The bioavailability of colloidal and dissolved organic phosphorus to the alga Pseudokirchneriella subcapitata in relation to analytical phosphorus measurements

The speciation of phosphorus (P) in freshwater affects its bioavailability. Analytical detection methods were compared for different colloidal and dissolved organic forms of phosphorus (P) in relation to the potential P bioavailability to Pseudokirchneriella subcapitata, determined with a 14 day growth response to P. Growth on these P-forms was referenced to supplies of inorganic P (P_i) as operational definition of the relative potential bioavailability. The bioavailability of ten organic P molecules ranged 1–70% of P_i while these forms were generally not detected by colorimetric method (CM, malachite green) or ion chromatography (IC). The bioavailability of P associated with Fe- and Al oxides ranged 55–85% of P_i and these forms were completely detected by CM and partially by IC. The bioavailability of total dissolved P in the environmental samples ranged 7–85% (mean 43%) of P_i. The P detected by IC underestimates bioavailable P while CM and total dissolved P (inductively coupled plasma, ICP) overestimate P bioavailability by, on average 44% (CM) or 57% (ICP) in the environmental samples. We conclude that CM is the best index among the three tested for predicting long-term availability of environmental dissolved P in which colloidal P contributes more importantly than organic P.