[Coral-List] Identifying bleaching resistant reef location: Should we have expected the far north (remote) GBR to have been so vulnerable to coral bleaching?

Scott Wooldridge swooldri23 at gmail.com
Sun Apr 10 22:14:57 EDT 2016

Hi All,

I have received numerous emails asking whether the ‘BleachRisk’ model
(attached papers below) correctly identified the remote far northern reefs
of the GBR as sensitive to thermal stress – given the extensive bleaching
that has occurred this 2016 summer. The answer is YES – most definitely.



I am in the process of writing a short report that details the
environmental reasons why, and I will make this report available to
CoralList in the next day or so. In the interim it is worthwhile to think
on (and rationalise against current opinions) the physical reef attributes
that the BleachRisk framework predicts to be indicative of reefs with ‘low
bleaching resistance’ (i.e. high bleaching sensitivity).

Quoting directly from the manuscript, ‘Statistically significant linear
regression relationships linked ‘low’ *bleaching resistance* (i.e., high
bleaching sensitivity) reef areas with *Porites *colonies that have reduced
tissue layer thickness (Fig. 8a) and multi-decade sclerochronological
histories characterised by reduced skeletal density (Fig. 8b), and enhanced
skeletal extension rates (Fig. 8c).’

Therefore contrary to most people’s opinion, the BleachRisk model predicts
that a bleaching sensitive reef will have fast-growing corals (based on
regional comparisons). That is, fast skeletal growth at ambient/optimal
temperatures is actually a very good proxy for bleaching sensitivity under
thermal stress.

I have recently written a ‘Think again paper’ on this issue, which explains
why a fast growing reef with high coral cover may be judged healthy under
present conditions, whilst at the same time being extremely vulnerable to
future thermal stress levels.


Indeed, this explanation fits perfectly with the situation in the far
northern GBR, which for example, has Porites colonies with the highest
annual skeletal extension rates recorded across the entire GBR (Lough and
Barnes, 2000).  To highlight this fact, I have redrawn Fig. 8 to include
the skeletal growth attributes of Porites in the Far North.


This identified spatial linkage between an ‘inferred’ model response and an
independent ‘observable’ biological responses (not included within the
model calibration process), strengthens our confidence in the model,
including the suggestion that bleaching resistant (versus bleaching
sensitive) reefs are readily identifiable and explainable by characteristic
coral health signatures that exist outside of thermal stress conditions.

Such coherence between predicted and observed responses is far and away the
best test of the reliability of a model – independent of the choice of
predictive error score and the like.

Scott Wooldridge

Catchment to Reef Management Solutions, Newcastle, Australia (2280)


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