[Coral-List] poleward migration of corals

[David Hopley]

Dear Listers,

 

I have just been catching up with the discussion on the poleward expansion
of coral reefs in response to global warming and could not agree more with
the comments of Vassil Zlatarski about the need for an integrated approach
of all disciplines, including biological and geological, to the problems
posed.  Gene Shinn has noted the relevance of the geological past, that
major catastrophes have occurred in the early Holocene long before any
anthropogenic influences could be active.  Incidentally there may be a
history of early Holocene coral reef demise on the Great Barrier Reef
similar to that in the Caribbean. In my recent book (with Scott Smithers and
Kevin Parnell) we summarised the small amount of research carried out on a
massive area of shelf edge submerged reefs stretching along at least 75% of
the GBR margin and as far as we know, dead.  Current research by Jody
Webster, Peter Davies and others is highlighting the significance of these
reefs which may have an area of up to 50% of that of the GBR (reefal area
20,000km²) Because of their depth they may retain up to 85% of the
Quaternary history of the GBR compared to about 15% for the current shelf
reefs which were active only during interglacial high sea level periods.

 

Obviously the past can tell us much about reef response to present and
future climate change.  As Gene has noted, both the mid-Holocene and the
last interglacial had temperatures higher than today in many reefal areas.
However, temperature is only one of a whole range of parameters controlling
coral reef growth.  Certainly coral communities may move polewards with
warmer waters but only if these waters are not polluted by run-off with
enhanced sediment and most of all nutrient yield.  However, I am surprised
that one of the most important factors determining where our reefs are today
has not been raised in this discussion.  Nearly all our modern reefs in the
GBR (and I believe elsewhere in the world) are over Pleistocene reefal
foundations.  The Holocene reef is generally only 5m to 15m thick. Other
topographic highs occur in reefal waters but do not seem to have attracted
reef communities with a permanency to build a true reef and some rock types
appear to be particularly unsuitable.  This of course leads to the question
- are there reefal foundations currently unoccupied which are polewards of
present reefs?  In some areas the answer is yes but then a whole range of
local geographic factors come into play.  There is an excellent paper by
Michael O'Leary, Paul Hearty and Malcolm McCulloch about to be published in
Palaeogeog. Palaeoclim. and Palaeocol. Which illustrates this, (I hope the
authors do not mind me highlighting their paper). They examine the coast of
Western Australia where there are last interglacial reefs south of the
present reef growth but factors involved include changing geography and
bathymetry with different sea levels which in turn affected salinity and
other factors influencing reef growth.

 

Higher water temperatures alone will not see new poleward reefs developing
even during the next millennium and who is to say what other factors may
arise in the intervening time.  As analysed in Chapter 11 of Hopley,
Smithers and Parnell, 2007, even where appropriate reefal foundations
occurred and in totally non anthropogenically impacted environments, it took
at least 2000 years for recolonisation of these foundations and formation of
the first Holocene reefs.  Certainly if reef communities do move polewards
in the next few decades let us give them as much protection as we can, but
don't expect too much from them. 

 

David Hopley

 

 

Dr. David Hopley

Email:  david.hopley at bigpond.com