Responses of Amphistegina gibbosa populations in the Florida Keys (U.S.A.) to a multi-year stress event (1991-1996)

Abstract

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.

Ultrastructural Responses in Field‐Bleached and Experimentally Stressed Amphistegina gibbosa (Class Foraminifera)

“…In addition, we documented that bleaching visible in reflected light typically began in individuals ∼ 0.6 mm in diam. and increased as the foraminifers grew (Williams et al 1997). The second enigma was “How could these protists continue to grow at the same time their cytoplasm was deteriorating?” The answer may be in the ectoplasm, which appears to function somewhat independently of the endoplasm (see also Travis and Bowser 1991).…”

“…Because these protists are mobile and exhibit phototaxic behavior (Lee, McEnery, and Garrison 1980; Zimiri et al 1974), the dose of radiant energy they actually experience in their microhabitat is unknown. However, our experimental results strengthen previous field and laboratory evidence (Hallock et al 1995; Talge et al 1997; Williams et al 1997) implicating photic stress rather than thermal stress in bleaching of A. gibbosa in the Florida Keys in the 1990s.…”

“…Sites and collection techniques are described in previous publications (e.g. Hallock et al 1995; Talge et al 1997; Williams et al 1997). Field samples, which included foraminifers and associated sediment from the collection site, were maintained in seawater in 1‐liter, wide‐mouth screw‐cap jars in an environmental chamber at 25 °C, at intensities of photosynthetically‐active radiation (PAR) of ∼ 5–8 ixmole photons m −2 s −1 on a 12‐h light/dark cycle (i.e.…”

Effectiveness of benthic foraminiferal and coral assemblages as water quality indicators on inshore reefs of the Great Barrier Reef, Australia

“…In addition, smaller size and faster turnover will allow foraminiferal assemblages to recover and reach successional endpoints more rapidly. Although ecological studies on foraminiferal succession in the Pacific Region are lacking, ecological studies of symbiont-bearing foraminifera in the Florida reef tract have shown that populations recover from bleachingrelated mortality events or hurricane scouring in a matter of months to at most a year or two (Williams et al 1997;Hallock et al 2006, Baker et al 2009). This is in contrast with coral communities which may need decades to fully recover colony density and diversity (Done et al 2007;Wakeford et al 2008).…”

In Situ Observations of Foraminiferal Bleaching in the Maldives, Indian Ocean

“…Bleaching in foraminifers was first described in laboratory experiments (Hallock et al, 1986), however, it was unknown in field populations until Hallock et al (1993) collected several specimens of Amphistegina gibbosa that appeared "spotted" during a post-bleaching coral survey in 1988. Since 1991, some bleaching of A. gibbosa has been consistently observed in field samples from the Florida Keys during summer months (Hallock et al, 1995;Williams et al, 1997;Mendez-Ferrer et al, 2018). Symbiont loss may affect over 80% of the total adult Amphistegina population; individual specimens display unusual color sometimes restricted to a few white spots, or progressing to near absence of symbiont color (Hallock et al, 1995;Mendez-Ferrer et al, 2018).…”