In coastal Louisiana, bulltongue (Sagittaria lancifolia L.) has greatly expanded its distribution while broadleaf arrowhead (Sagittaria latifolia Willd.) and delta duck potato (Sagittaria platyphylla Engelm.) have decreased in abundance. Several factors may be important in these distributional patterns, including increased salinities, altered hydrologic regimes, and changed substrate types. In this study, we simultaneously manipulated these three factors under controlled greenhouse conditions. This three-year study consisted of an experiment that included equal amounts of each of the three species of Sagittaria planted approximately 15 cm apart in 200 liter mesocosm vessels. Dead aboveground biomass was harvested regularly and above- and belowground biomass was harvested at the end of the study. All three species began with relatively high turnover across salinities. While S. latifolia and S. platyphylla only grew well in the freshwater treatments, S. lancifolia grew in all three salinity levels (0, 3, and 6 ppt) and showed decline in the 6 ppt treatment, but not until the third year. Both S. latifolia and S. platyphylla produced less belowground biomass than aboveground biomass while S. lancifolia produced much more belowground biomass in comparison to above-ground biomass. These results suggest that the pattern of replacement of S. latifolia and S. platyphylla by S. lancifolia in coastal Louisiana may be a result of locally increased salinities. Because Sagittaria lancifolia produces massive tubers and rhizomes (belowground biomass) and is somewhat flood- and salt-tolerant, it may be a good candidate for use in restoration of freshwater marshes that are susceptible to saltwater intrusion.