Nutrient dynamics and seasonal vegetation growth were examined in a newly formed floating marsh dominated by Panicum virgatum in the Mississippi River delta. The floating marsh formed in a shallow aquatic environment receiving secondarily treated municipal effluent. Net Areal Primary Productivity (NAPP), total belowground biomass, NO₃, and plant-tissue δ¹⁵N ratios varied significantly (P < 0.05) along a 75-m marsh transect, while mean plant-tissue δ¹³C values differed between the dominant species. The area nearest the effluent discharge had the highest NAPP (3876 g m⁻² y⁻¹), total belowground biomass (4079.0 ± 298.5 g m⁻²), and mean NO₃ (5.4 ± 2.9 mg l⁻¹). The mean δ¹⁵N of Hydrocotyle umbellata floating marsh was less enriched at 0–75 m (9.7 ± 1.9‰) compared to 100–200 m (21.0 ± 3.8‰). The δ¹³C of the belowground peat mat of the floating marsh was similar to P. virgatum but not H. umbellata, indicating that P. virgatum was forming the mat. Nutrient availability affected NAPP and δ¹⁵N. NAPP was greater than most reported values for floating marsh from 0–45 m then decreased along with NO₃ concentrations and δ¹⁵N further from the effluent source. These results suggest that nutrient rich freshwater can promote restoration of some floating marshes.