Depth moderates loss of marine foundation species after an extreme marine heatwave: could deep temperate reefs act as a refuge?

Renae K Hovey,Gary A Kendrick,Ana Giraldo-Ospina

doi:10.1098/rspb.2020.0709

Renae K Hovey, Gary A Kendrick + Show 1 more

Open Access

https://doi.org/10.1098/rspb.2020.0709

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Abstract

Marine heatwaves (MHWs) have been documented around the world, causing widespread mortality of numerous benthic species on shallow reefs (less than 15 m depth). Deeper habitats are hypothesized to be a potential refuge from environmental extremes, though we have little understanding of the response of deeper benthic communities to MHWs. Here, we show how increasing depth moderates the response of seaweed- and coral-dominated benthic communities to an extreme MHW across a subtropical–temperate biogeographical transition zone. Benthic community composition and key habitat-building species were characterized across three depths (15, 25 and 40 m) before and several times after the 2011 Western Australian MHW to assess resistance during and recovery after the heatwave. We found high natural variability in benthic community composition along the biogeographic transition zone and across depths with a clear shift in the composition after the MHW in shallow (15 m) sites but a lot less in deeper communities (40 m). Most importantly, key habitat-building seaweeds such as Ecklonia radiata and Syctothalia dorycarpa which had catastrophic losses on shallow reefs, remained and were less affected in deeper communities. Evidently, deep reefs have the potential to act as a refuge during MHWs for the foundation species of shallow reefs in this region.

Highlights

In ecology, the term ‘refugium’ is used to describe regions that facilitate the temporal and spatial resilience of biological communities over evolutionary timescales or from past climate change [1,2]
The catastrophic loss of canopy-forming macroalgae, E. radiata and S. dorycarpa documented in shallow waters [15,23] was not shown from deeper offshore reefs between 25 and 40 m off Western Australia, supporting our hypothesis that deep water habitats exhibit a buffering effect from extreme climatic events, that allows the persistence of kelp-dominated communities
We analysed benthic community composition across a subtropical–temperate biogeographic transition zone and found a reduced response to an extreme Marine heatwaves (MHWs) in deeper reefs (25–40 m), despite a natural shift in community composition from mixed assemblages of tropical corals and kelps in the north (Abrolhos Islands) to a typical temperate community dominated by kelps in the south (Rottnest Island) [52,53]

Summary

Introduction

The term ‘refugium’ is used to describe regions that facilitate the temporal and spatial resilience of biological communities over evolutionary timescales or from past climate change [1,2]. Extreme climatic events (ECEs) can generate changes in species distributions, ecosystem structure and functioning [3,4,5,6,7] and are predicted to increase in magnitude and frequency because of climate change [8,9], and taxa are being impacted in ecological timescales. Biogeographic transition zones are biodiversity hotspots owing to the overlap of taxa present at the edge of their biogeographical distribution (tropical and temperate taxa). These zones are vulnerable to the disturbances of extreme climatic events as many species already live at the physiological limits of their distribution [23]

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