DOC concentrations across a depth gradient on a Caribbean coral reef

The dissolved organic carbon (DOC) pool on tropical coral reefs is mainly fueled by photosynthates released from benthic primary producers (BPP), such as reef algae and scleractinian corals. DOC concentrations near BPP have repeatedly been observed to be elevated compared to those in the surrounding water column. As the DOC release of BPP increases with increasing light availability, elevated DOC concentrations near them will, in part, also depend on light availability. Consequently, DOC concentrations are likely to be higher on the shallow, well-lit reef terrace than in deeper sections on the fore reef slope. We measured in situ DOC concentrations and light intensity in close proximity to the reef alga Dictyota sp. and the scleractinian coral Orbicella faveolata along a depth gradient from 5 to 20 m depth and compared these to background concentrations in the water column. DOC concentrations near Dictyota sp. were significantly higher at 10 m than at 5 and 20 m depth. Furthermore, at 10 m DOC concentrations near Dictyota sp. were elevated by 15 μmol C L-1 compared to background concentrations in the water column, but not at 5 and 20 m. DOC concentrations near O. faveolata and in the water column did not differ between depths and concentrations near O. faveolata were not elevated compared to background concentrations at any of the tested depths. Our results indicate that DOC concentrations near Dictyota sp. can differ along a depth gradient from 5 to 20 m. However, the occurrence of elevated DOC concentrations did not follow a natural light gradient across depth. Instead, a combination of light availability (including a restriction by photoinhibition) and water movement are proposed to interactively determine the DOC concentrations in the close vicinity of BPP across the reef slope.