BULLETIN OF MARINE SCIENCE, 83(3): 441–451, 2008 441 Bulletin of Marine Science © 2008 Rosenstiel School of Marine and Atmospheric Science of the University of Miami VARIATION IN ACROPORA PALMATA BLEACHING ACROSS BENTHIC ZONES AT BUCK ISLAND REEF NATIONAL MONUMENT (ST. CROIX, USVI) DURING THE 2005 THERMAL STRESS EVENT Ian Lundgren and Zandy Hillis-Starr ABSTRACT At Buck Island Reef National Monument (BIRNM) in St. Croix, U.S. Virgin Islands, Acropora palmata (Lamarck, 1816) experienced extensive bleaching in 2005. Water temperatures at BIRNM in 2005 were higher than at any time since at least 1991, and exceeded the bleaching threshold by over 2 °C, causing extensive coral bleaching. Forty-four A. palmata colonies at three sites in BIRNM were sampled before, during, and after the thermal stress event. Additionally, colonies at randomly located sites on the reef shelf were surveyed to extend results over a larger portion of BIRNM; these colonies were surveyed during and after the thermal stress event. Acropora palmata colonies located in the backreef benthic zone exhibited a diferent response to the thermal stress event than those located elsewhere in BIRNM: backreef colonies bleached earlier and sufered greater tissue loss than those located elsewhere. We hypothesized that increased water temperature coupled with reduced water low and increased light penetration caused higher mortality in backreef locations. Since A. palmata in the backreef benthic zone is least resistant and resilient to thermal stress events, backreef areas are a poor focal area for A. palmata marine protected areas. Coral bleaching refers to the loss of photosynthetic dinolagellate algal symbionts (zooxanthellae), the reduction of photosynthetic pigments, or both (Glynn, 1993; Brown, 1997). Many reef-building coral species live very close to their natural ther- mal tolerance limits (Jokiel and Coles, 1990; Lesser, 1997). When sea surface tem- peratures (SSTs) exceed the “bleaching threshold”, 1 °C above the mean monthly maximum SST (Goreau and Hayes, 1994), corals are likely to experience bleaching. If zooxanthellae are not reassimilated into coral tissues, the coral animal will die. Although bleaching is a generalized coral response to stress associated with a wide variety of factors such as thermal stress, irradiance, pollution, sedimentation, aeri- al exposure, and disease (Glynn, 1993; Brown, 1997; Hoegh-Guldberg, 1999), mass coral bleaching appears to be triggered by sustained elevated water temperature (thermal stress event) (Brown, 1997; Hoegh-Guldberg, 1999; Lesser, 2004). Predictive models correlate the magnitude and duration of water temperatures above bleaching thresholds to forecast thermal stress events (Gleeson and Strong, 1995; Strong et al., 2004; Liu et al., 2003). Average water temperatures have risen 0.4–0.8 °C since the late eighteenth century (Barton and Casey, 2005) and it is predicted that water tem- peratures will continue to rise approximatetly 1–2 °C per century (Hoegh-Guldberg, 1999; Barton and Casey, 2005), which suggests more frequent thermal stress events in the future. During thermal stress events, bleaching can be patchy, locally exacerbated, or in- hibited by multiple factors. For example, high solar irradiance coupled with thermal stress appears to be particularly efective at promoting mass coral bleaching (Brown, 1997; Lesser, 2004). Conversely, shading and cloud cover have been shown to decrease CORAL REEF PAPER