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

Esther Peters estherpeters at verizon.net
Fri Apr 20 10:39:41 EDT 2018

Thanks very much for the great information, Scott! We do need to look 
further for what might be occurring with interacting stressors and their 
sources. I wonder if the relationship of DIN to thermal stress might 
also explain Caribbean bleaching events? In particular, the South 
Florida bleaching events of 2014 and 2015, perhaps they coincided with 
additional DIN loading in the coastal waters? The real cause(s) may not 
ever be known, but deeper understanding of ocean phenomena must be our 
goal to help managers of both land and sea resources.

Esther Peters, Ph.D.
Department of Environmental Science & Policy
George Mason University

On 4/20/2018 6:26 AM, Scott Wooldridge wrote:
> 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.
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