Severe, Widespread El Niño–Associated Coral Bleaching in the US Phoenix Islands

Abstract

The present study provides an assessment of the bleaching response of corals at Baker and Howland Islands (central Pacific) following the 2009-2010 El Nino-Southern Oscillation event. Nearly 35% of colonies belonging to 17 coral genera surveyed within belt transects exhibited bleaching, predominantly along east Howland Island and east and northeast Baker Island, and within the 10-18 m isobaths. Bleaching conditions differed among coral taxa, with Pocillopora, Acropora, and Fungia exhibiting the greatest percentage of affected colonies, followed by Pavona, Porites, and Psammocora; Montipora appeared to be the most resistant to bleaching with < 2.5% of colonies affected. Colony size had direct effects on bleaching responses for Acropora, Pavona, and Pocillopora, with bleached colonies exhibiting greater mean diameters than unbleached corals; mixed effects were observed for Fungia, Porites, and Psammocora. Bleaching extent was also affected by colony size, but only in Acropora; the lack of differences in the other study corals is probably due to the high variability in the extent of bleaching within taxa. In situ subsurface loggers indicated that seawater temperatures increased steadily throughout 2009, peaking at 31.2 °c in early November, and remaining above the coral bleaching threshold (29.7 °c) beyond the time when instruments were recovered on 7 February, 2010. This is the first documented mass bleaching episode at baker and Howland Islands; however, anecdotal evidence suggests recurrent, widespread bleaching may be a principal source of disturbance, affecting coral reef species composition and structural dynamics at Howland and Baker over multi-year timescales.