Recovery from the 2004 Coral Bleaching Event in the Gilbert Islands, Kiribati

Abstract

Over the past three decades, periods of anomalously warm ocean temperatures have been observed to cause mass coral bleaching, a paling of corals and other reef organisms due to the loss of dinoflagellate Symbiodinium. During late 2004, warming of the surface waters in the Central Equatorial Pacific occurred in association with a moderate El Nino event. The accumulation of degree-heating-weeks, a measure of thermal stress experienced by corals, reached the highest level on record in parts of the Gilbert Islands of Kiribati in November, 2004. The warm water anomaly led to the first ever reports of mass coral bleaching from the Gilbert Islands. This study describes the development of the mass bleaching event, the extent and pattern of coral mortality, and the post-bleaching change in coral cover around little-studied Tarawa and Abaiang Atolls. Random surveys, conducted at shallow depths in 2004, 2005, and 2009 via free-diving due to limited local infrastructure, provide preliminary evidence that the outer reef coral communities have become increasingly dominated by more bleaching-resistant coral taxa since the 2004 bleaching event. There was a significant recovery in coral cover at the three Abaiang sites, which are less influenced by human disturbance and wave activity, from 2005 to 2009. This change included an increase in the area of the less bleaching-sensitive Heliopora coerulea and massive Porites and a widespread loss of the more bleaching-sensitive Pocillopora spp Recovery of coral cover was observed only at one Tarawa site, due to a significant increase of the resilient Porites rus. Historical meteorological and SST data also indicate that the coral reefs of the Gilbert Islands may have been subject to other episodes of thermal stress sufficient to cause coral bleaching before 2004, due to the occurrence of “Central Pacific” El Nino events. Further monitoring of this relatively unstudied island chain is necessary to evaluate whether the frequent El Nino-driven thermal stress has affected coral community structure and resilience to future warm water events.