Effect of light and brine shrimp on skeletal δC in the Hawaiian coral Porites compressa: a tank experiment

Previous experimental fieldwork showed that coral skeletal δ¹³C values decreased when solar intensity was reduced, and increased in the absence of zooplankton. However, actual seasonal changes in solar irradiance levels are typically less pronounced than those used in the previous experiment and the effect of increases in the consumption of zooplankton in the coral diet on skeletal δ¹³C remains relatively unknown. In the present study, the effects of four different light and heterotrophy regimes on coral skeletal δ¹³C values were measured. Porites compressa corals were grown in outdoor flow-through tanks under 112%, 100%, 75%, and 50% light conditions at the Hawaii Institute of Marine Biology, Hawaii. In addition, corals were fed either zero, low, medium, or high concentrations of brine shrimp. Decreases in light from 100% resulted in significant decreases in δ¹³C that is most likely due to a corresponding decrease in photosynthesis. Increases in light to 112% also resulted in a decrease in δ¹³C values. This latter response may be a consequence of photoinhibition. The overall curved response in δ¹³C values was described by a significant quadratic function. Increases in brine shrimp concentrations resulted in increased skeletal δ¹³C levels. This unexpected outcome appears to be attributable to enhanced nitrogen supply associated with the brine shrimp diet which led to increased zooxanthellae concentrations, increased photosynthesis rates, and thus increased δ¹³C values. This result highlights the potential influence of nutrients from heterotrophically acquired carbon in maintaining the zooxanthellae-host symbiosis in balance. In addition, evidence is presented that suggests that coral skeletal growth and δ¹³C are decoupled. These results increase our knowledge of how light and heterotrophy affects the δ¹³C of coral skeletons.