Hydromechanical boundary layers over a coral reef

Three hydrodynamic boundary layers were measured over a coral reef, dominated by Porites compressa and Montipora verrucosa corals, in Kaneohe Bay, Hawaii. These measurements were used to evaluate the applicability of present models describing boundary layers and to define the range in which processes governed by them takes place. The Diffusion Boundary Layer (DBL), related to diffusion-limited processes such as respiration and photosynthesis, was thicker over M. verrucosa than over P. compressa (2.00 ± 0.6 and 1.42 ± 0.4 mm, respectively). The Momentum Boundary Layer (MBL), controlling water movement in the proximity of the sessile organisms, was thicker over M. verrucosa than over P. compressa as well (97 ± 27 and 58 ± 24 mm, respectively), corresponding to the stronger requirement for water motion by the former, and was thicker by an order of magnitude than the DBL. The Benthic Boundary Layer (BBL), controlling the interactions of the reef with the open sea 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 sublayer with slow water motion throughout its height, an inner sublayer up to the height of the coral knolls, and a fully developed outer BBL. This structure of the BBL suggests that: (1) sedimentation at the lower segment of the BBL is contributing to the patchy structure of this reef; and (2) high corals colonies increase sedimentation while reducing water motion and food supply from lower colonies located within the lower and middle segments of the BBL.