outlet for the driest RCP 8.5 projection). Changes in water temperature are even
expected to superimpose with potential modifications of phytoplankton biomass and
composition as well as oxygen concentrations, as shown by the sensitivity analysis
performed by Wang et al. [73]. Acting on diffuse sources and, to a lesser extent, on
point sources (as discussed above) is therefore even more necessary in a context of
climate change.
6 Conclusions and Perspectives
The PIREN-Seine program is a rare example of an interdisciplinary scientific project
successfully spanning 30 years. Owing to its longevity, it has been able to document
the long-term changes experienced by the Seine River system and its metabolism.
At the time the program was launched, in the late 1980s, the major water quality
issue was the oxygen deficits caused by discharge of insufficiently treated urban
wastewater, particularly in the main branch of the Seine downstream from Paris. The
heterotrophic degradation of the organic matter and the nitrification of the reduced
nitrogen brought by these point sources of pollution were responsible for severe
oxygen depletion in these sectors.
One decade later, the problem of algal blooms in the large tributaries of the Seine
upstream from Paris appeared as a major threat for drinking water production from
surface water (about half the supply of the Paris agglomeration) and also as a
significant contributor to organic matter loading in the downstream sectors. The
abundance of phosphorus, coming from point sources, and the resulting lack of any
limitation of algal growth appeared as the major cause of this river’s eutrophication
problem. The strong contrast between autotrophy in these upstream sectors and the
heterotrophy of the downstream sectors of the river network was particularly
striking.
In the early 2000s, major progress in wastewater treatment, including the drastic
reduction of organic matter loading, as well as the treatment of phosphorus,
completely changed the situation. Good oxygen levels were recovered nearly everywhere, while algal blooms were considerably reduced as a result of the limitation of
phosphorus.
While the WWTP outflows of nutrients were drastically reduced following the
European regulations of the Water Framework Directive [65], diffuse pollutions
have not yet produced the same success story. Nitrate (and pesticide) contamination
is still a major threat for groundwater quality and the drinking water supply, for river
water quality and the diversity of its biological communities and for marine coastal
zones and the occurrence of toxic algal blooms.
In essence, controlling diffuse pollutions is much more difficult to achieve than
containing point source pollutions. It requires not only technical measures but also a
complete overhaul of the structure of the agro-food system and hence of the
socioecological metabolism of the watershed. Moreover, inertia of groundwater
210
J. Garnier et al.
expected to superimpose with potential modifications of phytoplankton biomass and
composition as well as oxygen concentrations, as shown by the sensitivity analysis
performed by Wang et al. [73]. Acting on diffuse sources and, to a lesser extent, on
point sources (as discussed above) is therefore even more necessary in a context of
climate change.
6 Conclusions and Perspectives
The PIREN-Seine program is a rare example of an interdisciplinary scientific project
successfully spanning 30 years. Owing to its longevity, it has been able to document
the long-term changes experienced by the Seine River system and its metabolism.
At the time the program was launched, in the late 1980s, the major water quality
issue was the oxygen deficits caused by discharge of insufficiently treated urban
wastewater, particularly in the main branch of the Seine downstream from Paris. The
heterotrophic degradation of the organic matter and the nitrification of the reduced
nitrogen brought by these point sources of pollution were responsible for severe
oxygen depletion in these sectors.
One decade later, the problem of algal blooms in the large tributaries of the Seine
upstream from Paris appeared as a major threat for drinking water production from
surface water (about half the supply of the Paris agglomeration) and also as a
significant contributor to organic matter loading in the downstream sectors. The
abundance of phosphorus, coming from point sources, and the resulting lack of any
limitation of algal growth appeared as the major cause of this river’s eutrophication
problem. The strong contrast between autotrophy in these upstream sectors and the
heterotrophy of the downstream sectors of the river network was particularly
striking.
In the early 2000s, major progress in wastewater treatment, including the drastic
reduction of organic matter loading, as well as the treatment of phosphorus,
completely changed the situation. Good oxygen levels were recovered nearly everywhere, while algal blooms were considerably reduced as a result of the limitation of
phosphorus.
While the WWTP outflows of nutrients were drastically reduced following the
European regulations of the Water Framework Directive [65], diffuse pollutions
have not yet produced the same success story. Nitrate (and pesticide) contamination
is still a major threat for groundwater quality and the drinking water supply, for river
water quality and the diversity of its biological communities and for marine coastal
zones and the occurrence of toxic algal blooms.
In essence, controlling diffuse pollutions is much more difficult to achieve than
containing point source pollutions. It requires not only technical measures but also a
complete overhaul of the structure of the agro-food system and hence of the
socioecological metabolism of the watershed. Moreover, inertia of groundwater
210
J. Garnier et al.