Coral distribution and bleaching vulnerability areas in Southwestern Atlantic under ocean warming

Abstract

Global climate change is a major threat to reefs by increasing the frequency and severity of coral bleaching events over time, reducing coral cover and diversity. Ocean warming may cause shifts in coral communities by increasing temperatures above coral's upper thermal limits in tropical regions, and by making extratropical regions (marginal reefs) more suitable and potential refugia. We used Bayesian models to project coral occurrence, cover and bleaching probabilities in Southwestern Atlantic and predicted how these probabilities will change under a high-emission scenario (RCP8.5). By overlapping these projections, we categorized areas that combine high probabilities of coral occurrence, cover and bleaching as vulnerability-hotspots. Current coral occurrence and cover probabilities were higher in the tropics (1°S-20°S) but both will decrease and shift to new suitable extratropical reefs (20°S-27°S; tropicalization) with ocean warming. Over 90% of the area present low and mild vulnerability, while the vulnerability-hotspots represent ~ 3% under current and future scenarios, but include the most biodiverse reef complex in South Atlantic (13°S-18°S; Abrolhos Bank). As bleaching probabilities increase with warming, the least vulnerable areas that could act as potential refugia are predicted to reduce by 50%. Predicting potential refugia and highly vulnerable areas can inform conservation actions to face climate change.

Figure 1

Probability maps for current projections and delta maps of projected changes between current and future scenarios of coral occurrence, coral cover and coral bleaching along the Southwestern Atlantic coastline (1°N–27°S latitude). (a) The probability maps for coral occurrence, cover, and bleaching for the current scenario (2000–2014) and (b,c) delta maps between current and the future scenarios (2040–2050 minus 2000–2014, left; and 2090–2100 minus 2000–2014, right) under a “business as usual” warming rate (RCP 8.5). The blue-red and green-purple scale bars represent absolute probability values and delta probability values, respectively. Maps created in ArcMap version 10.2 (https://desktop.arcgis.com/en/arcmap/).

Figure 2

Current and future projections of coral occurrence, coral cover and coral bleaching in the transitioning zone from tropical to subtropical areas in the Brazilian province (20°S–27°S). (a) Current and (b,c) future probability projections of the transitioning zone from the tropical to the subtropical region with bathymetry isobaths represented by the different contours, from 50 to 200 m depth, which are specified in the legend (gln_batimetria_cprm_30 shapefiles, from (https://geoservicos.inde.gov.br/geoserver/web/). The blue-red scale bar represents absolute probability values. Maps created in ArcMap version 10.2 (https://desktop.arcgis.com/en/arcmap/).

Figure 3

Probability map of vulnerability for the current scenario and delta maps of projected changes between current and future scenarios. (a) Vulnerability areas based on the overlap of coral occurrence, coral cover and coral bleaching probabilities of the Brazilian coast for the current (2000–2014) scenario and (b,c) delta maps between current and the future scenarios (2040–2050 minus 2000–2014, left; and 2090–2100 minus 2000–2014, right) under a “business as usual” warming rate (RCP 8.5). The blue-red and green-purple scale bars represent absolute probability values and delta probability values, respectively. Maps created in ArcMap version 10.2 (https://desktop.arcgis.com/en/arcmap/).

Figure 4

Current and future sea surface temperature (SST) of the Brazilian coast from extracted values of the Bio-ORACLE database. (a) Current mean annual SST at the Brazilian coast and (b) the density of mean SST extracted along the Brazilian coast for current (2000–2014), and future 2040–2050 and 2090–2100 projected SST under a “business as usual” warming rate (RCP 8.5). The vertical dotted line and arrow points the average threshold SST (28.7 °C) recorded during global coral bleaching events between 2007 and 2017 (Sully et al., 2019). Note that the majority of the regions of mean SST for 2040–2050 and 2090–2100 will exceed mean SST bleaching for the current decade. Density was performed using the Kernel Density Estimation. The Map was created in ArcMap version 10.2 (https://desktop.arcgis.com/en/arcmap/) and the density graph was plotted using the package “yarrr” in R software.