Quantifying ENSO-related bleaching on nearshore, turbid-zone coral reefs: a test of the turbid-zone reef climate change refugia hypothesis.

Lead Research Organisation: University of Exeter
Department Name: Geography


Over the last two months there have been increasing reports of the current El Niño causing massive coral bleaching along Australia's Great Barrier Reef (GBR), with aerial surveys reporting that >90% of reefs are bleaching, and with more that 50% of corals on these reefs already dead. However, these surveys cannot assess what is happening on the nearshore turbid-zone reefs, firstly because turbidity levels inhibit aerial assessments, and secondly because there is little ecological data against which to compare change. The bleaching response of corals on these turbid-zone reefs is however of significant scientific interest. This interest relates specifically to the hypothesis that there may be particular marine environments that might act as important refugia sites from bleaching i.e., settings that are more effectively buffered from surface warming such that coral populations remain largely unaffected. Reefs forming in well-flushed, highly turbid settings are one such candidate location for these refugia. Increased bleaching resilience has been hypothesized because high particulate content in the water may limit UV stress, and because the corals may be more readily able to switch to predominantly heterotrophic feeding modes - reasons for enhanced protection from thermal stress events that were initially hypothesised in the early 2000's. However, recent modelling now provides a global-scale framework through which the spatial extent of such potential refugia can be defined. What is lacking however is any empirical field evidence definitively showing that these turbid-zone reefs are actually able to withstand major coral bleaching events.

Without doubt the best studied of these turbid-zone reefs are those along the nearshore areas of the GBR, which have been the focus of intensive study by the PI and his group since 2006. This work has largely focused on assessing rates and styles of reef growth, but our most recent work has had as its central aim an assessment of te spatial extent and contemporary ecological structure of these reefs. Working at sites in the central GBR we have undertaken an unprecedented mapping and ecological surveying campaign, collecting >130 km of seafloor swath survey data and >4,500 video still quadrats. The resulting datasets have enabled us to develop high resolution maps of reef structure and ecological composition, which show that despite their narrow bathymetric extent, these reefs are characterised by a clear depth-controlled ecological zonation, and that they exhibit high live coral cover (mean: 38%, but up to ~80%). We are thus in a unique position to quantitatively assess the extent to which this major bleaching event has impacted these turbid-zone reefs, and to test the recently proposed hypothesis that such reefs may act as critical climate change refugia sites.

In this project we will undertake a rapid assessment of the impacts of bleaching on the turbid-zone reefs in the vicinity of Paluma Shoals (central Halifax Bay). We will re-examine a suite of six proximal reefs using remotely-operated underwater video survey methods and collect ecological data along replicate transects across each reef. Video data will be used to determine species abundance and bleaching intensity. This will allow us to ascertain: 1) the total extent of bleaching-induced mortality; 2) the extent to which specific coral species have been impacted; and 3) any immediate impacts on the structural complexity and diversity of the reefs. We will also undertake comparable assessments at other turbid-zone reefs which have been the focus of our earlier studies e.g., further north around Dunk Island and to the south at Middle Reef- these reefs occupying similar geomorphic and sedimentary settings to the Paluma complex. The work would thus deliver not only data on the extent of turbid-zone reef bleaching, but also provide a robust test of the hypothesis that turbid-zone reefs may form critical climate change refugia sites.

Planned Impact

The main beneficiaries of this work will include: academics interested in understanding coral responses to elevated sea-surface temperatures; coastal and marine park managers interested in identifying key potential climate change refugia sites that require elevated levels of protection; and the media and general public interested in the response of marine ecosystems to environmental change. Given the short-timescales and the necessarily relatively restricted scope of the work we plan to undertake, the capacity to interact with these potential beneficiaries is kept modest. A key dissemination outlet for engaging with other academics, managers and the media will be via peer-reviewed outputs - and we would anticipate at least one paper arising from this work. In addition we propose to attend and present at the Reef Conservation UK meeting in December 2016, which is actually the only major coral reef meeting scheduled between the period when the work can take place and the end of 2016. This meeting is typically attended by around 150 reef scientists, NGO workers and students, and will thus provide an ideal forum for us to disseminate the findings of this rapid impact assessment. We intend to do this both through a conference presentation and by providing a summary document on the findings of this study. This will be made available to delegates at the meeting as a hard copy pamphlet, but pdfs will also be made available on the Exeter Geography website (details of this will be posted on coral-list to ensure wide circulation). To engage with the media and with the general public we will also release (as appropriate) press statements to accompany any resultant papers, and will aim to submit a piece to NERC's Planet Earth online.


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Description High sea surface temperatures (SSTs) on the Great Barrier Reef (GBR) during summer 2015/2016 caused extensive coral bleaching, with aerial and in-water surveys confirming high (but variable) bleaching-related coral mortality. In contrast, bleaching impacts on nearshore turbid-zone reefs, traditionally considered more "marginal" coral habitats, were poorly documented. This is because rapid ecological surveys are difficult in these turbid water settings, and baseline coral community data from which to quantify disturbance are rare. However, models suggest that the extreme environmental conditions characteristic of nearshore settings (e.g., fluctuating turbidity, light and temperature) may acclimate corals to the thermal anomalies associated with bleaching on offshore reefs. Through this project e undertook a a novel pre- (June 2013/2014) and post-warming (August 2016) assessment of turbid-zone coral communities and examined the response of corals to prolonged and acute heat stress within the Paluma Shoals reef complex, located on the central GBR. Our analysis of 2,288 still video frames (~1,200 m2) which include 11,374 coral colonies (24 coral genera) suggest a high tolerance of turbid-zone corals to bleaching, with no significant changes in coral cover (pre: 48 ± 20%; post: 55 ± 26%) or coral community structure (e.g., Acropora, Montipora, Turbinaria, Porites) following the warming event. Indeed, only one coral colony (Lobophyllia sp.) exhibited full colony bleaching, and just 1.5% of colonies displayed partial pigmentation loss (<20% colony surface). Taxa-specific responses to this thermal stress event contrast with clear-water assessments, as Acropora corals which are normally reported as highly susceptible to bleaching on clear-water reefs were least impacted at Paluma Shoals, a phenomena that has been observed within other turbid settings. Importantly, field surveys confirm regional SSTs were sufficiently high to induce coral bleaching (i.e., comparable number of degree heating days in nearshore and offshore areas), but bleaching severity was much higher at central GBR offshore sites. A more optimistic outlook than is generally offered for nearshore reefs on the central GBR may be implied by our results, which highlights the importance of these resilient but often overlooked coral reef habitats as potential refugia during climate-related disturbances.
Exploitation Route This is important working in relation to understanding the capacity of turbid-zone reefs to act as important refugia from climate change-driven SST event
Sectors Environment,Leisure Activities, including Sports, Recreation and Tourism

Description Reef refugia out of the shadows: dynamics of marginal coral reef ecosystems over the past 30 million years in the Coral Triangle
Amount £496,431 (GBP)
Funding ID NE/R011044/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 04/2018 
End 03/2021