The forces on microorganisms at surfaces in flowing water

SUMMARY. Surfaces in flowing water are covered by boundary layers within which the velocity of flow ranges between zero at the surface and the velocity of the main stream at the limits of the boundary layer. Many organisms are small enough to live entirely within the boundary layer and so are subjected to much smaller drag forces than if they were exposed to the main stream velocity. The form and extent of boundary layers at different stream velocities around selected structures, representing leaves and stems, have been calculated and presented graphically. These data provide a basis for the estimation of shear stresses on substrate surfaces and of forces exerted upon organisms of different sizes that live within the boundary layers around such surfaces, when exposed to different current velocities. Values for shear stresses on bacteria calculated in this way lie within the range of shear stresses required to detach bacteria from surfaces experimentally.