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

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, 740000 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, 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.

Nitrogen dynamics in the shallow groundwater of a riparian wetland zone of the Garonne, SW France: nitrate inputs, bacterial densities, organic matter supply and denitrification measurements

“…Denitrification occurs even though there are nitrifying bacteria, probably because of the widespread presence of biofilms within the porous aquifer. Denitrification can occur within biofilms in oxygenated water (Teissier et al, 2002). These biofilms can be enriched in bacteria cells by surface water inputs (Table 2).…”

Eutrophication and its effect on dissolved Si concentrations in the Garonne River (France)

“…Due to an important fraction of hard substrates in the river bed, turbidity of the Garonne River is low compared to other lowland rivers, with suspended matter concentrations usually being below 50 mg L -1 (Coynel et al 2004). This low turbidity combined with a shallow depth allows diatom biofilms to develop on the river bed (Eulin & Le Cohu 1998;Teissier et al 2002;Améziane et al 2003;Boulêtreau et al 2006). Diatom biofilms regularly detach when their biomass is high and are then mixed into the water column (Boulêtreau et al 2006).…”

A standardised method for measuring in situ denitrification in shallow aquifers: numerical validation and measurements in riparian wetlands

“…Despite the presence of high concentrations of dissolved oxygen (from 0.6 up to 8.1 mg O 2 L -1 ) during the experiments, denitrification detected in each sampled piezometer shows that microbial denitrification processes do not occur in interstitial water. Referring to previous studies on periphytic biofilms demonstrating that denitrification could occur within oxygenated water in superficial water (Nielsen et al, 1990;Teissier et al, 2002), it has been assumed that denitrification is localised within the biofilms growing around particles included in the sediments making up the porous aquifer.…”