  We investigate the evolution of particle ensembles in open chaotic
hydrodynamical flows. Active processes of the type A+B --> 2B and A+B --> 2C
are considered in the limit of weak diffusion. As an illustrative advection
dynamics we consider a model of the von K\'arm\'an vortex street, a time
periodic two-dimensional flow of a viscous fluid around a cylinder. We show
that a fractal unstable manifold acts as a catalyst for the process, and the
products cover fattened-up copies of this manifold. This may account for the
observed filamental intensification of activity in environmental flows. The
reaction equations valid in the wake are derived either in the form of
dissipative maps or differential equations depending on the regime under
consideration. They contain terms that are not present in the traditional
reaction equations of the same active process: the decay of the products is
slower while the productivity is much faster than in homogeneous flows. Both
effects appear as a consequence of underlying fractal structures. In the long
time limit, the system locks itself in a dynamic equilibrium state synchronized
to the flow for both types of reactions. For particles of finite size an
emptying transition might also occur leading to no products left in the wake.
