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

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.

Predicting Heat Stress to Inform Reef Management: NOAA Coral Reef Watch's 4-Month Coral Bleaching Outlook

“…These were confirmed by CRW's satellite monitoring ( Figure 12B) and field observations, indicating widespread bleaching, with the middle section of the NWHI experiencing unprecedented mass bleaching (Bahr et al, 2015;Couch et al, 2017;Eakin et al, 2017).…”

“…This event affected more reefs in the U.S. and worldwide than either previously documented global bleaching event (1998 and2010;Eakin et al, 2017). It has been the worst ever in some locations [e.g., the northern GBR (Hughes et al, 2017), Kiritimati Island (The Washington Post, 2016), Jarvis Island (Brainard et al, 2018), and the NWHI (Couch et al, 2017)]. Some reefs bleached extensively for the first time on record (e.g., the northern GBR; Hughes et al, 2017), and some reefs were affected in consecutive years [e.g., Hawaii, the Florida Keys (Eakin et al, 2017), and the CNMI (Heron et al, 2016b)].…”

Comparative thermal performance ofOrbicella franksiat its latitudinal range limits

“…; (DeCarlo et al, 2017; Harvell et al, 2002; Hughes et al, 2017)). Reef building corals and their dinoflagellate symbionts live close to their physiological thermal maximum and, as a result, warming of 1°C or more above local mean monthly maxima can reduce fitness and cause tissue loss or whole-colony mortality (Hoegh-Guldberg 1999, Baker et al 2008), with significant negative implications for reef structure and function (Couch et al, 2017; Hughes et al, 2018b; Stuart-Smith et al, 2018). In the case of corals, there are numerous mechanisms that could enable local thermal adaptation, including genetic adaptation and physiological acclimatization of the host, changes in Symbiodinaceae composition, and the bacterial microbiome (Putnam et al, 2017).…”

Depth‐Dependent Thermal Stress Around Corals in the Tropical Pacific Ocean

“…Warm conditions that may lead to thermal stress and coral bleaching currently are forecast using SST values derived from satellite observations (Heron et al, 2016;Liu et al, 2014Liu et al, , 2006 to be predictive of coral bleaching for shallow fore reef corals (Couch et al, 2017;Eakin et al, 2010;Glynn & D'croz, 1990;Liu et al, 2014;McClanahan et al, 2007). At the same time, quantifying accurate bleaching thresholds for corals between 10 and 30 m has been limited and assessments of thermal stress on mesophotic coral reefs have only been done in a few studies (Bruno et al, 2001;Nir et al, 2014).…”