Characterizing reef net metabolism via the diel co‐variation of pH and dissolved oxygen From high resolution in situ sensors.

Coral reefs are subject to degradation by multiple environmental stressors which are predicted to intensify. Stress can alter ecosystem composition, with shifts from hard coral to macroalgae dominated reefs often accompanied by an increase in soft corals and sponges. Such changes may alter net ecosystem metabolism and biogeochemistry by shifting the balance between photosynthesis, respiration, calcification and dissolution. We deployed high temporal resolution pH and dissolved oxygen (DO) sensors at four Caribbean reef sites with varying covers of hard and soft corals, sponges and macroalgae. The resultant data indicated that the strength of the “metabolic pulse”, specifically the co-variation in daily pH and DO oscillations, was driven by the net balance of light -dependent and -independent metabolism. pH and DO were positively correlated over the diel cycle at coral dominated sites, suggesting that photosynthesis and respiration were the major controlling processes, and further indicated by agreement with a simple production:respiration model. Whereas, at a site with high macroalgal cover, pH and DO decoupling was observed during daylight hours. This indicates that an unidentified light-driven process altered the expected pH:DO relationship. We hypothesize that this could be mediated by the higher levels of macroalgae, which either stimulated bacterial-mediated carbonate dissolution via the production and release of allelopathic compounds or retained oxygen, evolved during photosynthesis, in the gaseous form in seawater (ebullition). Our work demonstrates that high resolution monitoring of pH and DO provides insight into coral reef biogeochemical functioning and can be key for understanding long-term changes in coral reef metabolism.