We used oligotrophic, P-limited herbaceous wetlands of northern Belize to assess how changes in nutrient availability impact species composition and ecosystem processes. The P, N, and NP enrichment plots were established in replicated marshes of three salinity levels to document potential salinity constraints. Addition of P or combination of N and P resulted in rapid switch from a microphyte (cyanobacterial mats, CBM) to macrophyte (Eleocharis spp., Typha domingensis) domination, while N addition did not have any impact. The switch was caused by significant changes in Eleocharis stem density and height, and consequently, the aboveground biomass, which increased from an average 120 g m⁻² in control and N plots to > 500 g m⁻² in P and NP plots. Decreased light under the dense canopy of Eleocharis in P and NP plots caused significant reduction in CBM growth. Biomass of Eleocharis in P and NP plots decreased with increasing salinity, but salinity did not affect biomass production in control and N plots. Tissue P of Eleocharis from P and NP plots increased 4- to 5-fold compared to P content in plants from control and N plots. Tissue P remained high due to internal nutrient recycling even after P addition ceased. Typha transplanted into plots grew exponentially in P and NP plots, while in control and N plots it grew slowly or did not survive. There were significant differences in NH₄-N both in soil extracts and in the interstitial water with soil and water NH₄-N being significantly lower in P-addition plots. The elimination of N₂-fixing CBM is a potential reason for a decrease in available sediment N as documented by a negative correlation between CBM cover and interstitial NH₄-N.