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

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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 the 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.