In recent decades the invasive plant Phragmites australis (common reed) has spread throughout Chesapeake Bay marshes, lowering plant community biodiversity. Excess nutrient loading and salinity intrusion due to sea-level rise make these marshes vulnerable to invasions. This study examined the interaction between Phragmites australis and the native Spartina cynosuroides (big cordgrass) to determine whether dominance of one species was detected across a range of salinity and nitrogen treatments. Aboveground biomass production of P. australis was greater than S. cynosuroides at lower salinities; however, S. cynosuroides maintained biomass production as salinity increased. Fv/Fm ratios were measured as an indirect measurement of plant tissue physiological health; only Spartina maintained the ratio at higher salinities. Nitrogen addition increased Phragmites biomass and Fv/Fm ratio at higher salinities. Results suggest salinity and nitrogen interactively affect Phragmites biomass production, and that the negative effect of increased salinity on Phragmites spread can be mitigated by nitrogen runoff.