ECOLOGIC EFFECTS OF OZONE DEPLETION: A CASE STUDY USING THE SYMBIONT-BEARING FORAMINIFER AMPHISTEGINA GIBBOSA

The damaging effects of chlorofluorocarbons and other chemicals on stratospheric ozone are relatively well documented. Recognizing the influence of consequent biologically-damaging ultraviolet radiation (UVB) on marine ecosystems is confounded by the multitude of other anthropogenic impacts including global warming and aquatic acidification by rising atmospheric CO₂ concentrations, and pollution of coastal waters by nutrients and toxic chemicals. Nevertheless, a decade-long study of larger foraminiferal populations implicates ozone depletion in symbiont-loss ("bleaching") and associated stress symptoms observed in Amphistegina spp. worldwide since 1991. Amphistegina gibbosa populations in the Florida Keys began to exhibit bleaching stress in summer 1991, within weeks of the eruption of Mt. Pinatubo, which increased ozone depletion for several years. Since 1992, populations have exhibited bleaching stress that follows the solar cycle, with peak symptoms near the summer solstice, and up to two months before the late summer sea-surface temperature (SST) maximum. Comparisons among reefs indicate that symptoms increase with water transparency. Laboratory experiments show that growth rates in A. gibbosa are highest at irradiance levels very near those that induce bleaching, and that shorter wavelengths of light energy (blue to UVB) accelerate bleaching. Coral bleaching, very rare prior to 1981, now threatens reefs worldwide. Although mass bleaching events correlate with elevated SSTs, several lines of evidence also implicate increasing UVB. Coral bleaching is a photoxidative response, i.e., corals become more sensitive to light as temperature rises. Peak years for coral bleaching in the 1990s were also peak bleaching years for Amphistegina. Some coral species commonly exhibit bleaching symptoms 1-3 months before sea-surface temperatures become critical. A field study of molecular indicators of photoxidative stress in Montastrea from the Florida Keys revealed stress in May 1999, when SST was still relatively low. Amphistegina spp. live on coral reefs but are independent of coral biology. As indicator organisms, these protists provide critical evidence implicating ozone depletion, as well as global warming, in the rapid decline of coral reefs worldwide.