. In the vast oligotrophic intertropical open oceans, rhizarian biomass is estimated to be equivalent to that of all other mesozooplankton (plankton in the size range 0.2–20 mm). The photosymbiotic association of many rhizarians with microalgae may be an important factor in explaining their distribution. The previously overlooked importance of these giant protists across the widest ecosystem on the planet6 changes our understanding of marine planktonic ecosystems

Niche construction by non-diazotrophs for N2 fixers in the eastern tropical North Atlantic Ocean

Diazotrophic dinitrogen (N2) fixation contributes ~76% to "new" nitrogen inputs to the sunlit open ocean, but environmental factors determining N2 fixation rates are not well constrained. Excess phosphate (phosphate-nitrate/16 > 0) and iron availability control N2 fixation rates in the eastern tropical North Atlantic (ETNA), but it remains an open question how excess phosphate is generated within or supplied to the phosphate-depleted sunlit layer. Our observations in the ETNA region (8°N-15°N, 19°W-23°W) suggest that Prochlorococcus and Synechococcus, the two ubiquitous non-diazotrophic cyanobacteria with cellular N:P ratios higher than the Redfield ratio, create an environment of excess phosphate, which cannot be explained by diapycnal mixing, atmospheric, and riverine inputs. Thus, our results unveil a new biogeochemical niche construction mechanism by non-diazotrophic cyanobacteria for their diazotrophic phylum group members (N2 fixers). Our observations may help to understand the prevalence of diazotrophy in low-phosphate, oligotrophic regions

Évolution décennale du zooplancton de la Mer Ligure en relation avec les fluctuations environnementales. De l'imagerie à la modélisation basée en taille.

Imaging methods have recently emerged as tools to measure the zooplankton dynamics. These methods give both taxonomic and size structure information on the community. The two kinds of information are necessary to understand the zooplankton dynamics and its links to the environment. Yet, the present work will focalize more on the zooplankton size structuration information. The manuscript is organized in three chapters: methodology, ecological analysis and description of an eleven years time series, and modeling. The methodological chapter assesses the validity of imaging methods as tools to measure zooplankton size spectra by investigating the effect of new biases onto the said size spectra (touching objects during the image acquisition process, efficiency of the automatic classification, model use to compute individual's biovolume/biomasses). It is a discussion about what the scientist want to measure and what he finally analyze giving the methodological constraints. The second chapter relates to the analysis of the temporal evolution (1995-2005) of Ligurian Sea ecosystems, mainly from data recorded at the coastal monitoring station of the bay of Villefranche-sur-Mer. Combining the data (taxonomic and size structure, zooplankton, climatic, meteorological, hydrological and biological), physical, chemical and biological factors determining the ecosystem dynamic are proposed — mostly precipitations and temperatures in winter as well as solar irradiation in spring/summer. The interactions between them lead to different phases of the zooplankton community. In the end, the last chapter proposes a zooplankton size-based model. Since classic box models generally failed in representing correctly the zooplankton, size-based approaches are suggested as alternatives. The presented model is exploratory, so as to understand what determines the zooplankton size structuration. It includes growth of zooplankton through “dynamic energy budget” like modeling and predation through size dependent prey/predator relationships. The size-based model simulations are then compared to the dynamics of the zooplankton size-spectra in the main phases identified in the preceding chapter. This is a first step toward an integration of such models into global ones in order to improve the quality of the representation of zooplankton. The present work enriches our knowledge of the more global concept of size-spectra observations and theory, and it also helps to better understand the links between zooplankton and environment.L'imagerie a récemment émergée comme outil de mesure de la dynamique du zooplancton en donnant des informations à la fois taxonomiques et de structure en taille de la communauté — nécessaires à la bonne compréhension de la dynamique du zooplancton au sein de l'écosystème pélagique. Ce travail est cependant davantage centré sur l'étude de la structure en taille du zooplancton dont différents aspects seront examinés au cours de trois chapitres: méthodes, analyse et description écologique d'une série temporelle de 11 ans, modélisation. Le chapitre méthodologique évalue la validité de l'imagerie en examinant l'effet de nouveaux biais, directement issus de l'imagerie, sur ledit spectre de taille (objets en contact au cours de l'acquisition d'image, efficacité de la classification automatique, modèle utilisé pour le calcul des biovolumes et biomasses). Il s'agit d'une discussion sur ce que le scientifique veut mesurer et sur ce qu'il analyse finalement compte tenu des contraintes méthodologiques. Le second chapitre est consacré à l'évolution temporelle (1995-2005) de l'écosystème pélagique de la mer Ligure — plus spécifiquement de la station d'observation côtière de la rade de Villefranche-sur-Mer. Par l'étude combinée des données disponibles (taxonomie, structure en taille, climat, météorologie locale, hydrologie et biologie), les facteurs physiques, chimiques et biologiques déterminant pour l'écosystème sont extraits. Il s'agit principalement du climat hivernal, précipitations et températures, ainsi que de l'irradiation solaire reçu au printemps et en été. Les interactions entre ces facteurs déterminent différents états de la communauté zooplanctonique. Enfin, le dernier chapitre propose un modèle du zooplancton basé sur sa structuration en taille. Puisque les modèles classiques, en boîte, représentent difficilement la dynamique du zooplancton, l'approche par la taille fut proposée comme alternative. Le modèle présenté est exploratoire, incluant la croissance du zooplancton selon une modélisation de type “budget énergétique dynamique” et la prédation selon des rapports de taille proies/prédateurs. Le but étant de comprendre ce qui détermine la structure en taille du zooplancton dans l'environnement. Les simulations effectuées sont ainsi comparées à la dynamique observée dans les phases majeures identifiées au chapitre précédent. C'est un premier pas vers l'intégration de ce type de modèles au sein de modèles plus généraux en vue d'améliorer la qualité de représentation du zooplancton. Le présent travail enrichit la connaissance du concept plus vaste de l'observation et de l'analyse théorique de la structuration en taille du zooplancton, mais il enrichit aussi la compréhension des liens entre le zooplancton et son environnement

A Continuous size-structured model applied to zooplankton comunity

International audienceSize appears to be a very good indicator of physiological behaviour in the zooplankton community, like predation or even detritus production. In order to get a better understanding of planktonic ecosystem dynamics, we build a continuous sizestructured model. This approach is very popular in fish studies (Benoit and Rochet,2004; Andersen and Pedersen, 2009). Our model incorporates predation on phytoplankton and cannibalism within the zooplankton community, and various predation behaviours are tested. Mathematical results are in the most cases hard to get in such models. Nevertheless for some choice of predation function, and external mortality, we can obtain mathematically equilibria. Numerical simulations show that these equilibria are not always stable, and Hopf bifurcations appear when model parameters vary

DECADAL VARIABILITY OF ZOOPLANKTON COMMUNITIES OF THE NW MEDITERRANEAN SEA (WEEKLY SAMPLED FROM 1995 TO 2006) IN RELATION WITH CLIMATIC FORCING.

participantThe inter-annual variability of the pelagic ecosystems of the Ligurian Sea is investigated combining original datasets (from 1995 to 2006 collected weekly) of zooplankton abundances, hydrology and local weather conditions obtained in the bay of Villefranche-sur-mer. Two main patterns of zooplankton dynamics were observed with a shift between 1999 and 2000. The first period was characterized by high precipitation and mild air temperature during the winter. This induced lower salinity and higher seawater temperature and low density of surface seawater during the winter. These waters were characterize by low loads of nutrients. During these years, zooplankton total biovolume was also lower as shown by the strong negative anomalies in the time series. Starting in 2000, the climate changed toward drier and colder winters with denser surface water and more intense convections as suggested by higher nutrients concentrations. An increase of the abundances of all zooplankton categories was observed with a doubling of the total zooplankton average annual means and a change in the zooplankton phenology with a spring development happening 2.5 weeks earlier during these years. These results could be explained by a strong bottom-up control on the pelagic ecosystem of the Ligurian Sea at the inter-annual scale. Whereas the summer thermal stratification increase was often suggested to drive long-term dynamic in Ligurian Sea zooplankton, our results highlight the strong influence of the Winter convection properties as the main factor governing inter-annual changes in zooplankton abundance. The effect of global climate cycles will be discussed as being possible factors driving the pelagic ecosystem in the NW Mediterranean Sea

Assessing biases in computing size spectra of automatically classified zooplankton from imaging systems: A case study with the ZooScan integrated system

International audienceBody size constrains prey-predator interactions and physiology, therefore plankton size spectra have been appointed as synthetic descriptors of plankton community structure and functioning. Recently developed imaging systems and supervised classification tools provide size measurements of any object in situ or in net samples and automatically classify them into previously defined categories. But because the nature of objects detected by these imaging systems is diverse, from non-living detritus to organisms of different plankton taxa, and because the steps in the analysis could introduce specific biases, a careful analysis of such plankton size spectra is needed before going deeper into ecological considerations. Using a WP2 net time series, we propose a general framework to analyze and validate zooplankton size spectra collected with nets and analyzed with the ZooScan integrated system that includes supervised classification. Size spectra were controlled, at each step of the procedure, to assess the modification of their shape due to several possible biases: (i) the effect of objects touching each other during the image acquisition, (ii) the error of the automatic classification differing among size classes and (iii) the choice of model to estimate body biovolume