Ultraviolet Light and Temperature Effects on Reef-Building Corals

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Recent explanations for the 1987 Caribbean-wide, mass coral bleaching event suggest that increased sea surface temperatures and enhanced ultraviolet light penetration were major contributing factors. This project will investigate this hypothesis by conducting a series of experiments designed to partition the relative effects that UV light and temperature have on coral growth and survivorship. Appropriate experimental levels for UV irradiation will be based on data derived from: i) long-term UV flux measurements recorded in 1989, ii) preliminary field experiments assessing effects of enhanced UV exposures, and iii) UV measurements to be taken in clear tropical oceanic waters. Experimental corals (Montastrea annularis and Agaricia lamarcki) held outdoors in UV-transparent acrylic aquaria will be subject to two UV light regimes: 1) ambient UV flux, based on values obtained form prior long-term monitoring (controls), and 2) maximum UV flux simulating the conditions that occurred during the 1987 bleaching event (based on results from preliminary field experiments identifying UV levels leading to coral stress, or on treatment, corals will be subjected to three controlled temperature regimes: 28oC (control), 30oC and 31oC (peak or near- peak temperatures recorded on reefs in the Caribbean during 1987) for up to two months. Treatment effects will be assessed by quantifying changes in colony color, chlorophyll concentration, zooxanthellae densities, mitotic indices and the concentrations of UV blocking pigments form coral tissues (using HPLC). A second experiment will separate the differential, and combined, effects of UV-A and -B. In addition to these experiments, we will establish a long- term UV monitoring program on Grand Cayman Island, Cayman Island (site of the NOAA Aquarius habitat program) and will relate these findings to observed intra- and interspecific differences in concentrations of UV pigments in reef corals along a depth gradient. These data will constitute an important baseline that can be used to document yearly and decadal changes in UV light levels resulting from continued deletion of stratospheric ozone.