Limnol. Oceanogr., 44(8), 1999, 1993–1999

We have examined the role of microbial communities on the surface of submersed macrophytes and in the underlying sediment for nitrification and denitrification in light and dark in NH -enriched microcosm systems using isotope 4 pairing and dilution techniques. Potamogeton pectinatus L. and intact sediment cores were collected in a shallow reservoir receiving treated municipal wastewater and containing dense submersed vegetation. Chambers containing 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. 14NH and 15NO were added at ambient concentra1 2 4 3 tions of 15 and 5 mg N liter21, respectively. NH was pri4 marily nitrified in the epiphytic microbial communities, and NO was denitrified in the underlying sediment. In chambers 3 containing macrophytes, there was a net production of O2 and NO in light and a net consumption in dark, and nitrification 3 was higher in light than in dark. In chambers with only sediment, there was always a net consumption of NO , and nitri3 fication was similar in light and dark. The results show that submersed macrophytes can be important for the N metabolism in NH -rich freshwaters (e.g., wastewater treatment sys4 tems) by stimulating nitrification through providing surfaces for attached nitrifying bacteria and possibly also through diurnal changes in the water chemistry. Submersed macrophytes provide a large accessible surface area for attached microorganisms (Sculthorpe 1967). Epiphytic microbial communities can play a major role in the transformation of inorganic nutrients in shallow freshwater environments with submersed vegetation (Mickle and Wetzel 1978). Submersed macrophytes can sustain high denitrification rates in nutrient-rich freshwater environments (Eriksson and Weisner 1996, 1997) and may support high abundances of attached nitrifying bacteria (Eighmy and Bishop 1989). Several studies involving N budgets in freshwater systems support the idea that submersed macrophytes can enhance N removal by offering surfaces that can hold populations of both nitrifiers and denitrifiers (Reddy and De Busk 1985; Eighmy and Bishop 1989; Körner 1997). In submersed vegetation, there is an exchange of photosynthetic gases, i.e., O2 and CO2, between leaf surfaces and surrounding water. Due to the often reduced water movements in dense stands of submersed macrophytes (e.g., Loose and Wetzel 1993), limiting gas exchange with the atmosphere and water exchange with areas of air-equilibrated water, the metabolic activity of submersed macrophytes and their epiphytes can produce conspicuous changes in the concentrations of O2, dissolved inorganic carbon (DIC), and pH (Pokorny et al. 1984; Prahl et al. 1991). However, although changes in the water chemistry produced by submersed macrophytes have been recognized in several studies, they have not been put together with activity measurements of bacterial N transformations. The gas exchange at submersed leaf surfaces may have a strong impact on bacterial N transformations in epiphytic microbial communities and, through water column–sediment interactions, on N transformations in the sediment. Macrophytes may affect sediment processes not only through the growth and metabolism of their roots, which has been shown earlier (e.g., Reddy et al. 1989), but also by influencing the characteristics of the overlying waters. The transition of NH to NO in sediment and epiphytic populations of ni1 2 4 3 trifiers may, in daylight, be stimulated by the photosynthetic release of O2 from submersed vegetation. In addition to producing O2, the primary production of the epiphyte–macrophyte association may, because of the dependence of NH 4 oxidizing bacteria on an alkaline pH and of the generation of acid that necessarily accompanies nitrification, promote epiphytic nitrification by raising pH of the interstitial water within the epiphytic communities. Epiphytic nitrifying bacteria in eutrophic waters may also be supported by the presence of precipitated particulate CaCO3 on the leaves, which may buffer pH within the epiphytic community during nighttime when primary production is absent. Thus, the metabolism of the macrophyte substrata may create a favorable environment for attached nitrifiers. As opposed to daytime, when there is a net production of O2, the macrophyte–epiphyton complex at night may be of importance in lowering the O2 concentration of the water by respiratory consumption (Sand-Jensen et al. 1985). Respiration in dense stands of submersed vegetation at night may cause a shift from aerobic to anaerobic bacterial respiration, i.e., denitrification in epiphytic communities, and may also stimulate sediment denitrification by lowering O2 concentrations of the overlying water. Consequently, it appears that the metabolic activity of submersed vegetation in shallow aquatic environments and the associated macrophyte–water gas exchange may promote a coupling between nitrification and denitrification, i.e., the sequence of NH oxidation to NO by nitrifying bacteria 1 2 4 3 and subsequent denitrification of NO to N2. 3 In the aquatic ecosystem from which we collected macrophytes and sediment (see below), Eriksson and Weisner (1997) measured denitrification in epiphytic microbial communities and in the sediment. However, denitrification was measured only in darkness and in systems containing sediment or macrophytes, but not both. Furthermore, Eriksson and Weisner (1997) did not measure nitrification and coupled