ABSTRACT:

Negative interactions between corals and algae have been implicated in preventing coral reef recovery. A recent hypothesis is that algae indirectly cause coral tissue mortality by fueling microbial growth, which drives hypoxic conditions at the interface between corals and algae. However, previous studies have largely ignored how water flow influences oxygen concentrations at the interface between corals and algae. This study quantified the effect of water flow on oxygen concentrations above the zone of interaction between massive Porites spp. and algal turf during the day and at night. Coral-algal interactions were exposed to 3 flow speeds (0, 7.7, 14.5 cm s^-1), and diffusive boundary layer (DBL) thicknesses were measured in the light and dark. DBL thickness (<500 to 2500 µm) decreased with increasing water flow, but remained thicker above algal turf and the zone of interaction (~1000 µm) than over corals (370 µm) in the dark. Oxygen extremes above the zone of interaction were most similar to concentrations above algal turf in low flow (34 to 43% oxygen saturation). The influence of microbes on oxygen conditions was estimated by adding antibiotic (50 µg ampicillin ml^-1) to algal turf-massive Porites interactions. The results showed no indication of microbially mediated hypoxic conditions. These results suggest that water flow likely limits the effectiveness of hypoxia or hyperoxia-mediated coral-algal competition under natural reef conditions.