  We demonstrate that active carpets of bacteria or self-propelled colloids
generate coherent flows towards the substrate, and propose that these currents
provide efficient pathways to replenish nutrients that feed back into activity.
A full theory is developed in terms of gradients in the active matter density
and velocity, and applied to bacterial turbulence, topological defects and
clustering. Currents with complex spatiotemporal patterns are obtained, which
are tuneable through confinement. Our findings show that diversity in carpet
architecture is essential to maintain biofunctionality.
