POSSIBLE PROXIES FOR STRESS IN REEF-BUILDING CORALS

Environmental stressors must have operated on coral reefs in the Pleistocene and Holocene epochs, but indicators in the fossil record are rare. Stress may not always result in a large scale marker such as a community phase shift. Reef corals are sessile organisms which produce incremental structures with the potential to preserve information about their growth and interaction with the surrounding environment. The skeletal density, stable isotope fraction content, and trace metal content of scleractinian coral skeletons have been used as a proxy for paleoclimatic conditions. These indicators may also record stress, disturbance, and/or disease of reef-building corals in the fossil record.

Potential skeletal indicators of environmental stress were investigated. Skeletal material from Acropora cervicornis, known to have affected by bleaching and white band disease, were compared to skeletal material from A. cervicornis known to have been healthy. Trace metal content of the skeletal material was measured by ICPMS. Preliminary results show elevated concentrations of Al, Cd, K, Mg, Mn, P, and Se in the sample of material from the diseased A. cervicornis compared with the sample from the healthy coral. Elevated manganese concentration in coral skeletons has been correlated to elevated sea surface temperatures. Elevated cadmium in coral skeletons has been correlated to the presence of phosphate and nitrate in surface waters, which, in excess, can cause stress in reef-building corals.

Morphological differences were investigated by sectioning and observation with a light microscope. Corallites on samples from the diseased A. cervicornis appear to have weakened structural elements, but, because of possible post-mortem erosion, this result is inconclusive.

If proxies for coral stress exist, the Pleistocene and Holocene fossil record could become a source of information about the permanence of the damage done to coral reef communities, the effect on the biodiversity of the reef community, and the prognosis for recovery of today's reef systems.