Drivers of carbon isotopic fractionation in a coral reef lagoon: Predominance of demand over supply

The carbon isotopic signature (δ¹³C) of coral skeletons is influenced by isotopic fractionation (ε_org) during photosynthetic dissolved inorganic carbon (DIC) fixation, but only a few direct ε_org measurements are available in coral communities. In particular, observations of ε_org at the ecosystem scale are lacking. Here we present high frequency (hourly) measurements of DIC and its δ¹³C in the water column and benthic chambers in a highly productive coral reef lagoon (Heron Island, Great Barrier Reef, Australia) and apply simple molar balance calculations to infer community ε_org. Variation in ε_org was between 3.7% and 25.2% in the open lagoon, with lower values during the mid-afternoon and higher values in early morning and evening. The ε_org range was broader (0.3-30.1%) in enclosed benthic chambers with a similar diel pattern. There was a strong correlation between carbon uptake rates and ε_org in closed incubations, suggesting that C demand largely controlled ε_org. Benthic chamber incubations revealed increased ε_org as water circulation increased, implying that C supply to photosynthesizing algae on the sediment also influenced ε_org. Hysteresis in carbon uptake through the day complicated the expected straightforward influence of irradiance on C demand, and consequently on ε_org. These results highlight the need for more in depth understanding on carbon uptake rates to fully understand δ¹³C variation in coral paleo-records.