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. 2017 Sep 27;12(9):e0185121.
doi: 10.1371/journal.pone.0185121. eCollection 2017.

Mass coral bleaching due to unprecedented marine heatwave in Papahānaumokuākea Marine National Monument (Northwestern Hawaiian Islands)

Courtney S Couch  1   2 , John H R Burns  1 , Gang Liu  3   4 , Kanoelani Steward  5 , Tiffany Nicole Gutlay  1 , Jean Kenyon  6 , C Mark Eakin  3 , Randall K Kosaki  7
Affiliations

Mass coral bleaching due to unprecedented marine heatwave in Papahānaumokuākea Marine National Monument (Northwestern Hawaiian Islands)

Courtney S Couch et al. PLoS One. .

Abstract

2014 marked the sixth and most widespread mass bleaching event reported in the Northwestern Hawaiian Islands, home to the Papahānaumokuākea Marine National Monument (PMNM), the world's second largest marine reserve. This event was associated with an unusual basin-scale warming in the North Pacific Ocean, with an unprecedented peak intensity of around 20°C-weeks of cumulative heat stress at Lisianksi Island. In situ bleaching surveys and satellite data were used to evaluate the relative importance of potential drivers of bleaching patterns in 2014, assess the subsequent morality and its effects on coral communities and 3D complexity, test for signs of regional acclimation, and investigate long-term change in heat stress in PMNM. Surveys conducted at four island/atoll (French Frigate Shoals, Lisianski Island, Pearl and Hermes Atoll, and Midway Atoll) showed that in 2014, percent bleaching varied considerably between islands/atolls and habitats (back reef/fore reef and depth), and was up to 91% in shallow habitats at Lisianski. The percent bleaching during the 2014 event was best explained by a combination of duration of heat stress measured by Coral Reef Watch's satellite Degree Heating Week, relative community susceptibility (bleaching susceptibility score of each taxon * the taxon's abundance relative to the total number of colonies), depth and region. Mean coral cover at permanent Lisianski monitoring sites decreased by 68% due to severe losses of Montipora dilatata complex, resulting in rapid reductions in habitat complexity. Spatial distribution of the 2014 bleaching was significantly different from the 2002 and 2004 bleaching events likely due to a combination of differences in heat stress and local acclimatization. Historical satellite data demonstrated heat stress in 2014 was unlike any previous event and that the exposure of corals to the bleaching-level heat stress has increased significantly in the northern PMNM since 1982, highlighting the increasing threat of climate change to reefs.

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Conflict of interest statement

Competing Interests: None of the organizations and commercial affiliations involved in this study have competing interests. Global Science & Technology Inc.’s involvement does not alter our adherence to PLOS ONE policies on sharing data and materials.

Figures

Fig 1
Fig 1. Annual maximum composite of CRW’s dOISST.v2 (25km) Degree Heating Weeks, across the Hawaiian Archipelago for years (a) 1987, (b) 2002, (c) 2004, (d) 2007, (e) 2009, (f) 2010, (g) 2014, and (h) 2015), when DHW ≥ 4°C-weeks (the minimum level associated with significant bleaching) appeared in PMNM.
Fig 2
Fig 2. Median of annual maximum Degree Heating Weeks for pixels between 1982 and 2016 in the (a) southern islands (Nihoa (n = 1 pixel), Mokumanamana (n = 1 pixel), French Frigate Shoals (n = 3 pixels), (b) middle islands (Maro Reef (n = 1 pixel), Laysan Island (n = 1 pixel), Lisianski Island (n = 4 pixels) and (c) northern atolls (Pearl and Hermes (n = 2 pixels), Midway (n = 2 pixels) and Kure (n = 1 pixel).
Lines represent Sen’s slope estimates. Data derived from CRW’s 25 km dOISST.v2-based dataset. Northern atolls: Mann-Kendell, tau = 0.366, p = 0.0018, Sen’s slope = 0.074; middle islands: Mann-Kendell, tau = 0.15, p = 0.1956, Sen’s slope = 0; southern islands Mann-Kendell, tau = 0.148, p = 0.205, Sen’s slope = 0.
Fig 3
Fig 3
(a) Mean ± SE % bleaching at permanent study sites at French Frigate Shoals (FFS), Lisianksi Island (LIS), Pearl and Hermes Atoll (PHR) and Midway Atoll (MID) in 2002, 2004 and 2014. See S2 Table for survey dates. Letters indicate glht posthoc tests with Bonferroni Correction α = 0.016. N = 4 sites at FFS, 3 sites at LIS, 5 sites at PHR and 2 sites at MID. (b) Generalized linear regressions (binomial errors) of relationship between % bleaching and CRW’s 25 km Degree Heating Week at the time of the survey for 2002, 2004 and 2014. Solid lines: predicted bleaching (with binomial errors). Grey area: upper and lower 95% confidence intervals. (2002: R2 = 0.3081, p<0.0001; 2004: R2 = 0.2325, p<0.0001; 2014: R2 = 0.6979, p<0.0001). (c) Mean ± SE residuals from % bleaching vs. DHW GLM (S1 Fig) across regions and years. Letters indicate glht posthoc tests with Bonferroni Correction α = 0.016. (d) Nonmetric multidimensional scaling (nMDS) plot of coral community (% of colonies comprised of different coral taxa) differences clustered by year at Pearl and Hermes Atoll (stress = 0.057). Other regions not included because bleaching did not differ significantly between years after correcting for differences in heat stress or site-level replication was too low.
Fig 4
Fig 4. The dOISST.v2 sea surface temperature (non-red solid lines) and CRW’s dOISSTv.2-based Degree Heating Weeks (DHW, double lines) in 2002, 2004, 2014 and 2015 for (a) French Frigate Shoals, (b) Lisianski Island, (c) Pearl and Hermes Atoll, and (d) Midway Atoll.
Dashed horizontal red line: the maximum of monthly mean SST climatology (MMM); Solid horizontal red line: bleaching threshold SST (1°C above MMM).
Fig 5
Fig 5. Mean ± SE % bleaching at all study sites at French Frigate Shoals (FFS), Lisianksi Island (LIS), Pearl and Hermes Atoll (PHR) and Midway Atoll (MID) surveyed between August 14th to September 26th, 2014.
Data are averaged across all sites for a regional average (black horizontal lines) and for each habitat. N = 5–27 transects/habitat/region.
Fig 6
Fig 6. Generalized regressions of relationship between the proportion of bleached colonies and (a) CRW’s 5 km Degree Heating Week (DHW) during survey, (b) depth, and (c) the interaction of region and relative community susceptibility.
Solid line: predicted bleaching (with binomial errors). Grey area: upper and lower 95% confidence intervals.
Fig 7
Fig 7
(a) Mean ± SE coral cover at permanent sites in September 2014 and August 2015. N = 12–18 transects per region/year. Asterisk indicates significant difference between years for each region (α = 0.0125). FFS = French Frigate Shoals, LIS = Lisianski Island, PHR = Pearl and Hermes Atoll, MID = Midway Atoll. (b) Mean % coral mortality (2014–2015% coral cover/2014% coral cover * 100) plotted against CRW’s maximum 5 km Degree Heating Weeks in 2014. Linear Regression: R2 = 0.3151, p = 0.0367.
Fig 8
Fig 8
Orthophoto mosaics (10 x 3-m) representing the coral reef habitat at LIS-4067 (a) before (August 2014) and (b) after the bleaching event (August 2015). (c) Mean values (± SE) of community composition and 3D structural metrics before and after the bleaching event at Lisianski. Asterisk denotes significant differences (two sample t-test, p<0.05). The M3C2 algorithm shows measured change (d) in habitat volume as loss (red) and gain (green) for the reef substrate compared between the 3D point clouds from 2014 and 2015. The point cloud comparison also shows areas of significant 3D structural change (E) that were greater than the 95% level of detection (red).
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Grants and funding

This study was supported by NOAA/NOS-HIMB agreement code MOA-2009-039 to provide salary to CSC, the STAR Fellowship Assistance Agreement no. FP-91768001-0 awarded by the US Environmental Protection Agency (EPA) to provide salary to JB. Funding was also provided by the ESPCoR Hawaiʻi (EPS-0903833) Track-I research grant to The University of Hawaii to provide salary to JB and KS, the NOAA Coral Reef Conservation Program to Coral Reef Watch to CME and the National Environmental Satellite, Data, and Information Service (NESDIS) for CME. This manuscript has not been formally reviewed by the EPA and the EPA does not endorse any products or commercial services mentioned in this publication. The scientific results and conclusions, as well as any views or opinions expressed herein, are those of the authors and do not necessarily reflect the views of NOAA or the Department of Commerce. Global Science & Technology Inc. provided support in the form of salaries for GL, but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of this author are articulated in the 'author contributions' section.

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