Hi Andrea,

 I don't know whether the following is of any use.  I know the paper refers
to Finland, but the isotopic values may be of some use.

Osmond C.B. et al. 1981.  Comparisons of d13C values in leaves of aquatic
macrophytes from different habitats in Britain and Finland; some
implications for photosynthetic processes in aquatic plants.  Oecologia,
50, 117-124

Cheers,

Chris

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Dr. Chris S.M. Turney
Research School of Earth Sciences
The Australian National University
Canberra, ACT 0200
Australia

Fax: 02 6249 0738                     Tel: (wk) 02 6249 3412
                                                (+61 2 6249 3412)
    (+61 2 6249 0738)

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