Do plankton chains swim upward better through turbulent water?
Although chains of plankton cells are less stable than single cells, their long shape lets them stay pointed upward and ride upward-moving water, so in weak turbulence they migrate much faster.
Source
Chain formation can enhance the vertical migration of phytoplankton through turbulence
Study at a glance
- Design
- Computational / modelling — Individual-based model of gyrotactic swimmers embedded in a direct numerical simulation of isotropic turbulence, varying elongation, stability and swimming speed.
- N
- No participants; each simulation tracked 100,000 model swimmers (convergence checked with 300,000).
- Population
- Simulated motile phytoplankton cells and chains in homogeneous isotropic turbulence
- Outcome
- Mean vertical swimming orientation, vertical fluid velocity sampled by swimmers, and net vertical migration rate
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What they did
The authors simulated fully resolved turbulence by solving the Navier-Stokes equations and released 100,000 model swimmers that are turned by flow vorticity, by flow strain (for elongated shapes) and by a stabilising torque that points them upward. They first varied elongation alone, then used literature data on chain swimming speed and stability to predict migration for chains of different lengths across realistic ocean turbulence levels.
What they found
Elongation helped weakly stable swimmers keep pointing upward, increasing mean vertical orientation by 38% at one swimming speed and 96% at a roughly three times faster speed. Turbulence sorted swimmers by shape: spheres drifted into downwelling water while elongated chains collected in upwelling water. In weak turbulence, chains of two to eight cells migrated 35 to 130% faster than single cells, and two-cell chains were faster at every turbulence level tested, but in strong turbulence longer chains migrated more slowly than single cells.
The limits
What it doesn't show
This is a simulation, not an observation of real plankton, and it treats cells as rigid, point-like, inertia-free particles. Swimming speed and stability as a function of chain length come from literature regressions and simple models, with a range of possible scaling exponents across species. The flow is idealised isotropic turbulence at one Reynolds number, and the suggestion that plankton sense turbulence to regulate chain length is speculative.
Key terms
- Gyrotaxis
- Swimming direction set by a balance between a stabilising torque (e.g. bottom-heaviness) and torques from the surrounding flow.
- Direct numerical simulation (DNS)
- Solving the full Navier-Stokes equations on a grid fine enough to resolve all eddies, without a turbulence model.
- Kolmogorov scale
- The smallest length and time scales of turbulence, where viscosity dissipates kinetic energy.
- Stability number (Ψ)
- Reorientation time multiplied by the Kolmogorov vorticity; large values mean flow easily overturns the swimmer.
- Rate of strain
- The stretching part of the local velocity gradient, which tends to align elongated bodies.
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Quiz yourself
What effect does chain formation have on intrinsic orientational stability?
Common questions
Why would chains be expected to do worse?
Longer chains resist rotation more, so they take much longer to right themselves after being tipped, which normally makes turbulence more disruptive.
What compensates for the lost stability?
Strain aligns the elongated chains, helping them stay oriented upward, and they tend to accumulate in upwelling water that carries them in their direction of travel.
Are chains always better?
No. In strong turbulence chains longer than two cells migrated more slowly than single cells.
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