Observations of coherent bands of algae in a surface shear layer

Abstract

Shear layers 1–2 cm thick were found immediately below the surface of lakes under “glassy” calm conditions. Driven by very faint breezes, these thin shear layers contained bands of algae aligned with the shear. The bands were being actively generated in the shear layer which was uncoupled from the underlying flow. The neutrally buoyant algae spanned the shear layer vertically within the bands. Virtually no algae were observed in the spaces between bands. The spacing between bands of algae was much smaller in scale than surface slicks and Langmuir circulations previously reported on lakes and oceans. No evidence was found of downwelling beneath the bands nor of an array of counter‐rotating helical vortices with axes aligned with the algal bands. In fact, the surface‐layer flow appeared essentially two dimensional and very different from the underlying flow which was clearly three dimensional and turbulent. Similar shear layers may occur at the surfaces of lakes and oceans under glassy calm conditions but go unnoticed because of the absence of a suitable tracer such as algae.