The Rarity of Depth Refugia from Coral Bleaching Heat Stress in the Western and Central Pacific Islands

Roberto M Venegas,Thomas Oliver,Scott F Heron,C Mark Eakin,S Jeanette Clark,Gang Liu,Noah Pomeroy,Russell E Brainard,Charles Young

doi:10.1038/s41598-019-56232-1

Abstract

Some researchers have suggested that corals living in deeper reefs may escape heat stress experienced by shallow corals. We evaluated the potential of deep coral reef refugia from bleaching stress by leveraging a long record of satellite-derived sea surface temperature data with a temporal, spatial, and depth precision of in situ temperature records. We calculated an in situ stress metric using a depth bias-adjusted threshold for 457 coral reef sites among 49 islands in the western and central Pacific Ocean over the period 2001–2017. Analysis of 1,453 heating events found no meaningful depth refuge from heat stress down to 38 m, and no significant association between depth and subsurface heat stress. Further, the surface metric underestimated subsurface stress by an average of 39.3%, across all depths. Combining satellite and in situ temperature data can provide bleaching-relevant heat stress results to avoid misrepresentation of heat stress exposure at shallow reefs.

Highlights

Mass coral bleaching events due to anomalously warm ocean water have increased in both frequency and severity, and represent a significant threat to coral reef ecosystems worldwide[1,2,3,4]
The National Oceanic and Atmospheric Administration (NOAA) Pacific Reef Assessment and Monitoring Program (Pacific RAMP) has been assessing and monitoring coral reef ecosystems since 2000 across 49 islands, atolls and subsurface reefs on the western and central Pacific spanning an area of over 30 million square kilometers (Fig. 1). We divided this vast domain into six regions (Table 1): the Mariana Archipelago; the Northwestern Hawaiian Islands (NWHI); the Main Hawaiian Islands (MHI); American Samoa (AMSM); the northern Pacific Remote Island Areas (N-PRIA); and the equatorial Pacific Remote Island Areas (EQ-PRIA). We utilized this large dataset of in situ temperature measurements from subsurface temperature recorder (STR) data collected as part of Pacific RAMP, and sea surface temperature (SST)-derived products from the NOAA Coral Reef Watch program at 5-km resolution to quantify the levels of heat stress that influenced a wide range of coral reef ecosystems distributed broadly across the Pacific
Using the resulting in situ heat stress exposure metric, we explored the patterns of heat stress and refugia with depth across the western and central Pacific

Summary

Introduction

Mass coral bleaching events due to anomalously warm ocean water have increased in both frequency and severity, and represent a significant threat to coral reef ecosystems worldwide[1,2,3,4]. In turn, can reduce coral diversity and habitat complexity[8,9] and negatively impact species richness, abundance, and biomass of coral reef fishes and other associated biodiversity[8,10,11] These attendant ecological consequences of mass coral bleaching and the corresponding declines in ecosystem services (e.g., food source, fisheries, and tourism) support the need for an accurate understanding of heat stress exposure on corals. We utilized this large dataset of in situ temperature measurements from subsurface temperature recorder (STR) data collected as part of Pacific RAMP, and SST-derived products from the NOAA Coral Reef Watch program at 5-km resolution to quantify the levels of heat stress that influenced a wide range of coral reef ecosystems distributed broadly across the Pacific

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Conclusion