The Third Global Coral Bleaching Event on the Marginal Coral Reefs of the Southwestern Indian Ocean and Factors That Contribute to Their Resistance and Resilience

Sean N Porter,Michael H Schleyer,Kerry J Sink

doi:10.3390/d13100464

Abstract

Coral reefs reach their southernmost limits in the southwestern Indian Ocean in Maputaland, South Africa. Here, we investigate the recent global coral bleaching event of 2016, the thermal dynamics of these marginal high-latitude reefs and the potential environmental factors regulating the responses of coral communities. Pre-, peak- and post-bleaching surveys of over 9850 coral colonies from 29 genera were undertaken over 3 years across 14 sites spanning 120 km of coastline using point-intercept and visual bleaching index survey methodologies. Bleaching data were related to several environmental variables including temperature, degree heating weeks (DHW), depth, latitude, and upwelling intensity. These reefs have experienced a history of relatively low thermal stress based on DHW. Long-term in situ temperature records nevertheless showed no obvious trend of increase. In situ temperatures also displayed poor relationships, with temperatures predicted by the Representative Concentration Pathway models. Mild coral bleaching with no significant mortality was recorded across sites with taxon-specific bleaching responses evident. Latitude and cumulative daily DHW were significantly related to the bleaching index whereas depth and interactions of depth with latitude and DHW were not. While upwelling of cooler water may offer some refuge to coral communities, especially in the Central and Southern Reef Complexes where it is more pronounced, this may only be transient as the upwelled water may also experience some degree of warming in future, thereby limiting such protection from global warming.

Highlights

Coral reefs are highly valued natural assets [1,2]
Over the period of available degree heating week (DHW) data, the high-latitude coral reefs of the southwestern Indian Ocean have experienced a history of relatively low thermal stress
These reefs are likely to be differentially affected by cooler upwelled water in the region; evidence suggests that reefs in the Central and Southern Complexes are influenced more than reefs in the Northern Complex due to their closer proximity to canyons and the shelf edge

Summary

Introduction

Coral reefs are highly valued natural assets [1,2]. Despite their value, coral reefs are becoming increasingly threatened by both global and local stressors [3]. Warming by anthropogenic climate change is a significant global threat and has resulted in high losses of live coral [4]. Warm temperatures can cause coral bleaching which is a consequence of the loss of zooxanthellar algae from the host tissue and often results in mortality [5]. There have been several global bleaching events with one of the most severe occurring in 2014–2017 [6], which affected parts of the south-west Indian Ocean [7]. The southern-most coral reefs in the south-west Indian Ocean are located in South

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