We studied the effect of water flow on the reduction of the photosynthetic electron transport rates (ETR) of the seagrass Thalassia testudinum and the brown algae Lobophora variegata and Padina sanctae-crucis. The main objective of this study was to contribute to the understanding of the magnitude of carbon limitation in a sheltered, shallow coral reef environment, while improving the use of the fluorescence technique. The results confirm that photosynthesis, in the 3 species, is sink limited

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... normal seawater pH. Considering that the actinic light intensity chosen for the experimental design was close to saturation irradiance for the 3 species, stronger ETR declines are expected under larger carbon requirements at supersaturating irradiance. The results suggest that photosynthesis may be strongly carbon limited in sheltered coral reef environments. Nevertheless, pharmacological experiments revealed the occurrence of carbon concentration mechanisms (CCM) related to: (1) the external presence of the enzyme carbonic anhydrase under very low hydrodynamics, when the diffusive boundary layers stabilize and enzyme losses are minimal (L. variegata and T. testudinum) and (2) the ability to take up HCO 3 -(T. testudinum and P. sanctae-crucis). The possibility that the capacity to calcify may be associated with the CCM of P. sanctae-crucis needs further testing. These results suggest that photosynthetic organisms adapted to coral reef systems, where calm, cloudless days with minimal water turbulence and maximum light penetration are common, have developed efficient CCMs. The study also supports the utility of the fluorescence technique for the analysis of the photosynthetic response to hydrodynamics within the limits imposed by the linearity between oxygen-based and fluorescence-based photosynthesis determinations. It also emphasizes the importance of light absorption determinations for accurate estimations of ETR values.