At this time discharged ammonium led to concentrations up to 8–10 mg N-NH 4
L
À1 downstream of SAV, decreasing to 2 mg N-NH 4 L
À1 in the estuary (Fig. 4).
Oxygen depletion immediately downstream of Paris occurred until the early 1990s,
when the proportion of treated effluent increased (Fig. 3a). Oxygen depletion farther
in the estuary was only interrupted by the implementation of a nitrification treatment
in the SAV WWTP in 2007. All these efforts had a very positive effect on the
oxygenation of the river, which today can boast a good ecological status with respect
to oxygen concentrations [62].
Exactly as simulated in Garnier et al. [28], nitrifying treatment, implemented in
WWTPs since 2007, considerably lowered the ammonium discharged in the river
(a tenfold decrease) [27] and favoured full reoxygenation of the estuary (Fig. 4)
[29]. In addition, molecular methods show that ammonia-oxidising bacteria (AOB),
(Nitrosomonas oligotropha and Nitrosomonas ureae-like bacteria), introduced by
the WWTP effluents, survived and actively participated in in-river NH 4 oxidation far
downstream of the WWTP outflow [27, 63].
Although such changes in water treatment were mandatory [64, 65], the studies
carried out as a part of the PIREN-Seine program have helped decision-makers over
the past 30 years.
3.2 Long-Term Nutrient Contamination and Algal Growth
As seen above, improved water treatments reduced ammonium levels and increased
oxygenation of the lower Seine River (Fig. 5). Although water treatments have also
efficiently reduced phosphates since the mid-1990s, phytoplankton blooms
(>100 μg Chla L
À1 ) still occurred until 2005, causing nuisance for the drinking
water production and producing, after bloom decline, large organic loadings, which
contributed to oxygen depletion [66].
A reduction of phosphorus load by a factor 10, reached only after 2005, has been
necessary to significantly decrease algal growth and avoid eutrophication. Nowadays, algal blooms as high as observed 20 years ago have disappeared. Despite these
spectacular improvements, the Seine River remains fragile. For example, low summer water curtails the dilution of point source pollutions, increasing nutrient concentrations above growth limitation levels. Additionally, malfunctioning of WWTPs
for maintenance purposes occasionally occurs, possibly accentuating temporary
degradation of water quality [29].
While point source reductions led to a significant reduction of ammonium and
phosphate pollution, nitrate from diffuse agricultural sources steadily increased from
an average of winter month concentrations of about 3 mgN-NO 3 L
À1 in 1971 to
6.3 mgN-NO 3 L
À1 in 2011, i.e. more than 3 mgN-NO 3 L
À1 over 40 years. The recent
trend is towards stabilisation, and even a decrease in the recent years, although NO 3
concentrations peaked again at 8 mgN-NO 3 L
À1 during the mild and dry 2017 winter
(Fig. 5).
Ecological Functioning of the Seine River: From Long-Term Modelling. . .
199
L
À1 downstream of SAV, decreasing to 2 mg N-NH 4 L
À1 in the estuary (Fig. 4).
Oxygen depletion immediately downstream of Paris occurred until the early 1990s,
when the proportion of treated effluent increased (Fig. 3a). Oxygen depletion farther
in the estuary was only interrupted by the implementation of a nitrification treatment
in the SAV WWTP in 2007. All these efforts had a very positive effect on the
oxygenation of the river, which today can boast a good ecological status with respect
to oxygen concentrations [62].
Exactly as simulated in Garnier et al. [28], nitrifying treatment, implemented in
WWTPs since 2007, considerably lowered the ammonium discharged in the river
(a tenfold decrease) [27] and favoured full reoxygenation of the estuary (Fig. 4)
[29]. In addition, molecular methods show that ammonia-oxidising bacteria (AOB),
(Nitrosomonas oligotropha and Nitrosomonas ureae-like bacteria), introduced by
the WWTP effluents, survived and actively participated in in-river NH 4 oxidation far
downstream of the WWTP outflow [27, 63].
Although such changes in water treatment were mandatory [64, 65], the studies
carried out as a part of the PIREN-Seine program have helped decision-makers over
the past 30 years.
3.2 Long-Term Nutrient Contamination and Algal Growth
As seen above, improved water treatments reduced ammonium levels and increased
oxygenation of the lower Seine River (Fig. 5). Although water treatments have also
efficiently reduced phosphates since the mid-1990s, phytoplankton blooms
(>100 μg Chla L
À1 ) still occurred until 2005, causing nuisance for the drinking
water production and producing, after bloom decline, large organic loadings, which
contributed to oxygen depletion [66].
A reduction of phosphorus load by a factor 10, reached only after 2005, has been
necessary to significantly decrease algal growth and avoid eutrophication. Nowadays, algal blooms as high as observed 20 years ago have disappeared. Despite these
spectacular improvements, the Seine River remains fragile. For example, low summer water curtails the dilution of point source pollutions, increasing nutrient concentrations above growth limitation levels. Additionally, malfunctioning of WWTPs
for maintenance purposes occasionally occurs, possibly accentuating temporary
degradation of water quality [29].
While point source reductions led to a significant reduction of ammonium and
phosphate pollution, nitrate from diffuse agricultural sources steadily increased from
an average of winter month concentrations of about 3 mgN-NO 3 L
À1 in 1971 to
6.3 mgN-NO 3 L
À1 in 2011, i.e. more than 3 mgN-NO 3 L
À1 over 40 years. The recent
trend is towards stabilisation, and even a decrease in the recent years, although NO 3
concentrations peaked again at 8 mgN-NO 3 L
À1 during the mild and dry 2017 winter
(Fig. 5).
Ecological Functioning of the Seine River: From Long-Term Modelling. . .
199