Limnetica 37

Phosphate retention processes in streams of a subtropical and a temperate region were studied.  Our hypothesis was that retention differs in the two regions.  We expect more retention in the subtropical region than in the temperate one. If that were confirmed the differences in stream phosphate uptake may be driven by an interaction between phosphate concentration and algal biomass, associated with differences in temperature.  Phosphate retention was studied by carrying out constant rate additions. Nutrient uptake rate (U) and mass transfer coefficient (Vf: 2.67 E-04 y 9.27 E-05 m/s to subtropical streams and temperate one respectively) were associated with background phosphate, macrophyte biomass and transient zone storage according to PCA analysis. Thus, streams of temperate regions respond more to macrophyte biomass than streams of subtropical regions which were associated principally to benthic algae. These results are novel for South America and they reflect the relative importance of benthic algal or macrophyte biomass and production in relation to the P uptake rate at the different environments