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In summer 2010, a bleaching event decimated the abundant reef flat coral Stylophora pistillata in some areas of the central Red Sea, where a series of coral reefs 100-300 m wide by several kilometers long extends from the coastline to about 20 km offshore. Mortality of corals along the exposed and protected sides of inner (inshore) and mid and outer (offshore) reefs and in situ and satellite sea surface temperatures (SSTs) revealed that the variability in the mortality event corresponded to two spatial scales of temperature variability: 300 m across the reef flat and 20 km across a series of reefs. However, the relationship between coral mortality and habitat thermal severity was opposite at the two scales. SSTs in summer 2010 were similar or increased modestly (0.5uC) in the outer and mid reefs relative to 2009. In the inner reef, 2010 temperatures were 1.4uC above the 2009 seasonal maximum for several weeks. We detected little or no coral mortality in mid and outer reefs. In the inner reef, mortality depended on exposure. Within the inner reef, mortality was modest on the protected (shoreward) side, the most severe thermal environment, with highest overall mean and maximum temperatures. In contrast, acute mortality was observed in the exposed (seaward) side, where temperature fluctuations and upper water temperature values were relatively less extreme. Refuges to thermally induced coral bleaching may include sites where extreme, high-frequency thermal variability may select for coral holobionts preadapted to, and physiologically condition corals to withstand, regional increases in water temperature.

Ostrowski, M., da Silva, J. C. B., and Bazik-Sangolay, B. 2009. The response of sound scatterers to El Niño- and La Niña-like oceanographic regimes in the southeastern Atlantic. – ICES Journal of Marine Science, 66: 1063–1072. Oceanographic conditions off Angola alternate seasonally between upwelling in the austral winter and El Niño-like intrusions and downwelling in summer. During winter in regions deeper than 30 m, the water column consists of a top layer of warm, tropical water overlying cold, nutrient-rich, and hypoxic South Atlantic Central Water (SACW). Closer inshore the water becomes well mixed. In the stratified region, acoustic backscatter at 38 kHz matches the oceanographic structure. It is strong in the top layer, but declines sharply in the SACW. During summer, the water column is continuously stratified, and the SACW is absent from the shelf. The backscatter reveals multiple thin layers extending across much of the shelf. The scattering layers are often perturbed by internal waves. The combined evidence from multiple acoustic surveys and the existing synthetic-aperture radar imagery suggests that tidal internal waves are a pervasive phenomenon in Angolan waters.

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