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The recent literature on the bioavailability of different forms of P in freshwater systems is reviewed. Bioavailable P is defined as the sum of immediately available P and the P that can be transformed into an available form by naturally occurring processes. Methods used to estimate the bioavailable P pool, which vary between studies largely depending on the time perspective applied, are critically evaluated. Most studies on particulate P aim to determine the potentially available P pool. Potential bioavailability of particulate P is normally analysed in bioassays with algal yield determinations and the available P fraction is characterized from interpretations of results of sequential chemical extractions. NaOHextractable P is in most studies the most algal-available P fraction. For soil samples and 'tributary water particulate matter, NaOH-P has often been found to be equal to algal extractable P. In other studies depletions of NaOH-P have accounted for the algal P uptake, but only a minor proportion of the fraction has been utilized. Organic P in lake water particulate matter and bed sediments of eutrophic lakes can also be algal-available to a significant extent.Studies on the bioavailability of dissolved P have often been concerned with immediate availability, or the minimum amount of available P. Such studies need other types of experimental design and normally assays with radiotracers are used. Immediately available P is frequently found to be less than P chemically assessed as dissolved reactive P (DRP) at low (< 10 pg DRP * l-') concentrations. However, immediate availability may also approach or exceed DRP concentrations, especially at higher concentrations. Potential bioavailability, assayed as for particulate P, may generally render higher bioavailability than P assayed as immediately available. Large fractions of dissolved P remain unutilized and are primarily found in the high molecular weight fraction of dissolved P. Introductionthe total P load on lakes will be available to the lake biota for production of organic matter. In Empirical relations between phosphorus loading, recent years increased attention has therefore phosphorus concentrations and algal biomasses been paid to determinations of the bioavailable or in lakes are normally based on total phosphorus, algal-available proportion of P from different cf. the OECD report on eutrophication of waters sources, with the aim of improving understanding (OECD, 1982). However, it is well known, and of P cycling in lakes, refining empirical models or also stated in the OCED report, than only part of improving lake management strategies.

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