Variability of the surface boundary layer of reef-building coral species

The coral-seawater interface is an important, highly dynamic microenvironment for reef-building corals. Also known as the concentration boundary layer (CBL), it is a thin layer of seawater bordering the coral surface that dictates the biochemical exchange between the coral colony and bulk seawater. The CBL is thus a key feature that modulates coral metabolism. However, CBL variation among small-polyped coral species remains largely unknown. Therefore, we recorded over 100 profiles of dissolved O 2 concentration using microsensors to characterise CBL traits (thickness, surface O 2 concentration, and flux) of three small-polyped branching coral species, Acropora cytherea , Pocillopora verrucosa , and Porites cylindrica . Measurements were conducted during light and darkness combined with low or moderate water flow (2 and 6 cm s -1 ). We found that CBL traits differed among species. CBL thickness was lowest in A. cytherea , while P. verrucosa showed the largest depletion of surface O 2 in dark and highest dark flux. In addition, we found that O 2 concentration gradients in the CBL occurred with three main profile shapes: diffusive, S-shaped, and complex. While diffusive profiles were the most common profile type, S-shaped and complex profiles were more frequent in P. verrucosa and P. cylindrica , respectively, and prevailed under low flow. Furthermore, profile types differed in CBL thickness and flux. Finally, low flow thickened CBLs, enhanced changes in surface O 2 concentration, and reduced flux, compared to moderate flow. Overall, our findings reveal CBL variability among small-polyped branching corals and help understand CBL dynamics in response to environmental changes.