Abstract
The health and future of coral and coral reef ecosystems remains threatened by global climate change (GCC) and ocean acidification (OA). There is a need to better understand the current health of coral reef ecosystems, particularly those in threshold environments which will likely be affected first by pressures from GCC and OA. This need can be addressed by answering two critical questions: 1) what are the current net ecosystem calcification (NEC) rates in threshold environments? And 2) what physical, biological or chemical parameter is/are driving these calcification rates? Bermuda, located at the northern threshold to support tropical coral reefs, provides an ideal study site to address these questions. To address the first question requires new methods to estimate reef water residence times, a parameter required for estimating NEC rates, but one that has traditionally been difficult to quantify with the necessary accuracy and precision. Two such methods are introduced here, one based on a radio chemical tracer (beryllium-7, 7Be) and one based on a stable chemical tracer (δ18O). Both 7Be and δ18O samples were measured from water samples collected on the Bermuda coral reef platform and yielded spatially and temporally integrated model residence times ranging between 1 and 10 days. To address the second question coral calcification rates were assessed simultaneously with physical (temperature, light) and chemical (seawater aragonite saturation states, Ωarag) properties at both seasonal and hourly timescales to assess how calcification is affected by these physical and chemical driving factors. Seasonal differences in Ωarag only explained a 2.4% change in coral growth, while seasonal differences in light and temperature explained 20% and 26% of the observed 81 mmolCaCO3 m-2 d-1 seasonal change in calcification rates of Porites astreoides and Diploria strigosa. This work contributes to the understanding impacts of GCC and OA on coral reef ecosystems by quantifying current NEC rates across the Bermuda coral reef platform, against which future rates can be assessed. This work also demonstrates that there are multiple driving factors affecting coral calcification rates in situ, though it does not suggest that OA may place a less influential role in governing future changes in coral growth rates as the expected change in Ωarag for 2100 far exceeds the observed seasonal difference in this study.