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 Skeletal δ¹³C levels in symbiotic reef corals are believed to be predominantly influenced by metabolic fractionation. Therefore, environmental variables influencing coral metabolism should also affect skeletal δ¹³C levels. To test this hypothesis, we measured the effects of light (which drives photosynthesis) and relative zooplankton levels (heterotrophy) on skeletal δ¹³C 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 δ¹³C levels. A significant decrease in δ¹³C values with depth was observed in Pavona gigantea only. Thus, light and zooplankton directly affect coral skeletal δ¹³C values, supporting the hypothesis that metabolic fractionation significantly contributes to skeletal δ¹³C levels. Simultaneous decreases in both light and zooplankton resulted in …