  We consider two systems of active swimmers moving close to a solid surface,
one being a living population of wild-type \textit{E. coli} and the other being
an assembly of self-propelled Au-Pt rods. In both situations, we have
identified two different types of motion at the surface and evaluated the
fraction of the population that displayed ballistic trajectories (active
swimmers) with respect to those showing random-like behavior. We studied the
effect of this complex swimming activity on the diffusivity of passive tracers
also present at the surface. We found that the tracer diffusivity is enhanced
with respect to standard Brownian motion and increases linearly with the
activity of the fluid, defined as the product of the fraction of active
swimmers and their mean velocity. This result can be understood in terms of
series of elementary encounters between the active swimmers and the tracers.
