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NOAA Biogeography Branch, SSMC-4 N/SCI-1 Silver Spring , MD 20910 Telephone: 301.713.3028 Web: http://ccma.nos.noaa.gov/about/biogeography/ |
Researchers from NOAA’s Biogeography Branch and the National Park Service’s South Florida Caribbean Network (NPS-SFCN) recently observed widespread bleaching in 22 species of hard corals at Buck Island Reef National Monument (BUIS) and the Virgin Islands East End Marine Park (EEMP) in St. Croix, U.S. Virgin Islands (USVI) (Table 1). The observed bleaching was part of a larger event occurring throughout the tropical western Atlantic (Coral Reef Watch). Bleaching, or the loss of symbiotic algae from coral tissue, commonly occurs when corals are exposed to 1 °C above annual maximum water temperature for an extended amount of time (Houegh-Guldberg, 1999). Satellite information and water quality data provided by NESDIS’ Coral Reef Watch have shown that water temperatures in the northeastern Caribbean exceeded the bleaching threshold (29.5 °C, 83.3 °F) for approximately 12 weeks prior to November 12, 2005 (Figure 1). Data on bleaching were collected as part of NOAA’s biannual monitoring of coral reef ecosystems in St. Croix, which is part of a larger program to monitor coral reef ecosystems in the U.S. Caribbean.
During October 16-30, 2005, bleaching occurred in 53% of the coral cover observed at 91 of 94 randomly selected hard bottom sites, which ranged in depth from 9 to 91 ft (2.7 and 27.5 m). Twelve coral species had bleaching greater than 50%, with four species being more than 90% bleached (Diploria labrynthiformis, Agaricia spp., Mycetophyllia spp., and Montastraea annularis) (Table 1). Several abundant corals also were severely bleached; four corals (M. annularis, D. strigosa, Porites astreoides, and Agaracia spp.) had cover greater than 9,850 cm2 and experienced bleaching from 40 – 95%. Fire corals (Millepora spp.) comprised 11,400 cm2 of cover of which 60% were bleached. Acropora palmata covered 21,800 cm2 of the sampled area but was less severely bleached (23%). Some corals had bleaching less than 20% (Stephanocoenia interceptis, Madracis spp., Dichocoenia stokesii, Meandrina meandrites), whereas other corals had no bleaching (Acropora cervicornis; Dendrogyra cylindricus, Eusmilia spp., and Scolymia spp., Table 1).
In general, the October 2005 coral bleaching event was widespread and occurred all around Buck Island but with no obvious spatial patterns (Figure 2). Also, no spatial patterns were observed in the occurrence of bleaching for four abundant coral species (Figure 3). A weak but significant correlation (r2 = 0.159, P = 0.0023) occurred between overall bleaching and depth, such that corals at shallower sites had higher rates of bleaching than those at deeper sites (Figure 4). If coral bleaching is temperature related, the observed correlation between depth and bleaching suggests that water temperature at depth would be lower than at the surface. However, water temperature, measured with a dive computer (Oceanic ProPlus 2) was not significantly lower at deeper sites than at shallower sites (r2 = 0.004, P = 0.633, F = 0.230, df = 60). Overall, the water temperature ranged from 82 to 95 °F with a mean = 88.9 ± 0.19 °F (from 27.7 to 35 °C, mean = 31.6 ± 0.1 °C).
A follow-up study was conducted during December 12-14, 2005 to evaluate possible coral recovery or mortality. Data were collected on coral cover and condition at 18 randomly selected hardbottom sites within and adjacent to the BUIS and the EEMP (hereafter protected areas, Figure 5). Bleaching was observed at 15 of the 18 sites in only 8% of the observed coral cover (Table 1). Most corals were unbleached (67%), but 20% exhibited abnormal coloration and were mottled or pale in appearance (Table 1). About 4% of the observed coral were colonized by Cyanobacteria or algae and were considered dead. Sixteen of 22 coral species were less than 10% bleached. Porites porites, Manicina aereolata, and favia fragum were 32-33% bleached, but these species had low abundance (< 0.2% total cover, Table 1). Agaricia coral occurred at 13 of 18 sites, had total cover of 0.24%, of which 80% was bleached. Dendrogyra cylindrus was 100% bleached but only occurred at one site (Table 1).
Overall, bleached coral cover decreased by 44.7% in areas from October to December 2005, however, bleaching and apparent recovering corals were still evident (Table 1, Click here to view photos of corals). Bleached cover decreased by 60% or more in several abundant corals (e.g., Diploria strigosa, Porites porites, Montastraea annularis, Mycetophyllia spp. and Millepora spp.). These corals also showed an increase in the occurrence of unbleached colonies between October and December 2005 (Table 1). Less abundant corals such as D. labyrinthiformis and Mycetophyllia spp. showed a decrease in bleached coral cover and a corresponding increase in normal coral cover (unbleached coloration).
Prolonged bleaching can be lethal to corals and, along with disease and pollution, may have contributed to the loss of about 16% of the world’s coral reefs within the past decade (Houegh-Gulberg 1999; Wilkinson 2000). A causal link between bleaching and coral death may not be quite obvious in this study because the effects of bleaching were not monitored through time at permanently marked coral heads. However, estimates of species’ coral cover and total coral cover were very similar between October and December 2005 when compared to the differences in the estimates of bleached and unbleached corals. These data suggest that more corals may be recovering than dying as a result of the September-October, 2005 bleaching event. Recent coral death (i.e. overgrowth by Cyanobacteria or algae) was observed in only a few species (Table 1). Porites porites and Millepora spp. had the highest occurrence of dead coral and may have been the species most affected by the bleaching event (see Table 1). Continued monitoring of BUIS and EEMP corals is necessary to determine recovery and the overall impact of such extreme natural events on coral reef ecosystems.
Researchers from the NPS-SFCN and US Geological Survey (USGS) also observed widespread coral bleaching at both the BUIS and the Virgin Islands Coral Reef National Monument (VICRNM) at permanent sites in St. John, where they have been monitoring reefs with video since 1999 (J. Miller, pers. comm.). Analyses of data collected (during bleaching maxima and at present) are still being conducted, but preliminary results indicate most sites had over 90% of coral cover bleached. NPS-SFCN researchers have noted also that mean monthly sea temperature (at depth) during September was the warmest seen in 15 years. Bleaching of A. palmata occurred much later than most other scleractinian corals; however mortality of bleached areas happened within weeks (J. Miller, pers. comm.). Bleaching severity and mortality from bleaching differed between individual colonies within a site. Most of the colonies that survived have regained previous pigmentation levels, with only a few colonies still having bleached tips (J. Miller, pers. comm.).
From October 16 to 30, 2005, coral bleaching was documented at 94 hard bottom sites during a biannual survey of benthic habitats at the BUIS. During December 12-14, 2005, only 18 sites were randomly selected for documentation of bleaching during benthic surveys because of limited logistical support. The goals of the December sampling mission were to (1) to spatially characterize the extent of coral bleaching that occurred September/October 2005 within and around the waters of the BUIS and the EEMP, and (2) to determine the mortality or recovery of corals from bleaching observed during October 2005. A stratified random sampling method was chosen instead of a repeated measures design at permanently marked locations to fully characterize 1) the spatial extent and occurrence of bleaching and 2) ecosystem levels of mortality or recovery from coral bleaching. Additionally, corals sampled during October 2005 were unmarked. Therefore, it was impossible to re-survey specific corals as would be required by a repeated measures design.
Documentation of bleaching occurred on hard bottom areas within and outside of protected areas, which were likely to have coral. At each site, the cover of bleached and unbleached coral within five 1-m2 quadrats [along a 25 m-transect] was estimated visually and recorded for each species. Coral cover is reported in cm2 and was converted to those units using the relationship: 1% cover = 100 cm2. The amount of bleaching of each species was calculated with the equation: bleached cover / [bleached cover + unbleached cover]) x 100%. The biannual benthic habitat survey is part of the Caribbean Coral Reef Ecosystem Monitoring project. The detailed methodology for the benthic habitat survey is located on line (Detailed Methods for Characterization and Monitoring of Coral Reef Ecosystems and Associated Biological Communities).
During October, 2005, managers from the NPS-SFCN and the EEMP requested that the Biogeography Branch explicitly collect data on coral bleaching during its annual monitoring of coral reef ecosystems in the U.S Caribbean. Plans are to continue future monitoring of coral bleaching and recovery during benthic surveys planned for April 2006, when the Biogeography Branch’s next biannual monitoring of coral reef ecosystems will occur.
During December 2005, NOAA’s Biogeography Program also partnered with NASA Ames Research Center and provided diving support for the collection of spectroradiometer measurements and data for the calibration of hyperspectral imagery (Airborne Visible Infrared Imaging Spectrometer, AVIRIS). Plans are to continue working closely with NASA to provide this information to park managers, thus further developing the knowledge base necessary to better respond to large-scale bleaching events. Ultimately, the data collected on reef habitats will provide information to address primary resource management questions of the Park and Monuments, and will aid NPS and local management in assessing the effectiveness of marine protected area regulations.
Table 1. Condition of corals in the Buck Island Reef National Monument, St. Croix during October 16-30 and December 12-14, 2005. Coral species are ranked by observed coral cover (Column 2). Photos of bleached coral are available at: http://ccma.nos.noaa.gov/products/biogeography/coral_bleaching/oct_rpt.html. Source of data and photos: NOAA NCCOS Biogeography Branch, unpublished data.
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| Figure 1. Sea surface temperature (SST) time series (solid dark-blue line) and coral bleaching Degree Heating Weeks (DHW) time series (solid red line) from January 2004 to December 2005 for the U.S. Virgin Islands. The corresponding thermal condition related to coral bleaching is color-coded and plotted below the time series. The thermal condition is categorized in the five bleaching alert levels defined by Coral Reef Watch's Satellite Bleaching Alert. Source: http://coralreefwatch.noaa.gov/satellite/current/sst_dhw_series_usvirgin_cur.html. |
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Figure 2. Spatial distribution of bleached coral determined from 94 sampled hardbottom sites at Buck Island, St. Croix, U.S. Virgin Islands in October 2005. |
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Figure 4 . Linear regression of mean bleached coral (% cover) against depth for 91 hardbottom sites sampled at Buck Island, St. Croix, U.S. Virgin Islands from October 16-30, 2005. |
