[Coral-List] Fast extension rates don’t always equate to a healthy coral

[Scott Wooldridge]

Terry Done (AIMS) took some amazing pictures showing that many of the branching  Acropora that survived the 2002 bleaching event on the southern Great Barrier Reef (GBR) where characterised by what he called 'skinny attenuated tips' (i.e. high extension / low density).

This is interesting, because this highly attenuated skeleton provides an ideal skeletal framework which can subsequently be 'infilled/thickened' in the months-years that follow - provided the coral survives. In this way, the 'bleaching stress' can be masked in the sclerochronological record by what appears (i.e. is measured as) fast growth - compared to its norm.

However, if the stress is severe, and the energy resources of the coral host near-depleted, skeletal extension ceases. This response has also been observed - resulting in 'stress bands' of high density / low extension skeleton. This response is also often recorded at the final skeletal growth front of corals that have died.  

The take home point being, that the skeletal growth response of symbiotic corals subject to autotrophic stress typically displays as a maximum before ‘turnoff’. This makes it extremely difficult to utilise the skeletal record (alone) to assess coral ‘health’ (i.e. the probability of persistence of a reef-building, reproducing entity).   However, the trajectory of a coral towards the species-specific ‘turnoff’ point can be charted. And it is this fact, which provides some worrisome conclusions for the GBR, as discussed further discussed further and references provided within:

Wooldridge SA (2014) Assessing coral health and resilience in a warming ocean: why looks can be deceptive. BioEssays 36(11):1041-1049

Cheers,
scott

________________________________________
From: Andrew Ross [ross.andrew at mac.com]
Sent: Saturday, 11 April 2015 12:14 AM
To: coral-list at coral.aoml.noaa.gov
Cc: Scott Wooldridge
Subject: Re: [Coral-List] Fast extension rates don’t always equate to a healthy coral

List,
Scott and I and others have discussed a little bit already and it might be worth bringing up here:
When I was young and energetic I measured Acropora cervicornis apical polyp extension weekly for about 5-months, including through death in the 2005 Caribbean bleaching event.
Extension rates increased in the days/weeks leading up to and into bleaching, including the times of highest water temperatures. Bleaching didn’t set in in earnest until temperatures had started to wane.
This was Montego Bay, Jamaica, where the water is generally warm (28C) and enriched.
In the week(s) of full bleaching leading to final death, I often noted shrinking inclusive of skeleton, though this was likely just in the soft apical skeleton.
The formal paper is maybe 60% complete. If anybody is curious, it’s in my thesis on ResearchGate.
Andrew

On Apr 9, 2015, at 6:25 PM, Scott Wooldridge <S.Wooldridge at aims.gov.au<mailto:S.Wooldridge at aims.gov.au>> wrote:

Just following on from Doug’s comments.  The paper can be downloaded from my ResearchGate page.

Wooldridge SA (2014) Assessing coral health and resilience in a warming ocean: why looks can be deceptive. BioEssays 36(11):1041-1049

Abstract: In this paper I challenge the notion that a healthy and resilient coral is (in all cases) a fast-growing coral, and by inference, that a reef characterised by a fast trajectory toward high coral cover is necessarily a healthy and resilient reef. Instead, I explain how emerging evidence links fast skeletal extension rates with elevated coral-algae (symbiotic) respiration rates, most-often mediated by nutrient-enlarged symbiont populations and/or rising sea temperatures. Elevated respiration rates can act to reduce the autotrophic capacity (photosynthesis:respiration ratio) of the symbiosis. This restricts the capacity of the coral host to build and maintain sufficient energy reserves (e.g. lipids) needed to sustain essential homeostatic functions, including sexual reproduction and biophysical stress resistance. Moreover, it explains the somewhat paradoxical scenario, whereby at the ecological instant before the reef-building capacity of the symbiosis is lost, a reef can look visually at its best and be accreting CaCO3 at its maximum.

I also have a soon-to-be published manuscript which highlights that bleaching-sensitive reef areas on the Great Barrier Reef during the 1998 and 2002 mass bleaching events include old massive Porites colonies that have multi-decade sclerochronological histories characterised by regionally-enhanced skeletal extension rates (and reduced skeletal densities) outside of the bleaching events.

Cheers,
scott
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