[Coral-List] PAM measurements with Acropora

[Monika Wilhelm]

Dear listers,

about a month ago I inquired about coral tank experiments for my 
master thesis and now that I have started my first test runs I would 
like to ask for your advice once more.
In fact, I am donig tank experiments on combined temperature and light 
stress with Acropora and Porites from two different sites in Reunion 
island. As part of these experiments I want to measure the Fv/Fm (dark 
adapted yield) and F/Fm'(light adapted yield) for both species. 
However, I currently face the problem that I have a very high 
variation of values at different positions on the branches of 
Acropora. On the sides the yield is still high after 2 days of stress, 
also during the day, whereas close to the top the yield is a lot 
lower. I have read in the literature that for Acropora one should 
avoid to measure at the top but then the sides of my test fragments 
are mainly shaded due to the shape of the branches, which might be a 
reason why the yield is still high.
Also I haven't found a solution yet to always measure at the same 
position of the branch, which I consider essential for my actual 
experiments.

Against this background, can anyone provide advice on the following 
questions:
1)Which is an appropriate position on the branch for Acropora in order 
to be able to compare results to a massive Porites?
2)Do you have an advice what kind of device (some sort of sample stand 
that I can position in the tank) could work to insure a set geometry 
and thus to measure always on the same position of the branch?

I would be really happy if you could help me with this.

Thanks a lot in advance.

Kind regards,
Monika