Geochemical aspects of aqueous iron, phosphorus and dissolved organic carbon in the humic Lake Tjeukemeer, The Netherlands

Abstract

SUMMARY. 1. The main source of P, Fe and DOC in the humic 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 acids (FA)) were determined by ultrafiltration and Sephadex G‐100 gel filtration. Throughout the year most dissolved P and Fe was in colloids >35 nm 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 fulvic acids 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 Fen‐1(FA)n.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 which might have been organically coated.