Production and Respiration in the Red Sea Coral Stylophora pistillata as a Function of Depth

Colony morphology, rates of production and respiration, translocation of carbon from symbiotic algae to host, and the daily contribution of carbon fixed by zooxanthellae to animal respiration demands (CZAR) in phenotypes of Stylophora pistillata from 3 and 35 m were compared. Corals from 35 m showed an increase in branch density, a decrease in zooxanthellae density, and an increase in chlorophyll a per algal cell when compared to colonies from 3 m. These changes are explained as adaptations to limited photosynthetically active radiation at the deeper depth. Photosynthetic efficiency was higher at 35 m, as evidenced by a production rate 25% that at 3 m, but with light only about 8% that of shallow water irradiance. Respiration of deeper corals decreased by a half. A depth-specific respiratory decline was displayed by both the algae and the animal fractions. Decreased coral animal respiration appears to be a direct function of decreased photosynthetically fixed carbon availability, and to be an immediate response to daily carbon input. Decreased carbon availability to the host animal at 35 m was a consequence of both decreased net carbon fixation and decreased percentage of net fixed carbon translocated to the host. The daily CZAR at 35 m was less than half that at 3 m. Mean CZAR at 35 m was 78%, suggesting that deeper corals have an obligate requirement for heterotrophically obtained carbon. By contrast, corals from 3 m, which displayed a mean CZAR of 157%, appeared to be phototrophic with respect to carbon required for respiration. Altered trophic strategies with depth were confirmed by daily carbon budgets calculated for average size corals from both depths. Multiple correlation tests of all parameters confirmed the utility of expressing production and respiration measures in terms of unit surface area. However, significant correlations with other normalizing parameters were found, and their usefulness discussed.