In 1979 and 1980, NH(,4)NO(,3) and (NH(,4))(,2)HPO(,4) were applied to surface water of a prairie marsh, Eagle Lake, Iowa, to determine the impact of nitrogen and phosphorus loading in a natural wetland on water chemistry and on emergent plant production and decomposition. Nitrogen and phosphorus fertilization of surface water in 1979 caused increases of NH(,4)-N (+144%), NO(,3)-N (+291%), and PO(,4)-P (+47%) in 15-cm deep interstitial water. In 1980, fertilization caused increases in NH(,4)-N (189%), PO(,4)-P (+60%), and the non-PO(,4)-P fraction of total phosphorus (+50%) at the same depth;Typha glauca and Sparganium eurycarpum responded to fertilization with increased concentrations of nitrogen in the shoots (+30 to 34% for Sparganium and +7% to 14% for Typha) and roots-rhizomes (+25% for Sparganium and +38% for Typha in 1980); increased shoot density in 1980 (+20% for Sparganium and +30% for Typha), and shoot production in 1980 (+57% for Sparganium and +19% for Typha). Root-rhizome production was not altered by fertilization;Sparganium shoot litter with an initial nitrogen concentration of 1.41% lost 27% more dry weight over a 505-day period than did Sparganium with an initial nitrogen concentration of 0.59%. Typha shoot litter with an initial nitrogen concentration of 0.55% lost 2% more dry weight during 505 days than Typha tissues with an initial nitrogen concentration of 0.48%. Application of nitrogen and phosphorus to marsh surface waters did not alter the rate of Sparganium or Typha shoot and root-rhizome decomposition;Nitrogen-rich Sparganium shoot tissue was a net exporter of nitrogen through 505 days of decomposition (-50% of original N quantity) and Typha shoot tissue was a net exporter of nitrogen through only 350 days of decomposition (-14%). Quantities of nitrogen increased for Sparganium (+31%) and decreased for Typha (-10%) shoot tissues under unfertilized conditions and increased 35% for both species with fertilization over 505 days. Under unfertilized conditions, phosphorus quantities decreased for Sparganium (-2%) and increased for Typha (+25%) shoot tissues over 505 days. Phosphorus quantities increased in Sparganium (+7%) and Typha (+121%) shoot tissues with fertilization over 505 days. Root-rhizome litter consistently lost nitrogen throughout the 505-day sequence under unfertilized and fertilized conditions (-24 to 54% for the two species).