[Coral-List] Coral on the Great Barrier Reef was 'cooked' during 2016 marine heatwave. REALLY? REALLY? REALLY? #2

[Scott Wooldridge]

Hi John,

Many thanks for your response. Sorry for my slow response. Seems i have a
email feed for corallist that is much slower than some.

First, can i just make very clear that I dont question in the slightest
that man-induced (rising atmospheric pCO2) warming is the driver of modern
mass coral bleaching events on the GBR and elsewhere. I have no hidden
agendas, and strongly support the hope of most scientists that global
temperature rises can be kept below 1.5-2.0 degrees celsius across the
coming century through aggressive global CO2 mitigation strategies
(principally a transition away from fossil fuel intensive economies).

My thesis is that warming ocean temperatures (and rising pCO2) have served
to make previously acceptable levels of nutrient-enrichment (from
terrestrial runoff and/or natural upwelling) now unacceptable and
destabilising for the coral-algae symbiosis. In this way, bleaching
sensitivity (per unit thermal stress) can be expected (and has been
observed by many authors) to increase in response to variable gradients of
nutrient enrichment. The driver (triggering event) of the event is the well
reported combination of anomalous SST and irradiance, but the sensitivity
of the bleaching (and possible mortality) response is co-dependent on
nutrient enrichment levels.

To see the above description represented visually, see my Fig. 2 in:

https://www.researchgate.net/publication/299939163_Excess_seawater_nutrients_enlarged_algal_symbiont_densities_and_bleaching_sensitive_reef_locations_2_A_regional-scale_predictive_model_for_the_Great_Barrier_Reef_Australia

n.b. my analysis for the GBR also highlights that the thermal history of a
site interacts with the nutrient status of the site in determining
bleaching sensitivity/resistance. All this is well described in the above.


I hope that is really all really clear.

In terms of the 2015/16 bleaching and SSTs on the GBR i think it would be
great to hear from Mark Eakin (NOAA). I consider Mark a good friend and
have the greatest of respect for the NOAA coral bleaching products. No one
should know better than Mark whether the degree of bleaching and mortality
in the far northern GBR in 2015/16 was expected given the observed level of
heating, and other precedent events such as 2008/09.

Any chance you could give a little summary Mark? And your take on whether
other factors (beyond heat stress alone) might be at play? As scientists,
do you believe we should be looking at other factors beyond SST alone


scott

https://www.researchgate.net/profile/Scott_Wooldridge






> Dear Scott,
>
> Thank you for sharing your idea about the real cause of mass bleaching on
the GBR. As your’e probably aware http://www.co2science.org (a fossil fuel
industry funded website) has been pushing this idea for a while. Arguing
that pollution, fishing, seaweed etc are the real reason corals are
bleaching and dying. Despite that, I’m open to the hypothesis. But based on
past experiences on the coral-list, it seems to me the more vocal and
confident people are about the role of nutrients, the less data / science
they have in support of their explanations.
>
> In your case, what makes you think thermal stress was low or lower in
2016? You claimed DHW was only 3-4, but that contradicts the evidence:
Hughes et al 2081 report that across the northern GBR, DHW was ~ 8-14
preceding the event and that severe bleaching occurred on reefs with DHW
values of ~6-10. The NOAA Coral Reef Watch portal indicates the same:
https://coralreefwatch.noaa.gov/vs/gauges/gbr_far_northern.php
>
> Moreover, 2016 was the warmest year on earth in recorded history (NOAA
ranks 2009 as the 8th warmest). The ABM confirms this for the GBR: 2016 was
the warmest ever, and far warmer than 2009: https://www.dropbox.com/s/
2q9uk1xewedtp58/Feb-2016-sea-surface-temperature.png?dl=0
>
> So your’e wrong about the thermal stress. And you don’t provide any
values on nutrient concentration. While chlorophyll conc. is often
generally indicative of nutrient conc., the relationship is very messy and
chlorophyll can’t be used to make precise predictions about DIN. For one,
other factors influence chlorophyll, including temperature, predation,
other nutrients, etc. And to make such a comparison, you’d have to control
for other factors demonstrated to have strong effects on community thermal
sensitivity, eg coral composition and cover.
>
> Again I’m open to the idea and anything we can do to meaningfully reduce
bleaching. But wouldn’t you think that if local N pollution increased
bleaching sensitivity by ~2C (Wooldridge<https://www.
researchgate.net/profile/Scott_Wooldridge> 2009) we’d be able to detect
that in nature? Why would reefs in pristine locations ever bleach? We've
seen so many highly isolated, “pristine” reefs bleach w mass coral
mortality over the last decade (not only the N GBR), I’ve become suspicious
of claims about local drivers of bleaching sensitivity. Moreover, we’re
losing coral as rapidly on isolated atolls as we are on reefs adjacent to
inhabited, industrialized coastlines (https://www.nature.com/
articles/srep29778).
>
> Regardless, I appreciate you sharing your passion and ideas. But maybe
next time come armed with some evidence.
>
>
> John Bruno
> Professor, Dept of Biology
> UNC Chapel Hill
> www.johnfbruno.com<http://www.johnfbruno <http://www.johnfbruno.com/>