The amount of phosphorus available to algae in the sediments of four lakes in the western part of the Netherlands has been assessed by means of chemical extraction and bioassay techniques. In addition to direct chemical sediment analyses, extractions were carried out with an NTA column method and a stepwise NH 4 CI-NaOH-HCI shaking method, the latter supposedly separating the weakly bound, the Fe-and Al-bound and the Ca-bound phosphates in the sediments. Bioassays, with sediment as the sole source of P, were made with Scenedesmus quadricauda in modified Skulberg's Z8 medium to determine the amount of phosphates available to algae.The average total P concentration of the sediments varied from 0.8 to 3.6 mg P g-l dry wt and correlated well with the net external P loading of the lakes. Uptake of P by algae in the bioassays varied from 0.4 to 36% while NTA extracted 36-69% of the total P. The ratio NH 4 C1 extracted/ NaOH extracted/ HCI extracted phosphates is different from lake to lake, although in all lakes the highest extractions (27-62% of total P) are found in the NaOH fraction. However, in the peaty sediments of these lakes, the NaOH step extracted not only the Fe-and Al-bound phosphates but, also, large amounts of humus compounds. Hence, this fraction also contains non-available organic P.The results are related to soil type and chemical characteristics of the sediments, and compared with data from other authors. A positive correlation was found between phosphate available to algae and NTA-and NaOH-extractable P, but the correlation with total phosphorus was higher. Moreover, algal-extractable P proved to be positively correlated with total iron and clay content and negatively with the amount of organic matter.It is concluded that the sediments in the investigated lakes show great variability and that the chemical extraction techniques cannot replace the bioassays to assess the amount of phosphorus available to algae.

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Most relatively large (> 250 ha) and shallow (< 2 m), Dutch lakes originate from large-scale peat mining. Originally, their water tables were regulated by natural processes such as evapotransporation, precipitation, drainage and seepage (up and downwards). These lakes used to be oligo to mesotrophic, and the phytoplankton consisted mainly of diatoms and green algae. However, 30 years ago filamentous cyanobacteria were also present.Owing to the intensification of agriculture in the surroundings of the lakes, their management developed more and more towards a man-made reservoir function. This management resulted in inflow of eutrophic water from the agricultural areas in winter. In summer the lakes were increasingly used as a water supply. This supply is compensated by water imported mainly from the River Rhine.The impacts of this reservoir function on the limnology of the Dutch fen lakes are discussed with the Tjeukemeer, the Loosdrechtse Plassen and the Reeuwijkse Plassen as examples. All these lakes have become typical eutrophic waters with high turbidity, dominance of filamentous cyanobacteria and bream and complete loss of submerged macrophytes. Finally, methods and results of manipulation procedures to restore these waters are discussed.

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