title: Evidence for water-mediated mechanisms in coral-algal interactions.
abstract: Although many coral reefs have shifted from coral-to-algal dominance, the consequence of such a transition for coral-algal interactions and their underlying mechanisms remain poorly understood. At the microscale, it is unclear how diffusive boundary layers (DBLs) and surface oxygen concentrations at the coral-algal interface vary with algal competitors and competitiveness. Using field observations and microsensor measurements in a flow chamber, we show that coral (massive Porites) interfaces with thick turf algae, macroalgae, and cyanobacteria, which are successful competitors against coral in the field, are characterized by a thick DBL and hypoxia at night. In contrast, coral interfaces with crustose coralline algae, conspecifics, and thin turf algae, which are poorer competitors, have a thin DBL and low hypoxia at night. Furthermore, DBL thickness and hypoxia at the interface with turf decreased with increasing flow speed, but not when thick turf was upstream. Our results support the importance of water-mediated transport mechanisms in coral-algal interactions. Shifts towards algal dominance, particularly dense assemblages, may lead to thicker DBLs, higher hypoxia, and higher concentrations of harmful metabolites and pathogens along coral borders, which in turn may facilitate algal overgrowth of live corals. These effects may be mediated by flow speed and orientation.
authors: EPHE, PSL Research University, UPVD, CNRS, USR 3278 CRIOBE, 66860 Perpignan, France Aquaculture and Fisheries Group, Wageningen University, PO Box 338, 6700 AH Wageningen, The Netherlands.
authors: Hendrikje
authors: H
authors: Jorissen
authors: EPHE, PSL Research University, UPVD, CNRS, USR 3278 CRIOBE, 66860 Perpignan, France School of Marine Science and Technology, Newcastle University, Armstrong Building, Newcastle upon Tyne NE1 7RU, UK.
authors: Christina
authors: C
authors: Skinner
authors: Marine Animal Ecology Group, Wageningen University, PO Box 338, 6700 AH Wageningen, The Netherlands.
authors: Ronald
authors: R
authors: Osinga
authors: Microsensor Research Group, Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany.
authors: Dirk
authors: D
authors: de Beer
authors: EPHE, PSL Research University, UPVD, CNRS, USR 3278 CRIOBE, 66860 Perpignan, France Laboratoire d'Excellence 'CORAIL', 58 Avenue Paul Alduy, 66860 Perpignan, France maggy.nugues@criobe.pf.
authors: Maggy M
authors: 0000-0002-1399-9787
authors: MM
authors: Nugues
mesh_terms: D000818:Animals; D037421:Anthozoa / Q000502:physiology*; D057894:Coral Reefs; D000458:Cyanobacteria / Q000502:physiology*; D000860:Hypoxia; D012623:Seawater / Q000737:chemistry*; D012624:Seaweed / Q000502:physiology*
keywords: coral reef; coral–algal interactions; oxygen concentrations; turf algae; water flow