Determining algal-available phosphorus of differing origin: routine phosphorus analyses versus algal assays

Eutrophication of surface waters can be accelerated by anthropogenic P-inputs, provided that P is in a form available to aquatic primary producers. Potentially algal-available P (P-aa) under aerobic conditions was determined with a dual-culture assay from 172 samples representing P in point and nonpoint sources and in lacustrine matter. The availability of P - expressed as the proportion of P-aa in total P (Tot-P) - ranged from 0 to 100%. In the different P sources, the mean availability ranged from 3.4 to 89% in descending order: wastewater of rural population > biologically treated urban wastewater > dairy house wastewater > biologically and chemically treated urban wastewater > field runoff > forest industrial effluent > fish fodder and feces > river water > field surface soil > forest runoff > lake settling matter > lake bottom sediments. Of the P fractions, dissolved reactive P (o-P) was highly available to algae, whereas particulate P ( P-part) and dissolved unreactive P (unr-P-diss) contributed to P-aa to a lower but varying degree. An approach based on source-dependent availability coefficients, derived from the algal assays, appeared promising in transforming the load of Tot- P into that of P-aa. Although the values for P-aa obtained by the dual-culture assay probably underestimate the true levels for ultimately available P, they may still give valuable information for eutrophication abatement.