Stokes Drift in Shallow Layers Above Coral Reefs: Theory, Field Observations, and Biological Implications.

Coral reefs in shallow water are exposed to wave-driven flow that carries nutrients, oxygen, and prey to the organisms living in the spaces within the reefs, carries away wastes, and transports released larvae from and settling larvae into reefs. Stokes drift is the net drifting of fluid undergoing wave motion due to a difference in Lagrangian and Eulerian velocities. It is a well-understood and well-researched topic, which will be physically demonstrated during the presentation, and has been applied to understand ocean processes like the motion of driftwood and the spreading of pollutants. We have modeled the consequences of a saturated porous bed layer (a coral reef) underlying essentially inviscid fluid (the ocean) on the surface of which are propagating gravity waves, and will explain how the damping effect of this porous bed affects the Stokes drift velocities both above and within the porous layer, leading to vertical drift into and out of the coral layer. We have tested our model predictions by comparing them with field measurements of water flow above and within shallow coral reefs of known permeability, and we explored and will explain the consequences for transport to coral reef organisms of the reduction in permeability caused by algal overgrowth of a reef.