Severe Heat Stress Resulted in High Coral Mortality on Maldivian Reefs following the 2015–2016 El Niño Event

Pia Bessell-Browne,Nora Hall,Patrick Buerger,Hannah E Epstein,Kathryn Berry

doi:10.3390/oceans2010014

Abstract

Coral cover worldwide has been declining due to heat stress caused by climate change. Here we report the impacts of the 2015–2016 El Niño mass coral bleaching event on the coral cover of reefs located on central and northern atolls of the Maldives. We surveyed six reef sites in the Alifu Alifu (Ari) and Baa (South Maalhosmadulu) Atolls using replicate 20 m benthic photo transects at two depths per reef site. Live and recently dead coral cover identified from images differed between reef sites and depth. Recently dead corals on average made up 33% of the coral assemblage at shallow sites and 24% at deep sites. This mortality was significantly lower in massive corals than in branching corals, reaching an average of only 6% compared to 41%, respectively. The best predictors of live coral cover were depth and morphology, with a greater percentage of live coral at deep sites and in massive corals. The same predictors best described the prevalence of recently dead coral, but showed inverse trends to live coral. However, there was high variability among reef sites, which could be attributed to additional local stressors. Coral bleaching and resulting coral mortalities, such as the ones reported here, are of particular concern for small island nations like the Maldives, which are reliant on coral reefs.

Highlights

Elevated seawater temperatures resulting from climate change are causing widespread coral bleaching events across tropical regions of the world [1,2,3]
Six coral reef sites were monitored in the Alifu Alifu and Baa Atolls in the Republic of Maldives (Figure 1) to assess post-bleaching coral mortality
The peak of the bleaching event occurred between mid-April and mid-June 2016 and some mortality was seen at this time, many colonies remained bleached for several months

Summary

Introduction

Elevated seawater temperatures resulting from climate change are causing widespread coral bleaching events across tropical regions of the world [1,2,3]. In 2015–2016, a severe El Niño event led to widespread coral bleaching across the Pacific and Indian Oceans [1,2,4,5,6,7,8], with some regions experiencing repeated heat stress events throughout this period Not quite as extreme, DHWs from the Western and Eastern Indian Ocean during the same period still greatly exceeded the bleaching threshold, with records ranging from four to 11 [11,12,13]. Coral bleaching occurs when corals expel their photosynthetic endosymbiotic algae (Symbiodiniaceae), often as a result of temperature stress, leaving a white skeleton visible through translucent tissue [14].

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Results

Conclusion