Temporal and spatial dynamics of nitrite accumulation in the River Lahn

Both the inorganic nitrogen pool and environmental factors controlling the spatial and temporal variability of nitrite concentration were monitored in the River Lahn at two sites with different sediment composition (coarse gravel/fine gravel and fine sediments). The field studies were conducted both in the free flowing and the interstitial water. Accumulation of nitrite in the River Lahn occurred in correlation to water temperatures over a range of 13°C during the warmer season and coincided with high concentrations of ammonium. An annual as well as daily variations of the concentrations could be determined with the highest data in the summer and lower values at the beginning of the colder season. Maxima were measured from May until July 1993, especially in the hours around noon, sometimes in ecotoxicologically critical concentrations (≥0.3 mg NO2−–N l−1). A spatial variability depending on the depth of the interstitial strata (8–28 cm) was only found at a muddy study site. Here the concentration of nitrite was four to sixfold of that in the free flowing water. At the other study-site consisting of coarse gravel, no accumulation of nitrite could be detected. In order to assess the importance of abiotic factors, e.g. temperature, on the nitrification process, laboratory experiments were also conducted. Nitrifying biofilms of a defined substrate from the River Lahn were used as a model for patterns and processes in the river-bed. The results indicate a critical temperature domain (16–22°C) in which the activity of Nitrobacter seems to be more inhibited than the nitritation by Nitrosomonas, resulting in a transient build-up of nitrite.