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. Plain Language Summary Coral reefs are regarded as the rainforests of the sea, supporting 25% of marine species and millions of people worldwide. Yet, they are facing a number of threats including rising sea surface temperature, decreasing ocean pH, overfishing and increased competition with algae. As coral reefs experience stress, the relative abundance of hard coral, algae, soft corals and sponges can change. A healthy reef is generally thought of as one with a high proportion of hard coral as their skeletons form the backbone of the reef environment. Under stress, however, a reef can transition from hard coral to algae dominated with significant implications for the entire reef ecosystem. In this paper, we suggest that the reef metabolism, which is driven by benthic composition, drives the natural variability and the relationship between pH and dissolved oxygen (DO) over a 24-hr cycle. Our data suggest that the daily range in pH and DO is larger at sites with more hard coral and less algae, and the relationship between pH and DO is stronger. High resolution measurements of pH and DO using autonomous sensors could potentially reveal vital information on ecosystem functioning and may be valuable tools in monitoring changes in coral reefs.