Nitrogen and phosphorus fertilizers were added over two growing seasons to whitetop marshes growing at the shallow and deep extremes of whitetop's water depth range in a northern prairie marsh to assess the effects of nutrient additions and water levels on species composition and understory species biomass. Nitrogen and nitrogen plus phosphorus additions reduced understory species biomass and caused only small changes in marsh species composition after 1 year, but dramatically decreased whitetop biomass and increased the biomass of the understory moist soil annual Atriplex patula at both water depths after 2 years. Phosphorus alone had little effect on species composition. Water levels influenced both the biomass of understory species and their potential response to nutrient additions. Biomass of understory species during both years was lower in deeper water. Nutrients had no effect on understory species biomass when standing water was present because standing water prevented seed germination. A mulching effect caused by greater whitetop litter accumulation and weaker whitetop stems associated with nitrogen fertilization appeared to be more important influences on understory species composition than differential understory species growth responses to added nutrients. Predicting responses of whitetop marshes to multiyear fertilization requires an understanding of the interactions between the physical conditions caused by increased biomass of whitetop, the relative abilities of understory moist soil species to grow under those conditions, and water levels, which control understory species seed germination and plant survival.