Calcification rate and the stable carbon, oxygen, and nitrogen isotopes in the skeleton, host tissue, and zooxanthellae of bleached and recovering Hawaiian corals

We tested the effectiveness of stable isotopes as recorders of physiological changes that occur during coral bleaching and recovery. Montipora capitata and Porites compressa fragments were bleached in outdoor tanks with seawater temperature raised to 30 degrees C (treatment corals) for one month. Additional fragments were maintained at 27 degrees C in separate tanks (control corals). After one month, (0 months recovery), buoyant weight was measured and a subset of fragments was frozen. Remaining fragments were returned to the reef for recovery. After 1.5, 4, and 8 months, fragments were collected, measured for buoyant weight, and frozen. Fragments were analyzed for stable carbon and oxygen isotopic compositions of the skeleton (delta C-13(s); delta O-18(s)) and nitrogen and carbon isotopic compositions of the host tissue (delta N-15(h); delta C-13(h)) and zooxanthellae (delta N-15(z); delta C-13(z)). delta C-13(s) decreased immediately after bleaching in M. capitata, but not in P. compressa. delta O-18(s), of both species failed to record the warming event. During the remaining months of recovery, delta C-13(s) and delta O-18(s) were more enriched in treatment than control corals due to decreases in calcification and metabolic fractionation during that time. Increased delta N-15(h) of treatment P. compressa may be due to expelled zooxanthellae during bleaching and recovery. Increased delta N-15(z) at 1.5 months in treatment fragments of both species reflects the increased incorporation of dissolved inorganic nitrogen to facilitate mitotic cell division and/or chl a/cell recovery. Changes in delta C-13(h) and delta C-13(z) at 1.5 months in treatment M. capitata indicated a large increase in heterotrophically acquired carbon relative to photosynthetically fixed carbon. We experimentally show that isotopes in coral skeleton, host tissue and zooxanthellae can be used to verify physiological changes during bleaching and recovery, but their use as a proxy for past bleaching events in the skeletal record is limited. (c) 2006 Elsevier Inc. All rights reserved.