The phosphate uptake behaviour of phytoplankton communities in two eutrophic Uruguayan lakes has been studied using [32p]phosphate. In order to measure net incorporation of this tracer, lake samples were prelabelled with successive pulses of radioactive phosphate prior to the uptake experiments proper. The data were analysed using a proportional flow-force relationship which allows the evaluation of two characteristic parameters: a conductivity coefficient LP that here reflects the overall activity of the uptake systems of the community, and a threshold concentration [Pe]A at which net incorporation of phosphate ceases for energetic reasons. Between December 1997 and July 1998 in Lago Rodo phosphate supply by inflowing groundwater was periodically interrupted resulting in distinct states of phosphate deficiency. This showed that the phytoplankton responded to alterations in the phosphorus status of the lake by characteristic changes in LP and [Pe]A. The uptake behaviour of the phytoplankton community thus obeyed the linear flow-force relationship over a wide concentration range, in contrast to expectations of non-equilibrium thermodynamics. The observed uptake characteristics exhibited by multi-species phytoplankton ensembles are discussed with respect to a possible coherent behaviour of the constituents of these communities.