Diffusive boundary layers and photosynthesis of the epilithic algal community of coral reefs

The effects of mass transfer resistance due to the presence of a diffusive boundary layer on the photosynthesis of the epilithic algal community (EAC) of a coral reef were studied. Photosynthesis and respiration of the EAC of dead coral surfaces were investigated for samples from two locations: the Gulf of Aqaba, Eilat (Israel), and One Tree Reef on the Great Barrier Reef (Australia). Microsensors were used to measure O2 and pH at the EAC surface and above. Oxygen profiles in the light and dark indicated a diffusive boundary layer (DBL) thickness of 180-590 microm under moderate flow (approximately 0.08 m s-1) and >2,000 microm under quasi-stagnant conditions. Under light saturation the oxygen concentration at the EAC surface rose within a few minutes to 200-550% air saturation levels under moderate flow and to 600-700% under quasi-stagnant conditions. High maximal rates of net photosynthesis of 8-25 mmol O2 m-2 h-1 were calculated from measured O2 concentration gradients, and dark respiration was 1.3-3.3 mmol O2 m-2 h-1. From light-dark shifts, the maximal rates of gross photosynthesis at the EAC surface were calculated to be 16.5 nmol O2 cm-3 s-1. Irradiance at the onset of saturation of photosynthesis, Ek, was