Turning up the Heat: Increasing Temperature and Coral Bleaching at the High Latitude Coral Reefs of the Houtman Abrolhos Islands

Abstract

BackgroundCoral reefs face increasing pressures particularly when on the edge of their distributions. The Houtman Abrolhos Islands (Abrolhos) are the southernmost coral reef system in the Indian Ocean, and one of the highest latitude reefs in the world. These reefs have a unique mix of tropical and temperate marine fauna and flora and support 184 species of coral, dominated by Acropora species. A significant La Niña event during 2011 produced anomalous conditions of increased temperature along the whole Western Australian coastline, producing the first-recorded widespread bleaching of corals at the Abrolhos.Methodology/ Principal FindingsWe examined long term trends in the marine climate at the Abrolhos using historical sea surface temperature data (HadISST data set) from 1900–2011. In addition in situ water temperature data for the Abrolhos (from data loggers installed in 2008, across four island groups) were used to determine temperature exposure profiles. Coupled with the results of coral cover surveys conducted annually since 2007; we calculated bleaching thresholds for monitoring sites across the four Abrolhos groups.Conclusions/ Significance In situ temperature data revealed maximum daily water temperatures reached 29.54°C in March 2011 which is 4.2°C above mean maximum daily temperatures (2008–2010). The level of bleaching varied across sites with an average of ∼12% of corals bleached. Mortality was high, with a mean ∼50% following the 2011 bleaching event. Prior to 2011, summer temperatures reached a mean (across all monitoring sites) of 25.1°C for 2.5 days. However, in 2011 temperatures reached a mean of 28.1°C for 3.3 days. Longer term trends (1900–2011) showed mean annual sea surface temperatures increase by 0.01°C per annum. Long-term temperature data along with short-term peaks in 2011, outline the potential for corals to be exposed to more frequent bleaching risk with consequences for this high latitude coral reef system at the edge of its distribution.

Unprecedented Mass Bleaching and Loss of Coral across 12° of Latitude in Western Australia in 2010–11

“…The relationship between depth and coral bleaching may partially relate to depth-mediated flow regimes [58], [60], and water column light attenuation properties which reduce incident light reaching the coral surface [61]. Although our study showed that bleaching was more prevalent in shallow water, deep water surveys (15–30 m) around Perth and the Houtman Abrolhos islands revealed considerable bleaching [62], [63], emphasising both the spatial extent and vertical distribution of the 2010/11 warm water event along the WA coast.…”

Corals record long-term Leeuwin current variability including Ningaloo Niño/Niña since 1795

“…The intra-annual variation in salinity of ~0.4 psu is also low12. However, interannual variability of mean annual HAI SST is high, with La Niña years being significantly warmer (annual mean temperature in 2011 was 1.5 °C above the twentieth century average SST) than El Niño years10.…”

Isolation by resistance across a complex coral reef seascape

“…The effects of the disturbance at the HAI were patchy, and bleaching varied widely within each island group and across the archipelago [85], making it difficult to draw any conclusions about the effects of this disturbance on the genetic structure of local coral populations, particularly because the small number of departures from HWE and LD suggests that populations are in mutation-drift equilibrium. In conclusion, there is an urgent need to better understand fine-scale processes driving larval dispersal and gene flow across coral reef archipelagos, so that spatial conservation management can be tailored to accurately reflect patterns of larval transport and contemporary source-sink dynamics.…”