The coastal plain of northern Belize consists of relatively undisturbed freshwater marshes that are strongly phosphorus-limited and characterized by monodominant stands or mixtures of emergent macrophytes. In order to assess the impact of agricultural activities on the nutrient dynamics and plant species composition in adjacent wetlands, we sampled along transects in 40 marshes—20 located downslope from agricultural fields and 20 bordered by scrub or broadleaf forest. Soil, water, and plant tissue samples were collected for nutrient analyses, and plant species composition was recorded at points along transects during both the wet and dry seasons. Marshes downslope from agricultural activity had significantly greater percent cover of Typha domingensis, a competitive dominant in the system (20% vs. 6%) and significantly more soil phosphorus (128–144 μg*cm⁻³) than marshes bordered by forest (70–98 μg*cm⁻³). Plant tissue P content was positively correlated with soil P content but not with soil N for Typha (r=0.55) and Eleocharis spp. (r=0.72). Canonical Correspondence Analysis (CCA) identified soil P as the variable most strongly affecting Typha abundance. Abundance of species common to unimpacted marshes (Eleocharis spp., Cladium jamaicense) was negatively correlated with soil P. Further intensification of agriculture in this region may result in expansion of T. domingensis, as has occurred in the Florida Everglades.