Reef Building Corals Rely on Symbionts over a Large Depth Range in an Internal Tidal Bore System, Florid Keys Reef Tract

Corals living in shallow waters typically acquire their nutrients and energy from their photosynthetic symbiotic zooxanthellae, whereas deeper corals may rely to a lower extent on photosynthetic derived materials due to lower light levels. Whether these deeper corals feed to a greater extent is hotly debated within the community. Our study was based in the Florida Keys Reef Tract, which is unique because it is characterized by upwelling or tidal bores that periodically bring nutrient rich waters onto the Florida Shelf which can be used by benthic organisms. We separately measured the stable carbon and nitrogen isotope signatures of the coral host and symbiotic zooxanthellae of three species of reef building corals (Porites astreoides, Montastraea cavernosa, and Montastraea faveolata) along a depth gradient (3-35 m) in order to determine the feeding strategies of these corals as well as to assess whether corals were able to utilize upwelled water as a source of nutrients. The δ13C and δ15N values of the zooxanthellae and host tissue of all three species became progressively more depleted in 13C and 15N with depth, and δ15N values were strikingly low. The trend in the δ15N values with depth suggest that feeding is less important at depth. Further, we found a strong correlation between the δ13C values of the host and their zooxanthellae at all depths, suggesting that even as photosynthetic rates decrease with depth, hosts continue to acquire most of their carbon from their symbionts and do not rely to any greater extent on feeding heterotrophically. Finally we suggest that the strikingly low δ15N values for deep water corals are due to the utilization of upwelled water as a source of nutrients