Féderation de recherche Fabri de Peiresc

Interplay between motility and turbulence in marine zooplankton

Orateur : François-Gaël Michalec
Institute of Environmental Engineering, ETH Zurich

Abstract : Plankton are small organisms that are by definition carried along by ocean currents without the means to swim against them. However, many members of this group do not behave as passive particles and propel themselves. They have evolved a variety of behavioral capabilities that confer more efficient exploitation of their heterogeneous and fluctuating environment. Examples range from the rapid chemotactic response of marine bacteria toward ephemeral nutrient patches to the vertical migrations of zooplankton across the mixed layer of the oceans. Plankton organisms are exposed to turbulence for most of their life and this has a multitude of implications for their ecology. Research on this theme has recently come to the forefront following the recent development of suitable conceptual frameworks and experimental techniques. The coupling between self-motility and turbulence is a particularly intriguing aspect of the fate of plankton in flow. This coupling generates transport that differs from that of entirety passive particles and this gives rise to complex dynamics at a range of spatial scales from the largest flow structures down to the Kolmogorov scale. Multiple examples from a variety of organisms illustrate this phenomenon, from the clustering of phytoplankton at the microscale to their gyrotactic trapping in kilometer-long layers. The interactions between flow and self-motility have therefore direct and profound impacts on the way marine ecosystems function, including biological productivity, predator-prey interactions and ecosystem dynamics. These interactions often result in an effective biased motion that is not necessarily in the intended direction. However, plankton organisms may also react to specific flow signals from their physical environment through changes in their behavior, and orientate toward preferred habitats. Rapid responses to the hydrodynamic features of the background flow may therefore represent important survival strategies for plankton organisms. Understanding these interactions requires a multidisciplinary approach and has proved a challenge both experimentally and numerically. This limitation is particularly apparent for larger zooplankton organisms that can depart strongly from the underlying flow and for which no conceptual models exist.

The aim of our research is to further our knowledge on the rich spectrum of dynamics generated by the interplay between flow motion and plankton behavior and on their consequences on plankton ecology. Research on this theme is dominated by studies on phytoplankton. We focus instead on larger zooplankton called calanoid copepods which are the most abundant multi-cellular organisms in the oceans. We use particle tracking techniques from the field of turbulence research to retrieve simultaneously both the Lagrangian information of plankton trajectories and the fine-scale structure of the underlying flow field.

I will present experimental results (a) on an active behavioral adaptation that allows these organisms to adjust their motility in response to background flow and to transition from being passively transported to being able of self-locomotion and (b) on the contribution of passive transport and active motion in determining their encounter rates and mating efficiency in turbulence. I will discuss the consequences of these processes at the scale of the organism and of the population.

Date et lieu : vendredi 9 novembre 2018 à 14h00, salle de séminaire IRPHE