Fluid motion, like in environmental flows, may generate chaotic advection: the particles transported by the flow typically exhibit chaotic dynamics. This manifests itself in that the particles trace out complicated geometrical objects, filamentary fractal structures. The appearance of these complex fractal structures plays an important role if the advected particles are chemically or biologically active. In fact, traditional chemical rate equations and population dynamical equations must be modified to incorporate this effect. For example, plankton population dynamics may be influenced by the emerging filamentary patterns. Other examples may include mixing and evaporation of oil spills, or melting of sea-ice.