The ratio of stable carbon isotopes 13C/12C (δ13C) in symbiotic reef coral skeletons is believed to be predominantly influenced by metabolic fractionation. Environmental variables influencing coral metabolism such as sunlight, which drives photosynthesis, and the abundance of zooplankton prey, will alter skeletal δ13C levels. Here, I experimentally evaluated the hypotheses that: 1) as solar irradiance decreases, skeletal δ13C decreases, and 2) as zooplankton increases, skeletal δ13C decreases. I then attempted to apply the results towards the paleoreconstruction of solar irradiance and zooplankton-rich upwelling events from coral δ13C records.
The effects of light and zooplankton levels on skeletal δ13C were evaluated with field experiments on the corals Pavona clavus and Pavona gigantea. Coral were reared in the field under 100% and 5% light with either ambient or reduced zooplankton. For both species, decreases in light or increases in zooplankton resulted in significant decreases in skeletal δ13C values. High resolution analysis of the intra-annual variation in skeletal δ13C indicated that Pavona is a reliable recorder of seasonal changes in irradiance and zooplankton.

The effects of intermediate changes in light and brine shrimp on the skeletal δ13C was evaluated with a tank experiment using Porites compressa corals. Coral were reared in an outdoor flow-through tank under 112%, 100%, 75% and 50% light conditions and fed either zero, low, medium or high concentrations of brine shrimp. Decreases in light from 100% resulted in significant decreases in δ13C. Increases in brine shrimp resulted in δ13C increases. This latter result seems attributable to enhanced nitrogen levels associated with brine shrimp.
A survey of skeletal δ13C across depth in the corals Porites compressa, Porites lobata and Montipora verrucosa was conducted. Skeletal δ13C significantly differed among species and decreased with depth in P. compressa and P. lobata. High resolution analysis on the intra-annual variation in skeletal δ13C in P. compressa reflected seasonal changes in solar irradiance.
This research shows that seasonal variation in solar irradiance is reliably recorded by coral skeletal δ13C levels. However, further research is need to decouple the indirect nutrient versus direct zooplankton effects on coral skeletal δ13C.