Morphological traits of zooplankton reveal ecological patterns along ice melt dynamics in the Arctic

Laure Vilgrain, Laboratoire d'Océanographie de Villefranche-sur-Mer, Villefranche-sur-Mer, France, Jean-Olivier Irisson, University Pierre and Marie Curie Paris VI, Lab. d'oceanographie, Villefranche, France, Sakina-Dorothée Ayata, LOV UPMC/CNRS, Villefranche sur mer Cedex, France, Marc Picheral, CNRS, Laboratoire d'Océanographie de Villefranche sur Mer, Paris Cedex 16, France, Marcel Babin, Takuvik Joint International Laboratory, Université Laval & CNRS, Québec, QC, Canada and Frederic Maps, Université Laval, Quebec, QC, Canada
Functional traits are individual characteristics that influence an organism's fitness and ecological functions. Thanks to technological progress, these traits can be measured at an individual level, completing with quantitative information the usual taxonomic approach to assess the structure and functioning of ecosystems.

We studied the surface waters of Baffin Bay, an Arctic marginal sea located between Greenland and Canada, at the moment of sea ice break-up. Strong environmental gradients are created by the confrontation of two water masses, sea ice melting, and the increase in temperature and irradiance. We focused on copepods that overwhelmingly dominate zooplankton communities there. We measured morphological descriptors of images (area, darkness, complexity, etc.) on about 28,000 copepod images taken by the Underwater Vision Profiler (UVP). A statistically-defined multidimensional morphological space allows to synthesize individual images into interpretable continuous traits (size, transparency, appendages, etc.). The spatial distribution of these traits revealed that large copepods are associated with ice-covered waters in the West while smaller are present in open waters in the East. Copepods of the eastern part also seem to have higher feeding activity, as inferred by appendage visibility. High phytoplankton concentrations and probable strong visual predation pressure on big copepods in well-lit open waters could be responsible for these traits distributions. Furthermore, copepods located right at the ice edge appeared more opaque on images, suggesting that these individuals have a strong red pigmentation (the UVP light source is red).

The combination of in situ imaging and individual trait-based approach revealed important ecological patterns that would have been inaccessible otherwise, including the role of copepod behaviour and ecological interactions on zooplankton ecosystem dynamics in the Arctic.