[Coral-List] Is there a time limit up to which buoyant weight based growth rates are useful in corals?

[Thomas Krueger]

Happy New Year to everyone and thanks for all the comments and 
suggestions to my question. Since a number of people were interested in 
the answer to this issue, I will do my best to give you the short 
version of an answer. Langdon, Gattuso & Anderson 2010 (Chapter 13 in 
Riebesell et al 2010 Guide to best practices for ocean acidification 
research and  data reporting) recommends to measure buoyant weight as 
the ratio of the change in dry skeletal weight and the number of days 
between weighing. While this is certainly a good approximation for 
weight increments especially on small time scales or small weight 
increments, one does inevitably run into the problems mentioned in my 
original post as highlighted by Goreau & Goreau 1960 (Biological 
Bulletin 118 (3) 419-429) and Kawaguti 1941 (Palao Trop. Biol. Sta. 2: 
309-317). I would like to thank Prof Rolf Bak for sending me his paper 
(Bak, R.P.M. 1976. The growth of coral colonies and the importance of 
crustose coralline algae and burrowing sponges in relation with 
carbonate accumulation. Neth. J. Sea Res. 10: 285-337) that provides 
repeated buoyant weight measurements from different corals and nicely 
illustrates this non-linear weight gain over time (Figs. 2-5 in the 
publication). As mentioned in my original post, the problem with the 
buoyant weight technique is that it is not normalized to any other 
metrics. The data of Bak 1976 clearly show that one should simply apply 
an exponential growth formulae on repeated weight measurements rather 
than a linear one to obtain a mg weight increment per day value. Prof 
Bak writes in this regard: "Comparisons of growth should be based on the 
production per unit of living surface area, or when data on living 
surface area are lacking on exponential growth formulae". For the 
practical application of buoyant weight measurements this means that one 
should perform occasional weighing over the period of interest and fit 
an exponential equation to derive a daily growth rate (as absolute 
weight increment per day), rather than dividing the difference between 
initial and end weight by the number of days. Again, for some corals 
that mainly grow along defined axes (e.g, vertical linear extension in 
digitate growth forms) the difference between the derived growth rates 
from a linear or exponential growth formula will probably be very small, 
but especially for massive and foliaceous corals were surface areas 
increase across the colony, the effects will be much bigger and warrant 
the use of the correct formula. If you are interested in reading the Bak 
1976 publication and do not have online access to it, you can email me 
or Prof Bak directly 
(http://www.researchstationcarmabi.org/research-station-carmabi/scientists/prof-dr-rolf-bak/). 
Thanks for all the feedback.


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*Thomas Krueger */Postdoctoral Researcher/

Laboratory for Biological Geochemistry | École polytechnique fédérale de 
Lausanne (EPFL)

*P*: (+41) 21 69 38039