Ecological Functioning of the Seine River: From Long-Term Modelling Approaches to High-Frequency Data Analysis

Abstract

At the start of the PIREN-Seine program, organic pollution by the effluent of the Parisian conurbation was responsible for episodic anoxia in the lower Seine River, while nutrients from both point and diffuse sources are used to cause eutrophication, a nuisance for drinking water production from surface water and biodiversity. The implementation of the EU Water Framework Directive led to a drastic decrease of organic carbon, phosphorus and ammonium concentrations in surface waters starting in the early 2000s and to a reduction of the frequency and the amplitude of phytoplankton blooms. However, nitrate contamination from fertiliserintensive agriculture continued to increase or at best levelled off, threatening groundwater resources and causing unbalanced nutrient ratios at the coastal zone where eutrophication still results in harmful algal blooms. High-frequency O 2 data combined with models, which have been developed for 30 years, can help discriminate the contribution of auto-vs. heterotrophic metabolism in the CO 2 supersaturation observed in the Seine River. Despite the impressive improvement in water quality of the Seine River, episodic crises such as summer low-flow conditions still threaten the good ecological status of both river and coastal waters. Modelling scenarios, including further wastewater treatments and structural changes in agriculture and future changes in hydrology under climate changes, provide the basis for a future vision of the ecological functioning of the Seine River network.

Managing the Agri-Food System of Watersheds to Combat Coastal Eutrophication: A Land-to-Sea Modelling Approach to the French Coastal English Channel

“…The 2002-2014 period was specifically characterised by significant changes in pressures. Domestic point sources have been considerably reduced since the application of the EU-WFD in the 2000s, especially for P and ammonium loads [64,65,86]. For example, P concentrations in the Seine basin upstream from Paris can now be lower than 0.05 mg L −1 in the water column, thus able to limit algal growth.…”

“…Regarding diffuse sources in the Seine River, nitrate concentrations have been stabilised at 5 mgN-NO 3 L −1 since the early 2000s, after a regular concentration increase resulting from fertiliser use for agriculture intensification (from 3.3 mgN-NO 3 L −1 in the late 1980s to 5.4 mgN-NO 3 L −1 in the early 2000s, i.e., about 10% per year, before good agricultural practices were recommended [87]. Recently, N concentrations appeared to stabilise owing to improved wastewater treatment (including denitrification [86]). As a result of all these changes, the marine coastal waters shifted from an N limitation to a P limitation [33].…”

“…It would be the most effective in reducing both eutrophication potential (see Table 7) and bloom toxicity (see Figure 10). In addition, the A/R/D scenario, which would improve river water quality, has been shown to reduce greenhouse gas emissions by 36% in the Seine Basin (50% for all of France [86]).…”

Trajectories of the Seine River Basin

“…In the 1870s, the collected wastewater started to be spread over sewage farms near Paris. Wastewater treatment plants (WWTPs) were then gradually built but insufficiently to treat the volume of wastewater generated by the Paris conurbation [41]. One of the sewage farms at Achères, located at 60 km downstream from Paris, was converted into the Seine-Aval WWTP between 1930 and the 1980s, treating up to 8 M inhab.…”

River Basin Visions: Tools and Approaches from Yesterday to Tomorrow

“…Two contrasting scenarios have recently been developed by Billen et al [28] for 2040. The first one represents a continuation of the current trends of opening and specialization (O/S scenario) into cropping systems based on chemically synthesized inputs, as is the case in the Seine basin.…”