The effects of waves and morphology on mass transfer within branched reef corals

Rates of mass transfer in coral reefs are governed both by the physical flow environment and the morphology of the coral. Laboratory experiments were conducted to estimate mass transfer in unidirectional and oscillatory flows by measuring the rate of dissolution of gypsum cylinders (clods) placed within the branching structure of three morphologically distinct coral species. Unidirectional flows were varied between 2.9 and 14.1 cm s^-1 and, as expected, mass transfer rates increased with increasing flow and a more open branch spacing. Depending on morphology and flow, mass transfer rates within the interior of the branching structure were 50 to 75% of that measured outside the coral in free-stream conditions. Oscillatory conditions showed relative mass transfer rates 1.6 to 2.9 times greater than equivalent unidirectional currents. This ratio increased with increasing wave frequency, likely due to the corresponding decrease in the diffusive boundary layer thickness. The ratio also increased with a greater compactness in branch spacing, with mass transfer rates within the coral structure up to 130% of free-stream conditions. We used planar laser-induced fluorescence imaging to study the instantaneous structure of mass advection through the coral. Oscillatory flow acts as a dominant forcing mechanism to generate water motion within the coral structure at levels not attainable with comparable unidirectional currents.