OBSERVATIONS OF COHERENT BANDS OF ALGAE IN A SURFACE SHEAR-LAYER

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 shea r. The bands were being actively generated in the shear layer which wa s uncoupled from the underlying flow. The neutrally buoyant algae span ned the shear layer vertically within the bands. Virtually no algae we re observed in the spaces between bands. The spacing between bands of algae was much smaller in scale than sur-face slicks and Langmuir circ ulations previously reported on lakes and oceans. No evidence was foun d 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 diff erent from the underlying flow which was clearly three dimensional and turbulent. Similar shear layers may occur at the surfaces of lakes an d oceans under glassy calm conditions but go unnoticed because of the absence of a suitable tracer such as algae.