An experimental study on effects of submersed macrophytes on nitrificationand denitrification in ammonium-rich aquatic systems

We have examined the role of microbial communities on the surface of submer sed macrophytes and in the underlying sediment for nitrification and denitr ification in light and dark in NH4+-enriched microcosm systems using isotop e pairing and dilution techniques. Potamogeton pectinatus L. and intact sed iment cores were collected in a shallow reservoir receiving treated municip al wastewater and containing dense submersed vegetation. Chambers containin g P. pectinatus shoots, sediment, or both P. pectinatus shoots and sediment were exposed to 6 h of darkness, 6 h of light, and 6 h of darkness. (NH4+) -N-14 and (NO3-)-N-15 were added at ambient concentrations of 15 and 5 mg N liter(-1), respectively. NH4+ was primarily nitrified in the epiphytic mic robial communities, and NO; was denitrified in the underlying sediment. In chambers containing macrophytes, there was a net production of O-2 and NO3- in light and a net consumption in dark, and nitrification was higher in li ght than in dark. In chambers with only sediment, there was always a net co nsumption of NO3-, and nitrification was similar in light and dark. The res ults show that submersed macrophytes can be important for the N metabolism in NH4+-rich freshwaters (e.g., wastewater treatment systems) by stimulatin g nitrification through providing surfaces for attached nitrifying bacteria and possibly also through diurnal changes in the water chemistry.