Effect of light and zooplankton on skeletal δ13C values in the eastern Pacific corals Pavona clavus and Pavona gigantea

Skeletal delta(13)C levels in symbiotic reef corals are believed to be predominantly influenced by metabolic fractionation. Therefore, environmental variables influencing coral metabolism should also affect skeletal delta(13)C levels. To test this hypothesis, we measured the effects of light (which drives photosynthesis) and relative zooplankton levels (heterotrophy) on skeletal delta(13)C values in the corals Pavona clavus and P. gigantea at two depths (1 m and 7 m). For both species, decreases in light or increases in zooplankton resulted in significant decreases in skeletal delta(13)C levels. A significant decrease in delta(13)C values with depth was observed in Pavona gigantea only. Thus, light and zooplankton directly affect coral skeletal delta(13)C values, supporting the hypothesis that metabolic fractionation significantly contributes to skeletal delta(13)C levels. Simultaneous decreases in both light and zooplankton resulted in decreases in skeletal delta(13)C values, reflecting decreases in light. In Pavona clavus, intra-annual variation in skeletal delta(13)C values over one year correlated with seasonal changes in irradiance. Further study is needed to resolve how skeletal delta(13)C values vary at intermediate levels of irradiance and zooplankton, and in other coral species.