Florida Reef Tract Scleractinian Coral Species-Specific Response to Sedimentation Stress

Major input sources, which introduce sediments into reef systems, include dredging, terrestrial runoff, river discharge, hurricane and storm activity, and beach nourishment projects. The effects of sedimentation are dependent on factors including sediment physical-chemical characteristics, frequency of exposure, coral species present, and their stress response capacity. Photo-physiological stress in corals can occur within hours of sediment exposure, and increases linearly with sediment duration, and amount. Particulate characteristics are important in the degree of coral stress response. A strong inverse relationship exists between particulate grain size, and positive correlation with sediment organic content. Only a few coral species can persist in wave-protected regions where silt-sized, nutrient-enriched sediments are deposited. However, wave-exposed areas with nutrient poor coarse-grained sediments are correlated with lower coral mortality, and many species can acclimate to moderate levels of sedimentation. Species-specific coral strategies are important in effecting tolerances, and may result in survival at high levels of, and extended exposure to, sedimentation and turbidity. There is substantial variability in sediment rejection among colonial coral species. For example, Acropora spp. may suffer irreversible mortality under sediment veneering. However, Porites spp. is highly sediment tolerant and has the ability to recover after complete burial. Some species, such as Montipora spp., adapt to sedimentation stress. This is consistent with the often nearly exclusive presence of Montipora spp. and/or Porites spp. in turbid-water habitats. Montastraea cavernosa is highly tolerant to long-term sediment stress. When corals initiate active sediment removal, energy expenditure is altered leading to a reduction in energy available for coral tissue formation, skeletal growth, and reproduction. In assessment of these effects, a relevant study area is one that is actively being modified by these factors. The Florida Reef Tract is suitable for this purpose. It is undergoing assemblage reconstruction, deteriorating health, and possible algal take-over. Loss of less resilient species has resulted in consequent reduction in diversity. Additional effects include decrease in coral cover, preferential colony sizes, and morphologies that indicate a reef system under stress. Coral morphology of colonial corals resilient to sediment input are typically (1) massive or platy formations with high skeletal plasticity containing large polyps, to effectively reject sediment, (2) branched species with small surface area are capable of being cleaned by weak currents, or (3) massive taxa with small polyps that tolerate intermittent veneering by sediments (Sanders and Baron-Szabo, 2005). Site location on the Florida Reef Tract reef is important in determining degree of effect. However, natural local and anthropogenic stressors are responsible for the Caribbean-wide declines. Ongoing onsite and remote sensing data has been collected and coral zonation is a direct response to sedimentation stress. Differences in species have led to distributional spatial variations, with more sediment tolerant corals in regions with high sedimentation rates, and corals adapted to high turbidity in regions of high sediment re-suspension. Carysfort Reef in the northern Keys is considered the most diverse in the Florida Reef Tract, and is the least directly disturbed by humans. However, the magnitude of disturbance on Carysfort Reef, although it is minimal compared other areas on the reef tract, has surpassed the tolerance levels of some corals causing a decrease in diversity and assemblage shifts. In the middle keys, an analysis of loss of A. palmata and A. cervicornis on Looe Key Reef from 1983-2000 showed a decline of 93% and 98% respectively. A higher percent coral cover is found in the Dry Tortugas compared to the Upper Keys. This observed difference could be due to the isolation of the Dry Tortugas from anthropogenic effects and proximity to gyres. The focus of this work is a comprehensive literature review and analyses of data that provides evidence of species-specific sedimentation response in corals, with emphasis on the Florida Reef Tract. The following hypotheses were examined and determined to be correct concerning coral adaptation to sediment stress: 1) Do corals in the Florida Reef Tract exhibit species-specific differences in response to sediment stress? 2) Do coral species have unique coping mechanisms employed to remove sediment? 3) Is the current distribution of corals on reefs within the Florida Reef Tract correlated with sedimentation stress