Abstract
IJsselmeer was formed in 1932 through the closure of the Afsluitdijk that separated the artificial lake from the former Zuiderzee estuary. The palaeoecology of IJsselmeer was studied on a 63-cm-long sediment core. Lithology and microfossil data, particularly the diatom flora, clearly show the transition from the marine Zuiderzee into the freshwater IJsselmeer. Trophic conditions in IJsselmeer since 1932 have been inferred by qualitative and quantitative diatom-based approaches: by plotting the distribution of trophic categories based on published trophic indicator values, by a canonical correspondence analysis (CCA) yielding relative total phosphorus (TP) changes and by applying a transfer function in order to calculate TP concentrations. All three approaches indicate that IJsselmeer initially was meso- to eutrophic. A first hypertrophic period is indicated for the mid-1940s, likely due to internal loading. After 1960, the phosphorus load steadily increased and TP in IJsselmeer reached highest concentrations (ca. 150 µg l−1) in the 1980s as confirmed by monitoring data since 1975. The monitored data show that the TP concentration continuously decreased after 1985 due to successful environmental protection measures. This trend is not (or not yet) evidenced by the diatom data and thus, the diatom-inferred TP concentration.
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