Natacha Brion

Supply of organic matter and bacteria to aquatic ecosystems through waste water effluents

In order to study the impact on the river Seine of the waste water effluents from the city of Paris and its suburbs, a detailed characterisation was made of both raw and treated effluents from the three main treatment plants in this area which differ both in size and type of treatment. The waste water samples were subjected to analyses of the following pools of carbon: dissolved organic carbon (DOC), particulate organic carbon (POC), biodegradable fractions of DOC (BDOC) and of POC (BPOC), the biochemical oxygen demand (BOD). Inorganic and organic forms of nitrogen and phosphorous, total bacterial biomass and nitrifying bacterial biomass were also investigated in parallel. On the basis of the results of the analysis performed, a specific load per inhabitant and per day (expressed in g inh−1 d−1) for raw and the different types of treated waste water was calculated for each variable considered in this study. For raw water, the specific loads of TOC ranged between 26.4 and 28.3 g C inh−1 d−1 with particulate organic matter constituting the main part (70–76%) and the biodegradable fraction representing between 60 and 75%. Concerning micro-organisms, the average specific load of total bacteria was around 2 g C of biomass inh−1 d−1, the nitrifying biomass represented 0.3–2.5% of the total bacterial biomass. Depending on the type of treatment, the specific load of TOC in treated water ranged between 3 and 10.8 g C inh−1 d−1, it corresponded to removal percentages in the range 59–89%. Total bacterial biomass (0.05–0.33 g C inh−1 d−1) was always lower in treated than in raw water. A significant correlation was observed between BTOC (sum of BDOC and BOPC) and BOD. BPOC represented 68% of BTOC in raw water and 43% in treated water. The total biomass of bacteria constituted 8% of the BTOC.

Lower Seine River and estuary (France) carbon and oxygen budgets during low flow

Ecological processes driving the oxygen budget were investigated in the downstream part of the Seine River and its estuary. Phytoplankton and bacterioplankton production were measured along longitudinal profiles (11 to 17 stations) in a range of low discharges from 300 m3 s−1 in 1993 and 1995 to 140 m3 s−1 in 1996. Values representative of the water column were based on investigations carried out during two tidal cycles. Net primary production was invariably greatest in the freshwater estuary, from Poses to Rouen (from 500 to 1,000 μg C l−1 d−1 between PK 202 and 240) and decreased sharply downstream (from 10 to 25 μg c l−1 d−1 between PK 250 and 310). This decrease was mainly due to the deterioration of the light conditions with the increase in depth and suspended matter concentrations. Heterotrophic activity was maximum in the reach where primary production declined. Judging by the production:respiration ratio (P:R), the system appeared clearly heterotrophic in the Seine River immediately downstream of the Paris region due to high allochthonous organic pollution by the incompletely treated Parisian effluents and in the part of the estuary characterized by intense degradation of autochthonous material. Because the effluents are not treated by a nitrification step, the oxygen consumption due to nitrification was much higher than expected from the P:R ratio. Oxidation of ammonium represented an oxygen consumption of between 1 and 14 g O2 m−2 d−1, almost equalling the sum of heterotrophic respirations that were barely balanced by photosynthesis. The reaeration flux at the water-atmosphere interface was deduced from the calculations and a reaeration coefficient was estimated.

Distribution of nitrifying activity in the Seine River (France) from Paris to the estuary

The distribution of nitrification has been measured with the H14CO3− incorporation method in the Seine River and its estuary during summer conditions. The Seine River below Paris receives large amounts of ammonium through wastewater discharge. In the river itself, this ammonium is only slowly nitrified, while in the estuary nitrification is rapid and complete. We show that this contrasting behavior is related to the different hydrosedimentary conditions of the two systems, as nitrifying bacteria are associated with suspended particles. In the river, particles and their attached bacteria either rapidly settle or have a sestonic behavior. Because of the short residence times of the water masses, the slow growing nitrifying population has no time to develop sufficiently to nitrify the available ammonium. The estuary is characterized by strong tidal dynamics. Particles settle and are resuspended continuously with the strong current inversions of ebb and flood. As a result of these dynamics, particles and their attached nitrifying bacteria experience longer residence times in a temporary suspended state than the water masses themselves, providing to slow growing nitrifying bacteria the opportunity to develop a large population capable of nitrifying all the available ammonium.