Effect of light and zooplankton on skeletal δ13C values in the eastern Pacific corals Pavona clavus and Pavona gigantea
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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 decreases in skeletal δ13C values, reflecting decreases in light. In Pavona clavus, intra-annual variation in skeletal δ13C values over one year correlated with seasonal changes in irradiance. Further study is needed to resolve how skeletal δ13C values vary at intermediate levels of irradiance and zooplankton, and in other coral species.
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