Coastal wetlands, such as marshes and mangroves, provide many valuable ecosystem services and are vulnerable to climate change and anthropogenic impacts. Marshes are dominant in temperate regions and mangroves within the tropics; both plant types co-occur in subtropical ecotones. Dynamics within marsh-mangrove ecotones may be linked in part to local environmental drivers like nutrient enrichment. Previous studies within monospecific marsh and mangrove stands have reported increases in productivity in response to fertilization; however, the allocation of excess nutrient resources within co-occurring stands of mature plants is not well understood. To investigate nutrient dynamics within the marsh-mangrove ecotone, we fertilized naturally co-occurring Spartina alterniflora and Avicennia germinans stands (the dominant marsh and mangrove species, respectively) within the Gulf of Mexico. After four growing seasons of continuous enrichment, above- and belowground biomass were 2.5 and 1.5 times greater, respectfully, in enriched plots for A. germinans; S. alterniflora biomass did not differ between enrichment treatments. Mangrove leaves indicated a large nutrient response with altered leaf stoichiometry, larger surface area, and greater SPAD (proxy for chlorophyll a) in enriched plots. The fertilization effects may suggest A. germinans has a greater ability to acquire and allocate excess nutrients into productivity and biomass, relative to S. alterniflora, and may therefore be the superior competitor for nutrient resources. In monospecific stands, both marsh and mangroves species have a high potential for nutrient uptake, but when they are co-occurring in the marsh-mangrove ecotone, nutrient retention may be more linked to mangrove ability to partition nutrient resources into various above- and belowground components. Global climate changes are driving mangrove coverage poleward into temperate salt marshes, expanding the areal extent of marsh-mangrove ecotones. At a local scale, these findings reveal that this mangrove expansion may be accelerated by anthropogenic nutrient input.