The ammonia-oxidizing bacterial population of the tidal River Elbe was analysed. In the freshwater section of the river, abundance was generally at a magnitude of lo4 cells per g dry weight and lo3 cells per ml in sediments and in water samples, respectively. In the brackish water region, counts decreased drastically with the increase in salinity caused by the low level of abundance of ammonia-oxidizers in the seawater. The contribution of attached ammonia-oxidizers to the total number ranged between 50 and approximately lOO%, depending on the respective load with suspended particulate matter (SPM) of the water. The presence of seven distinct species of the genus Nitrosomonas was established by DNA hybridizations. Based on 16s rRNA gene partial sequence analyses, four groups, comprising phylogenetically closely related species, were defined. Dependence of ammonia oxidation on the NH&l concentration, tolerance against increasing salinity and the possession of urease activity were found to be useful ecophysiological characteristics, being in accordance with phylogenetic relationships among the species.

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