Impact of an urban centre on the nitrogen cycle processes of epilithic biofilms during a summer low‐water period *

Abstract

Abstract Nitrogen transformations in epilithic biofilms of a large gravel bed river, the Garonne, France, has been studied upstream (one site) and downstream (four sites) of a large urban centre (Toulouse, 740 000 inhabitants). High biomass, up to 49 g AFDM m −2 (ashes free dry matter) and 300 mg chlorophyll a m −2 (Chl. a ), were recorded at 6 and 12 km downstream from the main wastewater treatment plant outlet. The lowest records upstream and larger downstream (less than 16 g AFDM m −2 or 120 mg Chl. a m −2 ) could be explained by recent water fall (early summer low‐water period). Measurements of nitrogen exchange at the biofilm–overlying water interface were performed in incubation chambers under light and dark conditions. The addition of acetylene at the mid‐incubation time allowed evaluation of both nitrification (variation in NH 4 + flux after the ammonium monooxygenase inhibition) and denitrification (N 2 O accumulation related to the inhibition of N 2 O reduction). Denitrification ( D w ) and nitrification rates were maximum at sites close to the city discharges in dark conditions (up to 9.1 and 5.6 mg N m −2 h −1 , respectively). Unexpected denitrification activities in light conditions (up to 1.4 mg N m −2 h −1 ) at these sites provided evidence for enhanced nitrogen self‐purification downstream. As confirmed by most probable number (MPN) counts, high nitrification rates in biofilm close downstream were related to enhanced (more than almost 3 log) nitrifying bacteria densities (up to 7.6×10 9 MPN m −2 ). Downstream of an urban centre, nitrogen transformations in the biofilm appeared to be influenced by the occurrence of an adapted microflora which is inoculated or stimulated by anthropic pollution. Copyright © 2002 John Wiley & Sons, Ltd.