Vertical secondary flows in submersed plant-like arrays

Obstructions that protruded from a laboratory test bed into the benthic bou ndary layer were exposed to gradients in longitudinal velocity (delta u/del ta z) that produced vertical pressure gradients along the surface of the ob struction. These pressure gradients generated vertical secondary flows that may have ecological significance for benthic fauna and aquatic macrophytes . Laboratory experiments demonstrated that secondary flows of up to 15% of the local longitudinal velocity were produced behind individual obstruction s within a submersed plant-like or animal tube array and for conditions Lik e those found in aquatic canopies or colonies. Our observations support the oretical predictions based on a reduced form of the Navier-Stokes equations . confirming that the ascending how is controlled by a local balance of ver tical pressure gradient, proportional to delta u(2)/delta z, and viscous st ress. The secondary flows were shown To transport dye from the bottom to a height dictated by the in-canopy current speed and turbulence intensity. By extension. the ascending flows can potentially contribute to the advection of nutrients from sediments, where they have been regenerated by microbial processes, to areas higher in the canopy, where they can be used by epiphy tes and macrophyte leaves. Pressure gradients generated near the stem base also produce pore-water exfiltration. The coupled pressure-driven exfiltrat ion and vertical advection have the potential to control nutrient availabil ity in the bed.