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L The main source of P, Fe and DOC in the humie Lake Tjeukemeer is superfluous water pumped from surrounding peaty polders. Most particulate P is intracellular but almost all particulate Fe is abiotic.2. The size and molecular weight of the P, Fe and DOC (mainly fulvic aeids (FA)) were determined by ultrafiltration and Sephadex G-lOO gel filtration. Throughout the year most dissolved P and Fe was in colloids >35nm with apparent molecular weight between 30,000 and 150.000. The bulk of FA occurred in particles <35nm.3. Calculating the atomic ratios of P. Fe and organic C in the different size classes revealed that Fe-FA chelates are a minor species of the Fe pool. Less than 10% of the fulvie aeids occurred as Fe-FA chelates.4. Based on their apparent size and molecular weight, the Fe-FA chelates are colloidal aggregates, probably with the formula 5. About 50% of the dissolved P had the same size as the Fe-FA aggregates. These aggregates were only noticeable in winter when humus-rich polder water was flushed through the lake. During the rest of the year the dissolved Fe and P consisted mainly of acid-labile inorganic colloids whieh might have been organically coated.

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