Unprecedented mass bleaching and loss of coral across 12° of latitude in Western Australia in 2010-11

Abstract

Background: Globally, coral bleaching has been responsible for a significant decline in both coral cover and diversity over the past two decades. During the summer of 2010-11, anomalous large-scale ocean warming induced unprecedented levels of coral bleaching accompanied by substantial storminess across more than 12° of latitude and 1200 kilometers of coastline in Western Australia (WA).

Methodology/principal findings: Extreme La-Niña conditions caused extensive warming of waters and drove considerable storminess and cyclonic activity across WA from October 2010 to May 2011. Satellite-derived sea surface temperature measurements recorded anomalies of up to 5°C above long-term averages. Benthic surveys quantified the extent of bleaching at 10 locations across four regions from tropical to temperate waters. Bleaching was recorded in all locations across regions and ranged between 17% (±5.5) in the temperate Perth region, to 95% (±3.5) in the Exmouth Gulf of the tropical Ningaloo region. Coincident with high levels of bleaching, three cyclones passed in close proximity to study locations around the time of peak temperatures. Follow-up surveys revealed spatial heterogeneity in coral cover change with four of ten locations recording significant loss of coral cover. Relative decreases ranged between 22%-83.9% of total coral cover, with the greatest losses in the Exmouth Gulf.

Conclusions/significance: The anomalous thermal stress of 2010-11 induced mass bleaching of corals along central and southern WA coral reefs. Significant coral bleaching was observed at multiple locations across the tropical-temperate divide spanning more than 1200 km of coastline. Resultant spatially patchy loss of coral cover under widespread and high levels of bleaching and cyclonic activity, suggests a degree of resilience for WA coral communities. However, the spatial extent of bleaching casts some doubt over hypotheses suggesting that future impacts to coral reefs under forecast warming regimes may in part be mitigated by southern thermal refugia.

Figure 1. Satellite SST anomalies, thermal stress and in situ temperature profiles.

(A) Onset and evolution of NOAA Coral Reef Watch Sea Surface Temperature Anomalies (SSTA) and Degree Heating Week (DHW) metrics at 0.5°-latitude resolution during the 6 month period over the summer of 2010–11 overlaid with location of surveyed regions. (B) Indicative regional in-situ temperature logger records. (▾) indicate timing of coral bleaching surveys. (Abbreviations: MBI – Montebello and Barrow Islands; NIN – Ningaloo Reef; HAI – Houtman Abrolhos Islands; and, PER – Perth metropolitan waters.).

Figure 2. Map of cyclone tracks and survey locations.

Map of cyclone tracks for the period November 2010– May 2011. Cyclone tracks shown as solid lines with thickness of the tracks proportional to strength category (maximum strength category 4); dashed-lines indicate tropical low before or after cyclone designation. Locations of quantitative and semi-quantitative survey sites shown as filled circles (see legend in graph).

Figure 3. Mean proportion of coral bleaching and absolute change in coral cover.

(A) Mean proportion of coral bleached by location. (B) Mean change in absolute coral cover (values in parentheses represent proportional change in coral cover). Error bars represent ±1SE. (*) denotes significant difference from 0 where 95% confidence intervals failed to intersect the horizontal axis.

Figure 4. Partial residual plots of best GAM model predictor variables for coral bleaching analysis.

Partial residual plots of the modeled relationships between proportion of bleaching (y-axes), (A) maxDHD, (B) depth, and (C) location (mean-centered). Grey lines represent model-fitted splines of the estimated smoothing functions bounded by 95% confidence limits (solid blue shading). Data points represent distribution of raw partial residuals.

Figure 5. Partial residual plots of best GAM model predictor variables for coral cover change analysis.

Partial residual plots of the modeled relationship between absolute change in coral cover (y-axes), (A) pre-bleaching coral cover, (B) minimum distance from cyclone track, and (C) location (mean-centered). Grey lines indicates model-fitted splines of the estimated smoothing function bounded by 95% confidence limits (solid blue shading). Data points represent distribution of raw partial residuals.