1. Dep. of Physical and Analytical Chemistry, Analytical Chemistry; Uppsala Univ.; Box 599 751 24 Uppsala Sweden
2. Inst. of Biology, Univ. of Southern Denmark; Campusvej 55 DK-5230 Odense M Denmark
3. Dep. of Ecology and Evolution, Limnology; Uppsala Univ.; Box 573 751 23 Uppsala Sweden
4. Dep. of Biochemistry and Organic Chemistry; Organic Chemistry, Uppsala Univ.; Box 599 751 24 Uppsala Sweden