[Coral-List] Is there a time limit up to which buoyant weight based growth rates are useful in corals?
Thomas Krueger
thomas.krueger at epfl.ch
Tue Jan 3 05:11:20 EST 2017
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.
--
*--*
*Thomas Krueger */Postdoctoral Researcher/
Laboratory for Biological Geochemistry | École polytechnique fédérale de
Lausanne (EPFL)
*P*: (+41) 21 69 38039
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