Water samples from all incubations were withdrawn periodi-cally over a period of 35 days. All samples were filtered with syringefilters (0.45 m, PTFE; Chromacol, Partille, Sweden); 400 L werethen transferred to a glass vial and stored at −18◦C. Prior to anal-ysis, the samples were thawed and 100 L of a solution containingthe isotope-substituted standards (final concentration: 100 g L−1)and HAc (final concentration: 10 mmol L−1) in ACN were added.