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 s21 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. Coral reefs exist under a wide range of physical environments and flow conditions, from high-energy environments where they are exposed to wave action as well as wave breaking, to low-energy environments where they are exposed to weak currents driven primarily by wind or tides (e.g., Hamner and Wolanski 1988; Kench 1998). Given this variety of physical environments, it is not surprising that studies have shown a link between variations in flow and habitat to coral growth patterns and morphologic zonation (Graus and Macintyre 1989; Sebens and Done 1992). Changes in growth form can occur in response to flow exposure (Lesser et al. 1994), with transformations from compact branching structures under exposed conditions, to a thinbranching shape in sheltered conditions (Kaandorp 1999). These morphologic modifications, resulting in changes to relative branch size, spacing, and branching pattern, can in turn alter the structure of flow itself, both around and through colonies (Chamberlain and Graus 1975; Sebens et al. 1997). Therefore, there is an apparent feedback between water flow patterns and coral morphology, which may not only play a critical role in coral survival, affecting colony overturning and breaking (Done 1982), but also have a sig1 Present address: Department of Integrative Biology, University of California, Berkeley, California 94720.