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

Abstract

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 μm under moderate flow (~0.08 m s−1) and >2,000 μm 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, E k, was <100 µmol photons m−2 s−1, indicating that the EAC is a shade-adapted community. The pH increased from 8.2 in the bulk seawater to 8.9 at the EAC surface, suggesting that very little carbon in the form of CO2 occurs at the EAC surface. Thus the major source of dissolved inorganic carbon (DIC) must be in the form of HCO3 −. Estimates of DIC fluxes across the DBL indicate that, throughout most of the daytime under in situ conditions, DIC is likely to be a major limiting factor for photosynthesis and therefore also for primary production and growth of the EAC.