Cyanobacterial mats, an important component of the oligotrophic alkaline wetland ecosystems of the Caribbean, are sensitive to nutrient enrichment. In order to elucidate their role in nutrient cycling, we measured extracellular enzyme activities in cyanobacterial mats and underlying sediment exposed to a long-term effect of different salinities and nutrient enrichment. Activities of alkaline phosphatase, leucine-aminopeptidase, arylsulphatase, and beta-glucosidase were measured fluorometrically. The distribution of phosphatase activity among different groups of microorganisms in the vertical structure of the mat was visualized using ELF (R) 97 phosphate. The activity of all enzymes, except arylsulphatase, was localized mainly in the mat itself and was several times higher than in the underlying sediment. Phosphatase always exhibited the highest activity, followed by leucine-aminopeptidase, arylsulphatase and beta-glucosidase. Phosphatase activity was significantly suppressed in P-addition plots under all salinity levels. The remaining enzymes were not significantly influenced by nutrient addition. Cyanobacteria, which formed most of the mat biomass, exhibited no phosphatase activity, with the exception of Chroococcus spp. Hence, we presume that the main sources of extracellular enzymes are different species of bacteria distributed in the mat and the mucilaginous sheaths of cyanobacteria. Sediment type, rather than salinity, seems to have an important influence on the extracellular enzyme production strategies.