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Nitrogen loading to the Bassin d'Arcachon coastal lagoon (SW France) was evaluated by studying land-use and nitrogen output in its 3001 km2 catchment. At present, the catchment is dominated by forestry (79%), while intensive agriculture occupies 9% of the surface. The N-output of two hydrological subunits, i.e. the Tagon subunit dominated by pine forestry and the Arriou II subunit comprising both forestry and intensive agriculture, were monitored for a seven year period (1996)(1997)(1998)(1999)(2000)(2001)(2002). From these observations it was calculated that forestry contributes on average 1.6 kg total N ha−1 yr−1, which is dominated by organic nitrogen (DON + PON are 70% of N). On an areal basis, intensive agriculture contributes 26 times more than forestry, i.e. 41.6 kg total N ha−1 yr−1, which is mainly in the form of nitrate (65% of N). These data were upscaled to the catchment and the upscaling was validated by comparison to gauged nitrogen throughputs for the catchment of the Leyre river that is the major tributary to the system. Taking into account the other known N sources and the interannual variability in the catchment it was estimated that nitrogen loading to the lagoon was on average 90 kg ha−1 yr−1 (range from 54 to 126 kg ha−1 yr−1). The sandy soils of the catchment have a clear potential for denitrification, but anoxic conditions (waterlogged) and input of organic matter to fuel this process are required. Currently, agricultural practices and spatial planning do not make use of this potential. Nitrogen loading in the Bassin d'Arcachon is reflected by 10-40 μM nitrate concentrations in winter, which became depleted during spring as a result of uptake by vegetation. Short-term uptake experiments showed that the macroalga Monostroma obscurum is well adapted to temperatures between 10 to 20 °C and competitive with respect to the seagrass Zostera noltii when the nitrate concentrations are above 10 μM. Spring conditions with high nitrate and high insolation are therefore favourable for M. obscurum and this species presents a high risk for algal blooming. In contrast, the macroalga Enteromorpha clathrata well adapted to summertime temperatures around 25 °C, forms occasionally blooms in the lagoon. This phenomenon is limited due to the low DIN concentrations in summer.

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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.

The nitrous oxide determination, after acetylene inhibition of its reduction, was made by GPC. We observed no N20 production in the water. The dénitrification rate, expressed as mg of nitrates. M~ 2 . d~ can be considered as a constant and is independent of the temperature variation, nitrate and dissolved oxygen concentrations of the sediment-water system.On a annual cycle, whithout the marked influence of environmental factors, the mean rate of dénitrification in the river Charente is about 682 mg.m -2 . d~ '.The comparison of the field measurements and evaluation by calculation of the dénitrification phenomenon shows that it is preferable to estimate the dénitrification rate by direct titration instead of nitrogen form analysis.