Plankton are small organisms that are by definition carried along by ocean currents. However, many species do not behave as passive particles and propel themselves with velocities ranging from 10 to 1000m/s. Turbulence is a ubiquitous feature of marine ecosystems which affects the spatial distribution of plankton, their behaviour, their ability to obtain food or locate a partner, which in turn influences ecosystem-scale processes.
1/ Phytoplankton is responsible for around 50% of the world's oxygen production and is the basic building block of the marine food chain. A detailed understanding of the physical mechanisms that lead the gyrotactic species of phytoplankton to migrate vertically towards the surface enables us to better quantify biogeochemical fluxes across the ocean. Observations at sea suggest that swimming in a chain may confer an ecological advantage for certain species that alternately seek light at the surface and nutrients at depth.
The simulation model we developed couples two phenomena: direct numerical simulation (DNS) of the Navier-Stokes equations governing the fluid motion in a turbulent flow, and Lagrangian tracking of the trajectories of several hundred thousand micro-organisms (individual or in the form of a chain of cells). Statistical analysis of the data reveals that in turbulence of moderate intensity, it is more efficient to move upwards in a chain of cells than to remain isolated.
2/ Copepods (planktonic crustaceans) make up most of the biomass in the oceans and are a major food source for fish. They alternate phases of active (jumping) and passive (cruising) transport. These behavioural responses to flow are still largely unknown due to a complex coupling between hydrodynamic forces and the morphology of the organisms (size close to the dissipative scale, non- spherical shape, and density different from that of the fluid).
All our experiments were carried out in a rectangular glass tank with dimensions 0.4 m×0.4m×1.2m filled with salted water at 18°C (salinity 15, density ρf = 1.01 g/cm3). The turbulent flow is generated by the vertical oscillation of two horizontal grids located 40cm apart. Fluid tracers and copepods trajectories are reconstructed by means of 3D Lagrangian particle tracking velocimetry (Lavision Shake-the-Box algorithm) allowing tracking at high seeding concentration.
Statistical properties (jump amplitude and duration) are used to characterize the interactions of copepods with turbulence. We found out that the jump frequency increases from 0.66Hz in calm water to 0.93Hz. This 40% increase indicates that copepods are able to adjust their swimming effort to the ambient flow by jumping more frequently while maintaining the other characteristics of their motion. Experiments with copepods exposed to microplastics (6-7 m plastic debris) have shown that these statistics vary significantly as a signature of the toxicity of microplastic particles ingested by copepods.
References
- Experimental investigation of preferential concentration in zooplankton swimming in turbulence. F.-G. Michalec, O. Praud, S. Cazin, and E. Climent (2022). Eur. Phys. J. E 45:12.
- Fluid inertia is an effective gyrotactic mechanism for settling elongated micro-swimmers J. Qiu, Z. Cui, E. Climent, and L. Zhao. (2022) Phys. Rev. Research 4, 023094.
- Chain formation can enhance the vertical migration of phytoplankton through turbulence. S. Lovecchio, E. Climent, R. Stocker and W. M. Durham. (2019) Science Advances. 5, eaaw7879.
- W. M. Durham, E. Climent, M. Barry, F. De Lillo, G. Boffetta, M. Cencini and R. Stocker, (2013) Turbulence drives microscale patches of motile phytoplankton, Nature Communications, 4: 2148.
