Gyrotactic trapping in laminar and turbulent Kolmogorov flow

TL;DR

This paper investigates how motile phytoplankton form thin layers in shear flows, analyzing the effects of flow regimes and stochastic influences on the stability and persistence of these layers.

Contribution

It provides a detailed dynamical systems analysis of gyrotactic trapping in laminar flows and explores how turbulence and Brownian motion affect layer formation and stability.

Findings

TPLs are fully characterized in laminar flows using dynamical systems.

Turbulence and Brownian motion make TPLs transient and affect their persistence.

Gyrotactic trapping is sensitive to flow regime and stochastic effects.

Abstract

Phytoplankton patchiness, namely the heterogeneous distribution of microalgae over multiple spatial scales, dramatically impacts marine ecology. A spectacular example of such heterogeneity occurs in thin phytoplankton layers (TPLs), where large numbers of photosynthetic microorganisms are found within a small depth interval. Some species of motile phytoplankton can form TPLs by gyrotactic trapping due to the interplay of their particular swimming style (directed motion biased against gravity) and the transport by a flow with shear along the direction of gravity. Here we consider gyrotactic swimmers in numerical simulations of the Kolmogorov shear flow, both in laminar and turbulent regimes. In the laminar case, we show that the swimmer motion is integrable and the formation of TPLs can be fully characterized by means of dynamical systems tools. We then study the effects of rotational Brownian motion or turbulent fluctuations (appearing when the Reynolds number is large enough) on TPLs. In both cases we show that TPLs become transient, and we characterize their persistence.