Global gradients of coral exposure to environmental stresses and implications for local management

Abstract

Background: The decline of coral reefs globally underscores the need for a spatial assessment of their exposure to multiple environmental stressors to estimate vulnerability and evaluate potential counter-measures.

Methodology/principal findings: This study combined global spatial gradients of coral exposure to radiation stress factors (temperature, UV light and doldrums), stress-reinforcing factors (sedimentation and eutrophication), and stress-reducing factors (temperature variability and tidal amplitude) to produce a global map of coral exposure and identify areas where exposure depends on factors that can be locally managed. A systems analytical approach was used to define interactions between radiation stress variables, stress reinforcing variables and stress reducing variables. Fuzzy logic and spatial ordinations were employed to quantify coral exposure to these stressors. Globally, corals are exposed to radiation and reinforcing stress, albeit with high spatial variability within regions. Based on ordination of exposure grades, regions group into two clusters. The first cluster was composed of severely exposed regions with high radiation and low reducing stress scores (South East Asia, Micronesia, Eastern Pacific and the central Indian Ocean) or alternatively high reinforcing stress scores (the Middle East and the Western Australia). The second cluster was composed of moderately to highly exposed regions with moderate to high scores in both radiation and reducing factors (Caribbean, Great Barrier Reef (GBR), Central Pacific, Polynesia and the western Indian Ocean) where the GBR was strongly associated with reinforcing stress.

Conclusions/significance: Despite radiation stress being the most dominant stressor, the exposure of coral reefs could be reduced by locally managing chronic human impacts that act to reinforce radiation stress. Future research and management efforts should focus on incorporating the factors that mitigate the effect of coral stressors until long-term carbon reductions are achieved through global negotiations.

Figure 1. Increasing (S curve) and decreasing (Z curve) sigmoidal membership functions, which were used to standardize the environmental data.

x _a and x _b are the control points for the lower and upper bounds along the stressor gradient; SD is the standard deviation, while is the mean.

Figure 2. Coral reef locations grouped into eleven oceanic provinces after Donner (2009).

Coral reef locations were obtained from Reefbase (http://reefgis.reefbase.org/), WCS coral monitoring sites in the western Indian Ocean, and from Ateweberhan & McClanahan (2010).

Figure 3. Correspondence bi-plots of the oceanic provinces based on the three stress groups (radiation, reducing, and reinforcing) and based on exposure severity class

(a & b respectively.

Figure 4. Box plots of distribution of, radiation, reducing, and reinforcing stress

(a, b, & c respectively).

Figure 5. Box plots of distribution of combined radiation stress and stress reducing variables, and the overall exposure model

(a & b respectively).

Figure 6. Exposure space bi-plots of reinforcing against combined radiation and reducing variables, with contours showing exposure grades (i.e. low, moderate, high, severe) based on the final exposure model.

Figure 7. Composite layers for radiation, reducing, reinforcing stress categories, and the overall stress model

(a, b, c & d respectively).