Internal circulation

Abstract

The driving mechanisms, the pathways, and the speeds of water movement through the reef framework are important for reef diagenesis, for the mass transport of solutes (such as nutrients) between the sub-surface (pore) waters, the benthic layer, and the surface waters (with biological and geochemical implications), and for the understanding of coral cay and fringing reef geohydrology (Buddemeier and Oberdorfer, 1988). For laminar flow, Darcy's Law that relates water movement through a homogeneous porous media to the hydraulic gradient and the hydraulic conductivity (primarily a function of permeability) applies. The assumption of laminar flow is likely to hold for all but a few environments, such as in large cavities in the vicinity of breaking waves. Coral reef environments, however, rarely have homogeneous framework at all but the smallest spatial scales, with sometimes extreme differences in both the horizontal and the vertical. Cavities and loose sediments composed of rubble and/or coarse sands exist alongside solid coral heads and densely packed fine sediments, meaning that flows can be variable over very short distances. Advective flows clearly dominate internal circulation, but molecular diffusion, with diffusion coefficients in the order of 10-10 – 10-9 ms-2 (Li and Gregory, 1974) must also be recognized.