The diffusion boundary layer is a thin (usually on the scale of micrometers) layer of fluid which is not mixed

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 with the main body of the fluid (or cellular arrangement in the case of organisms), by virtue of its proximity to the fluid's edge. In the case of flora the diffusion boundary layer is often associated with the leaf surface, where stomatal gas exchange occurs. In an estuary

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

this layer is immobile due to friction with the sediment.