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

[Scott Wooldridge]

Further to my earlier post, I provide here details which help reconcile why
corals in the far northern GBR were very sensitive to bleaching at low
thermal stress levels in 2015/16, whilst being bleaching tolerant at high
thermal stress in 2008/09. (see earlier post for details).


Across multiple past manuscripts, I have demonstrated that on the GBR the
environmental feature that unites reef sites which display enhanced
sensitivity to thermal stress is an excess of bioavailable dissolved
inorganic nitrogen (DIN = nitrate + nitrite + ammonium) in the surrounding
seawater. The GBR-specific DIN threshold of concern is associated with a
seawater concentration of chlorophyll-a >0.45 µg.L-1. Reef locations that
exceed this threshold are ~2-4 times more sensitive to bleaching at
progressive levels of thermal stress.


For most recent details see:

https://www.researchgate.net/publication/308746785_Excess_seawater_nutrients_enlarged_algal_symbiont_densities_and_bleaching_sensitive_reef_locations_1_Identifying_thresholds_of_concern_for_the_Great_Barrier_Reef_Australia


https://www.researchgate.net/publication/308746844_Excess_seawater_nutrients_enlarged_algal_symbiont_densities_and_bleaching_sensitive_reef_locations_2_A_regional-scale_predictive_model_for_the_Great_Barrier_Reef_Australia


This relationship is most easily demonstrated for the inshore reefs of the
central and southern GBR wherein terrestrial river discharge is the
dominant DIN source. Regional bleaching thresholds are seen to vary in
direct response to the degree of exposure to DIN-enriched discharge. See:

https://www.researchgate.net/publication/223780705_Water_quality_and_coral_bleaching_thresholds_Formalising_the_linkage_for_the_inshore_reefs_of_the_Great_Barrier_Reef_Australia


For the far-northern portion of the GBR, the situation is slightly
different, as the dominant DIN source is not terrestrial runoff but
offshore upwelling. Given the remoteness of these reefs few direct field
measurements are taken on a regular basis. Remote-sensing techniques can
provide some insight, but are challenged by inherent theoretical and
technical difficulties. By utilising a modified approach based on
remote-sensing data it is possible to demonstrate that natural shelf-edge
upwelling can cause the chlorophyll-a > 0.45 µg.L-1 (= enhanced bleaching
sensitivity) threshold to be exceeded on the majority of reefs in the far
northern GBR. In this way, summer offshore upwelling can predispose the
reefs in this region with a high sensitivity to thermal stress. For
details, see:

https://www.researchgate.net/publication/301614678_Understanding_coral_bleaching_risk_factors_in_the_remote_far-northern_Great_Barrier_Reef_GBR_that_make_obvious_the_best_'local'_management_option_for_maximising_the_capacity_of_the_GBR_to_resist_therm


However, as explained in the report, this upwelling process is dynamic on
an annual basis, based largely on fluctuations in the southern oscillation
index (SOI). When there is a strong El Niño (extremely negative SOI), sea
levels in the Western Pacific (including the GBR) are generally higher due
to warmer, expanded ocean waters and changing weather patterns (Steinberg
2007). Tides ‘ride’ on top of sea level and are influenced by what is
happening at any given time with climate and weather. This means that
normal everyday high tides are already higher because of El Niño. On days
when there are large spring ‘king’ tides, they become even higher. Higher
tidal ranges can be understood to enhance ‘tidal pumping’ pressures in
narrow channels, leading to greater upwelling forces at the shelf edge.
Furthermore, during El Niño, the thermocline is much closer to the surface
in the Western Pacific (Steinberg 2007), making access to the cooler,
nutrient-rich waters much more easily available to continental shelves. In
this way, we should expect, and have observed that the upwelling on the GBR
is enhanced by extreme El Niño events – such as occurred in 1982,1998,
2016. Equally, we should expect minimal/no upwelling in the opposing strong
La Nina (extremely positive SOI) condition.


With the expected pattern of summer upwelling (i.e. nutrient-enrichment) in
strong El Nino years and no upwelling in strong La Nina years, we are
therefore in a position to assess if this phenomenon can help explain why
corals in the far northern GBR were very sensitive to bleaching at low
thermal stress levels in 2015/16, whilst being bleaching tolerant at high
thermal stress in 2008/09.


Indeed, the results are clear and unequivocal. See:

https://www.researchgate.net/publication/324647722_Comparison_of_SOI_on_the_GBR_for_the_summer_of_200809_and_201516


The summer of 2008/09 was associated with a strong La Nina (no upwelling)
whilst the summer of 2015/06 was associated with an extremely strong El
Nino (nutrient rich upwelling). In this way, it is evident that reefs in
the far northern GBR can be reconciled as either sensitive or tolerant to
thermal stress, depending on the upwelling situation. And the degree of
difference is significant. With upwelling, DHW < 3-4 causes SEVERE
bleaching, without upwelling DHW>8-10 causes NO bleaching.


This is HUGELY significant.


Unfortunately it is unlikely that anything can be done to stop upwelling
occurring – given that it is an ocean-scale phenomenon. Thus, the annual
bleaching risk for reefs in the far northern GBR will always be a dynamic
risk profile (lottery) that is related to the chance of significant
upwelling (El Nino) intersecting with warm SSTs. In 2008/09 the reefs got
'lucky' whilst in 2015/16 that luck ran out.


But what of the inshore reefs of the central and southern GBR for which
terrestrial DIN is the dominant annual source of enrichment? This is a
process for which human intervention can work. Indeed, modelling results
highlight that it is well within the realm of management intervention to
bring the annual exeedence level for chlorophyll-a >0.45 µg.L-1 on these
reefs to very low levels. For example, a reduction of ~60% in river DIN
loads can ensure that these reefs are unlikely to experience chlorophyll-a
>0.45 µg.L-1 for anything but the largest of floods (i.e. flows greater
than a 1 in 15 year flood event). See:

https://www.researchgate.net/publication/278328873_Ecologically_based_targets_for_bioavailable_reactive_nitrogen_discharge_from_the_drainage_basins_of_the_Wet_Tropics_region_Great_Barrier_Reef


Indirectly, we already have good evidence of the potential improvement in
thermal tolerance based on river load reductions in DIN. For example a
severe drought resulted in zero discharge from the rivers around the
Townsville region of the GBR between 2001 and 2004 (representative of a
100% in river DIN load). Notably, Berkelmans (2008) records that during
2004 and 2005 numerous reefs in this region exceeded the thermal thresholds
by up to 2 degrees that had previously caused significant bleaching in 1998
and 2002, yet no bleaching was recorded.


There can be hope for some reefs. We can help to increase the ability of
corals to resist thermal stress. And the degree to which we can do it is
significant (~2 degrees). Indeed, modelling work highlights that local
water quality improvements (~60% DIN) in addition to restricting global
temperatures rises to less then 2 degrees Celsius can ensure that the
prospects for a healthy and resilient GBR in this particular region are
very strong even beyond 2100. See:

https://www.researchgate.net/publication/235762816_Safeguarding_coastal_coral_communities_on_the_central_Great_Barrier_Reef_Australia_against_climate_change_Realizable_local_and_global_actions


We don't need sensationalism. Rather, as a science (coral) community we
need to work together to deliver strong case study examples for reef
managers and policy makers which highlight the benefits of oligotrophic sea
water conditions for improving the thermal tolerance of coral reefs.


Scott

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


p.s. it is surely an overload, but for those who would like to understand
the physiology of the coral-algae symbiosis that underpins the above
phenomenon, and why seawater pCO2 > 260 ppm and irradiance levels > 900
umol.m-2.s-1 are also important thesholds to this story (and the testing
thereof), see:

https://www.researchgate.net/publication/317100418_Instability_and_breakdown_of_the_coral-algae_symbiosis_upon_exceedence_of_the_interglacial_pCO2_threshold_260_ppmv_the_''missing''_Earth-System_feedback_mechanism



Cited Literature

Berkelmans R (2009) Bleaching and mortality thresholds: how much is too
much? In: van Oppen MJH, Lough JM (eds) Coral bleaching: patterns,
processes, causes and consequences. Ecological Studies, Springer, pp 103–119

Steinberg C., 2007. Impacts of climate change on the physical oceanography
of the Great Barrier Reef. In: Johnson, J.E., and Marshall, P.A. (Eds.)
Climate Change and the Great Barrier Reef: a Vulnerability Assessment.
Great Barrier Reef Marine Park Authority and Australian Greenhouse Office,
Townsville, pp. 51-74.