[Coral-List] Cooling reefs to prevent bleaching

[Thomas Goreau]

>

Pumping water from below the thermocline to protect corals from  
bleaching seems to have become something of a fad lately, but this is  
very easily seen to be untenable. If the temperature differential  
between surface and deep waters is 10 degrees C, and coral reefs are  
elevated by 1 degree C above their bleaching thresholds, then one  
needs to pump enough water to mix at least 10% deep water into surface  
waters to get temperatures down to safe levels, but it needs to be  
understood that this implies a very large vertical flux because  
surface waters are not static, that is to say the water being pumped  
up must be at least 10% of all horizontal wind, wave, tidal, and  
current driven transport of surface waters at the site. However, deep  
waters in the Caribbean contain nutrient concentrations around 30  
times the eutrophication thresholds for coral reefs, and deep  
IndoPacific waters about 60 times these levels, so even if there are  
ZERO nutrients in surface waters (that is, places lacking ANY land  
based sources of nutrients), then only a 3.3% mixture of deep  
Caribbean waters or a 1.6% mixture of deep IndoPacific waters would  
trigger massive benthic algae blooms, and if there are any land based  
sources of nutrients the mixtures must be considerably smaller.  
Therefore one can't protect HotSpots from bleaching by artificial  
upwelling unless you want to smother the corals with weedy algae.  
These algal blooms are well known to follow cold water upwelling  
events, and have been especially well documented in reef locations  
with high upwelling such as the Gulf of Elat/Aqaba, the Arabian Gulf,  
Houtman Abrolhos, Lord Howe Island, Galapagos, and high algae reefs on  
remote uninhabited islands across the Central Pacific with very  
shallow thermoclines.

Thomas J. Goreau, PhD
President, Global Coral Reef Alliance
Coordinator, United Nations Commission on Sustainable Development  
Partnership in New Technologies for Small Island Developing States
37 Pleasant Street, Cambridge MA 02139
617-864-4226
goreau at bestweb.net
http://www.globalcoral.org

> Message: 1
> Date: Thu, 08 Jan 2009 12:21:17 -0500
> From: Mark Eakin <Mark.Eakin at noaa.gov>
> Subject: Re: [Coral-List] Cooling reefs to prevent bleaching
> To: Coral Listserver Listserver <coral-list at coral.aoml.noaa.gov>
> Message-ID: <39AF8056-8651-40A9-9E3A-0C52CEFD0518 at noaa.gov>
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>
> James,
>
> I suggest you revisit the Muscatine et al. (1991) study and consider
> the rates of cool water and nutrient delivery and dilution before
> dismissing ideas such as these.  The waters are unlikely to be more
> than 10 degrees cooler than the surface waters and considering the
> potential rates of water deliver, are likely to be diluted by 10:1
> before reaching the corals.  Even if we assume only 5:1 dilution, and
> remember that such proposals are for times of severe thermal stress
> only, this will barely bring the temperatures below the maximum
> monthly mean temperatures the corals experience as well.  To quote
> Muscatine et al. (1991): "Above about 16 "C, and up to 28 "C, the
> response is similar to that of unshocked controls. "  All proposed
> mechanisms to cool reefal waters that I have seen are unlikely to
> lower temperatures to much below 28?C and would only maintain the
> water flow until seasonal change brings the temperatures back below
> critical levels.  Nutrients and acidification are certainly concerns.
> (Please note though, that Cox & Ward (2002) increased and maintained
> ammonium concentrations in microcosms levels exceeding 20X ambient
> concentrations for a year.)  Potential enrichment and  acidification
> and their potential impact on the coral-algal symbiosis need to be
> evaluated.  This is the purpose of feasibility studies that are needed
> to evaluate potential side effects such as nutrients and acidity
> during temporary cooling (or shading, etc.) and weigh them against the
> effects of inaction (bleaching, disease, mortality).  Our current
> bleaching outlook calls for a high potential of bleaching-level
> thermal stress in the GBR and nearby regions over the next two  
> months (http://coralreefwatch.noaa.gov/satellite/bleachingoutlook/index.html
> ).  Protecting key reefs could be an important way to help corals
> survive and the hope is that tools and approaches to protect corals in
> times of high thermal stress will be available in the future.
>
> I also do not recall ever putting up a slide that showed acidic
> conditions being more of a concern on the "coral killing pecking
> order" than high temperatures and bleaching.  I certainly did not do
> so in my Ocean Sciences presentation to which you referred.  That talk
> focused on the extent and magnitude of the bleaching, disease, and
> mortality caused by thermal stress in the Caribbean in 2005.  It did
> contain a table from the Kleypas and Eakin (2007 Bulletin of Marine
> Science 80: 419-436) paper that listed the top threats to coral reefs
> as determined by a survey of coral reef scientists and managers at the
> 10th ICRS, which that ranked Climate Change / Bleaching at #9 (higher)
> and Ocean Acidification at #47 (lower) and discussed the importance of
> considering the two together based on our Hoegh-Guldberg et al. (2007
> Science 318:1737-1742) paper.  Bleaching certainly presents a more
> clear and immediate threat to coral reefs than acidification.
> However, the latter has the potential of more broad-reaching and long-
> lasting effects in marine ecosystems than the former, so both must be
> considered as important reasons why we need to get atmospheric CO2
> under control.
>
> The key point in my original post was that the threat to coral reefs
> is too great to eliminate potential approaches out of hand without
> testing and evaluation, whether the approaches be assisted adaptation
> or local mitigation.  This is the process of science and it should be
> allowed to run its course.  We are going to need a wide range of tools
> at our disposal and should test ideas and submit the results to peer
> review before stating unequivocally that they cannot work or
> dismissing them as crazy.
>
> Cheers,
> Mark