The effect of flow and morphology on boundary layers in the scleractiniansDichocoenia stokesii (Milne-Edwards and Haime) and Stephanocoenia michilini (Milne-Edwards and Haime)

Mass transfer characteristics of scleractinian corals are affected by their skeletal morphology and the concentration gradients that develop as a cons equence of the interactions of their morphology and biomass with the overly ing seawater. These interactions can have a profound effect on coral metabo lism. In this study, boundary layer characteristics were compared between d ifferent size colonies of the corals Dichocoenia stukesii and Srephanocoeni a michilini to determine the relative roles of colony size and corallite st ructures (i.e. surface roughness) in mass transfer. Colonies of both specie s were rounded in shape, but differed in small-scale roughness as measured by the elevation of corallites. Additionally, D. srokesii had a greater asp ect ratio than S. michilini, and their colonies were slightly taller fur a given diameter. Boundary layers were characterized by placing dead coral sk eletons in a flow tank and estimating shear velocities (u*) at different fl ow speeds. The effects of Row speed, size, and roughness on shear velocitie s were estimated for two juvenile size classes (10-20 and 30-40 mm diameter ) of each species that were exposed to unidirectional Row regimes (4 and 17 cm s(-1)). Shear velocities were significantly greater in high, compared t o low flow, and there was a significant interaction between colony size and surface roughness: the interaction was caused by a difference in magnitude , rather than direction, of the effect of roughness and size on u*. Thus, t here was a greater degree of turbulence at high flow compared to low flow, regardless of roughness or size, and the greatest turbulence occurred over large colonies of D. stokesii at high Row. Together, these results suggest that boundary layers around small corals are heavily influenced by upstream roughness elements, and more strongly affected by flow regimes than skelet al features. The relationship between colony morphology (i.e. aspect ratio and, possibly, surface roughness) and boundary layer characteristics may be non-linear in small corals. (C) 2001 Elsevier Science B.V. All rights rese rved.