HYDROMECHANICAL BOUNDARY-LAYERS OVER A CORAL-REEF

Three hydrodynamic boundary layers were measured over a coral reef, do minated by Porites compressa and Montipora verrucosa corals, in Kaneoh e Bay, Hawaii. These measurements were used to evaluate the applicabil ity of present models describing boundary layers and to define the ran ge in which processes governed by them takes place. The Diffusion Boun dary Layer (DBL), related to diffusion-limited processes such as respi ration and photosynthesis, was thicker over M. verrucosa than over P. compressa (2.00 +/- 0.6 and 1.42 +/- 0.4 mm, respectively). The Moment um Boundary Layer (MBL), controlling water movement in the proximity o f the sessile organisms, was thicker over M. verrucosa than over P. co mpressa as well (97 +/- 27 and 58 +/- 24 mm, respectively), correspond ing to the stronger requirement for water motion by the former, and wa s thicker by an order of magnitude than the DBL. The Benthic Boundary Layer (BBL), controlling the interactions of the reef with the open se a waters, was found to be more than 1 m thick and was characterized by a roughness height of 31 cm and a shear velocity (u) of 0.42 cm s(-1 ). The BBL was composed of three distinguished segments, a lower subla yer with slow water motion throughout its height, an inner sublayer up to the height of the coral knells, acid a fully developed outer BBL. This structure of the BBL suggests that: (1) sedimentation at the lowe r segment of the BBL is contributing to the patchy structure of this r eef; and (2) high corals colonies increase sedimentation while reducin g water motion and food supply from lower colonies located within the lower and middle segments of the BBL.