Global plankton diversity and size structure revealed by quantitative imaging approaches during the Tara Oceans expeditions

Fabien Lombard1, Fererico Ibarbalz2, Manoela Brandão3,4, Severine Martini5, Nicolas Henry6, Fabio Benedetti7,8, Amanda Elineau9, Laetitia Jalabert9, Marc Picheral5, Lars Stemmann10, Lucie Zinger2, Chris Bowler11 and G Gorsky12, (1)Sorbonne University, Laboratoire d'Oceanographie de Villefranche sur Mer, Villefranche Sur Mer, France, (2)Ecole Normale Supérieure Paris, Institut de biologie de l’École normale supérieure (IBENS), Paris, France, (3)Sorbonne université, Laboratoire d'océanographie de villefranche sur mer, Villefranche sur mer, France, (4)IFREMER, Centre Bretagne, Unité Dynamiques des Ecosystèmes Côtiers, Plouzané, France, (5)Laboratoire d’Océanographie de Villefranche (LOV), UMR 7093, Sorbonne Université, Villefranche-sur-Mer, France, (6)Sorbonne Université, CNRS, Station Biologique de Roscoff, Roscoff, France, (7)ETH Zurich Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (8)Sorbonne Université, Laboratoire d'Océanographie de Villefranche sur Mer, Villefranche Sur Mer, France, (9)Sorbonne Université, Institut de la Mer de Villefranche (IMEV), France, (10)Laboratoire d’Océanographie de Villefranche (LOV), UMR 7093, Sorbonne Université, Villefranche sur mer, France, (11)Ecole Normale Supérieure, Institut de Biologie/Ecology and Evolutionary Biology Section, Paris, France, (12)CNRS, Laboratoire d'Océanographie de Villefranche-sur-Mer, Villefranche-sur-Mer, France
The development of quantitative imaging opens new horizons for the study of plankton. Alongside the traditional sampling methods imaging methods improve our comprehension of the distribution and morphological properties of organisms. The strengths of quantitative imaging reside in its ability to estimate the abundances, the biovolume and the taxonomic composition of the plankton, simultaneously. However, current methods lack of cross comparisons, notably regarding taxonomic standards.

During the Tara Oceans expeditions (2009-2013), we deployed a holistic sampling scheme to sample the full size spectrum of plankton communities, from viruses to vertebrates. Quantitative imaging data were associated with -omics and contextual environmental data in order to determine how diversity, organisms’ size and abundance varies with physico-chemical and biological environment, but also to cross check results between them. We used these datasets to assess the latitudinal patterns and drivers of abundance, size structure, taxonomic and morphological diversity of planktonic communities on a global scale. The diversity indexes obtained form the different datasets were cross-compared and displayed comparable diversity patterns.

Most planktonic groups show a decline in diversity from the equator to the poles that is mainly driven by increases in ocean temperatures and, to a lesser extent, by decreases in nutrients availability and phytoplankton biomass. Conversely, the size structure and the abundance of zooplankton decrease towards warmer and more oligotrophic environments. This pattern is driven by copepods while large protists (mostly rhizarians) present the opposite pattern for both abundance and size. We ascribed such opposite patterns to morphological and/or physiological traits like the presence of photosymbionts, or the optimization of prey encounter rates under oligotrophic conditions. Our results depict latitudinal gradients in both diversity and size in plankton communities, assigned mainly by variations in sea surface temperature and trophic status in the global ocean.