sion was stirred with a magnetic stirrer. Then the suspension was
centrifuged with 4000 rpm at 4 C and filtered through glass fiber
filters (Whatman G6, Whatman, England). This means that, in compliance
to the definition of DOC, the filtrate contained colloidal and
even fine-particulate carbon. All stock solutions were diluted with
Berlin tap water until they had an identical light extinction. The
exposure concentrations lay between 1.5 mM (P. pungens) and
2.38 mM (P. nigra) (Table 1) due to different specific UV absorption
of the leachates. Concentrations of non-fractionated dissolved organic
carbon samples were determined by means of a TOC-505 A
analyzer (Shimadzu, Germany).