The ratio of stable carbon isotopes 13C/12C ( 13C) 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 13C levels. Here, I experimentally evaluated the hypotheses that: 1) as solar irradiance decreases, skeletal 13C decreases, and 2) as zooplankton increases, skeletal 13C decreases. I then attempted to apply the results towards the paleoreconstruction of solar irradiance and zooplankton-rich upwelling events from coral 13C records.

The effects of light and zooplankton levels on skeletal 13C were evaluated with field experiments on the Panamanian corals Pavona clavus and Pavona gigantea. Coral were reared in the field under 100% and 5% light with either ambient or reduced zooplankton for one year. For both species, decreases in light or increases in zooplankton resulted in significant decreases in skeletal 13C values. High resolution analysis of the intra-annual variation in skeletal 13C 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 13C was evaluated with a tank experiment at the Hawaii Institute of Marine Biology using Hawaiian 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 for three months. Decreases in light from 100% resulted in significant decreases in 13C. Increases in brine shrimp resulted in 13C increases. This latter result seems attributable to enhanced nitrogen levels associated with brine shrimp.

A survey of skeletal 13C across depth in the Hawaiian corals Porites compressa, Porites lobata and Montipora verrucosa was conducted in 1996-97. Skeletal 13C significantly differed among species and decreased with depth in P. compressa and P. lobata but did not vary between field sites. High resolution analysis on the intra-annual variation in skeletal 13C in P. compressa reflected seasonal changes in solar irradiance.

This research shows that seasonal variation in solar irradiance is reliably recorded by coral skeletal 13C levels. However, further research is need to decouple the indirect nutrient versus direct zooplankton effects on coral skeletal 13C.

Note: 13C should read greed delta, superscript 13, C.