DOC concentrations across a depth-dependent light gradient on a Caribbean coral reef

Area of Science:

• Marine Ecology
• Biogeochemistry
• Coral Reef Ecosystems

Background:

• Benthic primary producers (BPP) like algae and corals release photosynthates, fueling dissolved organic carbon (DOC) production on coral reefs.
• Elevated DOC concentrations are often observed near BPP, with release rates influenced by light availability.
• A depth-dependent light gradient on reefs suggests DOC concentrations might vary with light intensity.

Purpose of the Study:

• To investigate in situ DOC concentrations near the alga Dictyota sp. and the coral Orbicella faveolata along a depth gradient.
• To compare DOC concentrations near BPP with background levels in the water column under varying light conditions.
• To assess the role of light availability and potential photoinhibition in regulating DOC release from BPP.

Main Methods:

• Measured in situ DOC concentrations and light intensity near Dictyota sp. and O. faveolata at 5, 10, and 20 m depths.
• Collected water samples for background DOC concentration measurements in the surrounding water column.
• Conducted ex situ measurements of excitation pressure on photosystem II to assess photoinhibition.

Main Results:

• DOC concentrations near Dictyota sp. were elevated by 15 µmol C L^-1 at 10 m depth (intermediate light) but not at 5 m (high light) or 20 m (low light).
• No significant DOC elevation was observed near the coral O. faveolata at any depth.
• Photoinhibition of Dictyota sp. was suggested at high light intensities (5 m depth), potentially limiting DOC release.

Conclusions:

• Elevated DOC concentrations near BPP do not solely follow a simple light gradient across depth.
• Factors including water type, light availability, photoinhibition, and water movement interact to determine DOC concentrations near BPP.
• The physiological state of BPP, such as photoinhibition in algae, can modulate their DOC release and influence local DOC dynamics.