『Abstract
　Phosphate bound onto sediments still plays an important role in lake metabolism. Therefore, knowledge of the different compounds in sediments and their availability for algae is essential to control their growth.
　In this review different methods to fractionate phosphate in sediments of lakes and marshes are compared. The methods are operationally defined extractions with NaOH followed by HCl, an extraction with dithionite in bicarbonate with or without citric acid, or functional extractions with chelating agents such as NTA and EDTA. By complexing the NTA or EDTA with Ca these chelators dissolved only the Fe(OOH) adsorbed phosphate.
　It is shown that the operational extractions do not identify specific compounds such as ironhydroxide bound or Ca bound phosphate. The chelators, such as NTA or EDTA are more specific, can be used at the pH of the sediment, but have certain practical disadvantages. In sediments rich in organic matter, EDTA extracts also a certain amount of organic phosphate, the identity of which is unknown. NaOH as extractant gives the worst possible results as it incompletely dissolves iron bound phosphate and attacks an important quantity of organic phosphate. The quantity of phosphate extracted depends on the concentration of the NaOH and the duration of the extraction. Furthermore, during the extraction formation of CaCO3〜P is shown to take place. Thus, the organic pool is underestimated and the inorganic one overestimated. It is shown that for each extraction it must be checked whether a second extraction yields again some phosphate - this appears even to be the case with NaOH. After the extraction of inorganic phosphate some organic phosphates can be isolated such as humic- or fulvic bound phosphate and phytate. These compounds cannot be isolated and studied when NaOH is used.
　Bioavailability studies have shown that different quantities of phosphate in sediments can be used for phytoplankton growth. In this review different methods are compared and it is shown that the best method is to mix the algae with sediments and to do cell counting under the microscope. The cultures must be aerated in order to avoid an increase in pH. Usually the sum of iron- and calcium bound phosphates is available for the algae, unless the latter compound is in the form of detrital rock particles. Biogenic ‘apatite’ is available for the phytoplankton species tested. The chemical fractionation of the different forms of apatite is still difficult.

Key words: phosphate; sediments; bio-availability; fractionation; chelators』

Resumen
Introduction
Bioavailability
Fractionation of P-compounds
　Fractionation of inorganic P-compounds
　Fractionation of organic P-compounds
Comparison between bioavailable P and P-fraction
Conclusions
Links with the N cycle
　Modelling denitrification
Acknowledgement
References