Abstract
Drastic increases in global carbon emissions in the past century have led to elevated sea surface temperatures that negatively affect coral reef organisms. Worldwide coral bleaching-related mortality is increasing and data has shown even isolated and protected reefs are vulnerable to the effects of global climate change. In 2014 and 2015, coral reefs in the main Hawaiian Islands (MHI) suffered up to 90% bleaching, with higher than 50% subsequent mortality in some areas. The location and severity of bleaching and mortality was strongly influenced by the spatial and temporal patterns of elevated seawater temperatures. The main objective of this research was to understand the spatial extent of bleaching mortality in Hanauma Bay Nature Preserve (HBNP), O‘ahu, Hawai‘i to gain a baseline understanding of the physical processes that influence localized bleaching dynamics. Surveys at HBNP in October 2015 and January 2016 revealed extensive bleaching (47%) and high levels of coral mortality (9.8%). Bleaching was highly variable among the four HBNP sectors and ranged from a low of ∼31% in the central bay at Channel (CH) to a high of 57% in the area most frequented by visitors (Keyhole; KH). The highest levels of bleaching occurred in two sectors with different circulation patterns: KH experienced comparatively low circulation velocity and a low temperature increase while Witches Brew (WB) and Backdoors (BD) experienced higher circulation velocity and higher temperature increase. Cumulative mortality was highest at WB (5.0%) and at BD (2.9%) although WB circulation velocity is significantly higher. HBNP is minimally impacted by local factors that can lead to decline such as high fishing pressure or sedimentation although human use is high. Despite the lack of these influences, high coral mortality occurred. Visitor impacts are strikingly different in the two sectors that experienced the highest mortality evidenced by the differences in coral cover associated with visitor use however, coral mortality was similar. These results suggest that elevated temperature was more influential in coral bleaching and the associated mortality than high circulation or visitor use.
Highlights
Global sea surface temperatures (SSTs) have increased an average 0.9 ◦C in the past century due to an increase in anthropogenic atmospheric gases resulting mainly from fossil fuel burning (Sabine et al, 2004)
The pristine reefs of the northern Great Barrier Reef (GBR) were thought to be resistant to bleaching due to their remote location and low fishing and tourism pressure, over 99% of these reefs were observed bleached along a 1,000 km stretch (Normile, 2016)
The full extent of worldwide coral mortality has not yet been quantified, NOAA climate models predict another year of warming for the GBR, Kiritimati, and other Pacific Islands, in the southern hemisphere (NOAA Coral Reef Watch, 2017)
Summary
Global sea surface temperatures (SSTs) have increased an average 0.9 ◦C in the past century due to an increase in anthropogenic atmospheric gases resulting mainly from fossil fuel burning (Sabine et al, 2004). The greatest increases of 0.06–0.11 ◦C decade−1 have occurred since 1970 (EPA, 2016) and have resulted in mass coral bleaching events worldwide. Scientific documentation of these events began nearly a decade later (Jaap, 1979). The year 2014 marked the beginning of the longest global bleaching event on record, which currently continues and has affected more reefs than any previous worldwide bleaching event (Eakin et al, 2014). By November 2016, up to 90% of corals were dead (Baum Lab, 2016) This devastating loss of coral occurred on the relatively undisturbed reefs in the southeastern part of the atoll as well as in the degraded northwest (Sandin et al, 2008; Watson, Claar & Baum, 2016). The negative influence of prolonged elevated seawater temperature on coral reefs is not selective and appears to affect protected, pristine, and degraded reefs,
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