LARGER BENTHIC FORAMINIFERAL POPULATIONS ON THE FLORIDA REEF TRACT REVEAL THE INTERPLAY BETWEEN INSHORE AND OFFSHORE ENVIRONMENTAL STRESSORS

Larger benthic Foraminifera (LBF), which host algal symbionts, are useful bioindicators for coral reefs worldwide; their responses can indicate changes in water quality, as well as presence and intensity of photo-oxidative stress. This study examined LBF populations quarterly for two years at sites where selected parameters for scleractinian corals also were monitored. Population densities of 11 common LBF species were examined at four 6 m deep patch reefs within Biscayne National Park (BNP) and the upper Florida Keys National Marine Sanctuary (FKNMS), and at 3, 6, 9 and 18 m depths in FKNMS. Populations of Amphistegina gibbosa, the dominant species, were also assessed for size distribution and prevalence of bleaching. At the 6 m sites, densities of LBF ranged from 1x10³ to 1x10⁵ m^-2 and were typically highest at Algae Reef (AR), which is near an intact mangrove shoreline. The 6 m site in BNP had the lowest densities of LBF overall. At the 6 m sites, incidences of partial bleaching in adult A. gibbosa were typically below 40%, indicating chronic photo-oxidative stress. Along the depth transect, LBF densities ranged from 5x10³ to 5x10⁵ m^-2, with higher densities at the 9 and 18 m sites. Overall, the bank-margin reefs appeared to provide better habitat for these foraminifers than inshore patch reefs, yet the prevalence of bleaching was higher offshore during summer months. The apparent paradox of higher densities with more bleaching of A. gibbosa with depth along the onshore-offshore transect demonstrates the importance of high water quality to these foraminifers, even though clear water can result in chronic photo-oxidative stress.

Lesion-recovery data for colonies of Orbicella annularis species complex (coral) revealed the same ranking for 6 m sites as LBF abundances: highest at AR and lowest at BNP. In contrast, along the depth transect, lesion recovery was highest at the 3 m patch-reef site and lowest at the 18 m site, consistent with long-term trends of major loss of coral cover on offshore reefs. Algae Reef represented the “sweet spot” among the reefs examined, including highest coral cover and lesion recovery, as well as LBF abundances comparable to the offshore sites. Water transparency data indicated that tannins, originating from mangrove shorelines, can play an important photo-protective role in reef waters.