Transient turbid water mass reduces temperature-induced coral bleaching and mortality in Barbados

Abstract

Global warming is seen as one of the greatest threats to the world's coral reefs and, with the continued rise in sea surface temperature predicted into the future, there is a great need for further understanding of how to prevent and address the damaging impacts. This is particularly so for countries whose economies depend heavily on healthy reefs, such as those of the eastern Caribbean. Here, we compare the severity of bleaching and mortality for five dominant coral species at six representative reef sites in Barbados during the two most significant warm-water events ever recorded in the eastern Caribbean, i.e., 2005 and 2010, and describe prevailing island-scale sea water conditions during both events. In so doing, we demonstrate that coral bleaching and subsequent mortality were considerably lower in 2010 than in 2005 for all species, irrespective of site, even though the anomalously warm water temperature profiles were very similar between years. We also show that during the 2010 event, Barbados was engulfed by a transient dark green turbid water mass of riverine origin coming from South America. We suggest that reduced exposure to high solar radiation associated with this transient water mass was the primary contributing factor to the lower bleaching and mortality observed in all corals. We conclude that monitoring these episodic mesoscale oceanographic features might improve risk assessments of southeastern Caribbean reefs to warm-water events in the future.

Keywords: Anomalously warm SST; Barbados reefs; Coral bleaching; Coral mortality; Transient water masses.

Figure 1. Map of Barbados indicating location and characteristics of the six reef sites surveyed, and the two current velocity meters.

The inset shows the location of Barbados in the Caribbean. The pie charts show relative abundance of the five selected dominant coral species at each site. SSID, Siderastrea siderea; DSTR, Diploria strigosa; MCAV, Montastraea cavernosa; OANN, Orbicella annularis; PAST, Porites astreoides.

Figure 2. Average percent of bleached colonies during peak bleaching (September/October) and the following post-bleaching (February and June) survey periods in 2005–2006 (black squares) and 2010–2011 (white circles) for the five selected dominant coral species (A–E) across the six study sites (1–6).

Error bars denote 1 standard error. Lack of symbols in b3 panel indicates that no M. cavernosa colonies were sampled in Sept/Oct 2005 and in June 2006 at the North Bellairs site; and in c2 panel no O. annularis colonies were sampled in Sept/Oct 2005 at the Coconut Court site.

Figure 3. Average percent of recently dead coral tissue per colony during the post-bleaching survey periods (February and June) in 2006 (black squares) and 2011 (white circles) for the five selected dominant coral species (A–E) across the six study sites (1–6).

Error bars denote 1 standard error. Missing symbol in b3 panel indicates that no M. cavernosa colonies were sampled at the North Bellairs site in June 2006.

Figure 4. Comparison of the temperatures experienced by coral reefs in Barbados during the 2005/2006 (blue line) and 2010/2011 (red line) mass bleaching events.

(A) shows mean daily in situ water temperatures (data are only available from June 2005); (B) shows satellite-derived accumulated heating stress as bi-weekly computed degree heating weeks (http://coralreefwatch.noaa.gov/satellite/vs/data_timeseries/vs_ts_Barbados.txt).

Figure 5. Comparison of environmental conditions experienced by coral reefs in Barbados during the 2005 (blue) and 2010 (red) warm water events.

Data are from MODIS-Aqua satellite 4 km computed monthly means for (A) sea surface temperature (SST), (B) an index of coloured dissolved organic matter (CDOM), (C) particulate organic carbon concentration (POC), and (D) chlorophyll-a concentration in water surrounding Barbados (lat. 12.889–13.506°N, long. 59.900–59.237°W) (http://gdata1.sci.gsfc.nasa.gov/daac-bin/G3/gui.cgi?instance_id=ocean_month). A1--D1 show monthly means for both years and median values over an 8-year period (2003--2010) (black dashed line with bars indicating 1st and 3rd quartiles). A2--D2 show a comparison of the deviation from the median values for 2005 and 2010. (Maps of time-averaged data for CDOM, POC and Chl-a, are given in Figs. S1 and S2).

Figure 6. Photographs taken with the same camera, between 10 am and 3 pm during the coral bleaching surveys in October of the two warm-water events in 2005 (A1--D1) and 2010 (A2--D2) on Barbados reefs, showing the marked difference in water colour and light penetration.

(A) North Bellairs fringing reef; (B) Batts Rock patch reef; (C) Atlantis bank reef; (D) Coconut Court patch reef.