Marine Science Faculty Publications

Benthic Foraminiferal Assemblages from Current-Swept Carbonate Platforms of the Northern Nicaraguan Rise, Caribbean Sea

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Calcifying organisms with endosymbiotic algae produce a major portion of the calcium carbonate sediments in tropical oceans. In 1982-1983, global coral bleaching events revealed the emergence of serious breakdowns in these important symbiotic associations. In 1991-1992, similar problems appeared in Amphistegina spp., the most abundant reef dwelling algal symbiont-bearing foraminifer worldwide.

Since summer 1992, A. gibbosa populations from Conch Reef in the Florida Keys (USA) have been monitored for visible evidence of symbiont loss and associated symptoms. Previous reports have revealed seasonal and size trends in incidences of visible symbiont loss along with trends in reproductive anomalies and shell breakage through 1993. Using data collected through 1996, we statistically tested these postulated trends, with the goals of describing population responses to this multi-year stress event and gaining insight into the underlying cause.

Since 1992 the proportions of mottled individuals increased in early spring, peaked in mid summer, and declined in late summer and fall. The summer 1992 peak in bleaching was the most intense, affecting 82% of the population at 18 m whereas only 43% were affected in summer 1996. The disease appears to be degenerative; symbiont loss disproportionately affects the larger size classes.

Population densities crashed in fall 1991 and remained low through the spring and summer months of 1992, at which time juveniles (< 0.6 mm diameter) comprised less than 10% of the population. Because Amphistegina spp. typically reproduce asexually in the spring, the lack of juveniles suggested that attempts to reproduce failed, resulting in anomalously low population abundances. Since 1992, the proportion of juveniles has increased steadily and population density has partly recovered.

Though the stress has not been identified, the trends and type of damage to the population are consistent with a mechanism related to solar irradiance. Whatever the cause, the potential consequences range from impact on local sediment supply to global carbon budgets.

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Journal of Foraminiferal Research, v. 27, issue 1, p. 42-50