The factors controlling prokaryote abundance and activity along salinity gradients were investigated in the Bras del Port solar saltern system (Alacant, Spain) in May 1999. Specific growth rates were high and prokaryote abundance relatively low at the lowest (seawater) salinities; the opposite was found at higher salinities. Experiments were carried out in representative salterns at salinities of 4 to 37 %, to test whether prokaryote abundance and growth rate were (1) limited by inorganic or organic nutrients (nutrient addition experiments), (2) limited by cell abundance (dilution experiments), or (3) affected by zooplankton cascading down to affect the prokaryote predators. Low-salinity ponds were limited by organic nutrients, while high-salinity ponds responded slightly only to dilution. Zooplankton affected prokaryote growth rates particularly in the medium-salinity ponds. In the low salinity ponds, zooplankton effects were weak and probably indirect (through increased supply of organic matter). Neither organic matter limitation nor zooplankton predation pressure affected prokaryote development in the higher salinity ponds. We suggest that 3 types of functional communities occur in the same saltern system: (1) an active, substrate-limited community in the low salinity ponds; (2) an active, grazer-controlled community in the medium salinity ponds; and (3) a possibly dormant, probably substrate-limited, community in the high salinity ponds. However, the results at the highest salinities were equivocal, because the dilution manipulation had detrimental effects, artificially decreasing the contribution of the haloarchaea, which were essential contributors to the total activity in the saltern. Bacterial taxonomic community composition was also determined in these experiments by denaturing gradient gel electrophoresis (DGGE) analyses on 16S rRNA genes, and showed very small changes in community composition in the experimental manipulations. Together with the known microbial community structure and composition at differing salinities along the gradient, our results show that functional aspects of the microbial food web also vary between salterns.