Resilience of Central Pacific reefs subject to frequent heat stress and human disturbance

Simon D Donner,Jessica Carilli

doi:10.1038/s41598-019-40150-3

Abstract

Frequent occurrences of coral bleaching and associated coral mortality over recent decades have raised concerns about the survival of coral reefs in a warming planet. The El Niño-influenced coral reefs in the central Gilbert Islands of the Republic of Kiribati, which experience years with prolonged heat stress more frequently than 99% of the world’s reefs, may serve as a natural model for coral community response to frequent heat stress. Here we use nine years of survey data (2004–2012) and a suite of remote sensing variables from sites along gradients of climate variability and human disturbance in the region to evaluate the drivers of coral community response to, and recovery from, multiple heat stress events. The results indicate that the extent of bleaching was limited during the 2009–2010 El Niño event, in contrast to a similar 2004–2005 event, and was correlated with incoming light and historical temperature variability, rather than heat stress. Spatial and temporal patterns in benthic cover suggest growing resistance to bleaching-level heat stress among coral communities subject to high inter-annual temperature variability and local disturbance, due to the spread of “weedy” and temperature-tolerant species (e.g., Porites rus) and the cloudy conditions in the region during El Niño events.

Highlights

Episodes of heat stress and subsequent mass coral ‘bleaching’ over the past three decades have led to widespread coral mortality and raised questions about the viability of coral reef ecosystems during a period of rapid climate change[1,2,3,4]
The available survey data suggest that the outer reef coral communities in the central Gilbert Islands have been relatively resilient to repeat heat stress associated with Central Pacific El Niño events
The analyses presented here suggest that past climate experience and local human disturbance have together influenced the observed variability in bleaching response and coral community structure across the region

Summary

Introduction

Episodes of heat stress and subsequent mass coral ‘bleaching’ over the past three decades have led to widespread coral mortality and raised questions about the viability of coral reef ecosystems during a period of rapid climate change[1,2,3,4]. SSTs in the Gilbert Islands vary twice as much from year-to-year than from season-to-season, unlike 99% of the world’s coral reefs[12] This atypical temperature variability causes bleaching-level heat stress, according to standard metrics like the National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch’s Degree Heating Weeks (DHW) product, to occur at a frequency[12] that other coral reefs are unlikely to experience without several decades of further warming[13]. The unique geography of the northern Gilbert Islands presents an opportunity to examine the influence of both climate variability and local human disturbance on coral response to ENSO-driven heat stress events. The response of the coral community over time across the different atolls is still not well understood

Methods

Results

Conclusion