The last sector, downstream of the Paris urban area, was also strongly affected by
upstream urban discharges and experienced a very comparable water quality trajectory. In the 1970s, the oxygen deficit was particularly severe as far as the estuary;
only the area immediately downstream of navigation dams provided satisfactory
oxygenation conditions in very local areas. The oxygen deficit then gradually
decreased, reaching levels compatible with the survival of most fish species in the
2000s (Fig. 6).
Long-term variations in IPR scores across sectors are consistent with changes
in water quality in the Seine River (Fig. 7, Table 1). Thus, the sectors that have
experienced the greatest improvements in their water quality (sectors 4 and 3 and to
a lesser extent sector 2) also showed the most substantial signs of recovery in their
fish communities. In the upstream sector, IPR scores also show a significant decrease
over time, but this improvement trend remains slight (only four out of seven metrics
show improvement trends and one metric – density of omnivorous species – even
shows the opposite trend) but without any comparison with the magnitude of the
changes observed further downstream. The temporal trend curves (Fig. 7) suggest
that in the upstream urban sector (sector 2), a considerable community improvement
would have occurred in the early 1990s, while further downstream (sectors 3 and 4),
the improvement would have been more pronounced later, in the early 2000s. This
is consistent with the fact that organic pollution problems were solved earlier
upstream of the urban area than farther downstream. However, this statement must
be addressed with some caution given that the sites sampled may have changed over
time, which may bias temporal trends.
Despite the general improvement in the fish community composition, fish tissues
are still heavily contaminated by various pollutants such as mercury, PCBs and some
pesticides [5]. This widespread contamination has led to regulations prohibiting fish
consumption on a large part of the Seine River axis and its main tributaries. Despite
a significant reduction in organic pollution and eutrophication in recent decades,
pollution by various metal or organic contaminants remains a major problem in the
basin [34–36].
Beyond questions regarding the integrity of fish communities, a trend towards
increasing species richness has emerged in recent decades. First noted on the Seine
River in the Paris metropolitan area [5], this trend is actually more widespread and
also affects the sectors located upstream and downstream of the urban area. It implies
the establishment of new species or the extension of populations of species that were
initially much more restricted in their distributions. For example, the wels catfish
(S. glanis), a species introduced quite recently, started to be sampled occasionally in
the 2000s, mainly in the mid-Seine River. It is now commonly found along the entire
river course. Other species such as bitterling (Rhodeus amarus) and pumpkinseed
(Lepomis gibbosus), initially mainly located upstream of the urban part of the Seine,
gradually extended their distribution downstream over the recent decades as the
water quality improved. A similar pattern was also observed for some lithophilic and
rheophilic species such as barbel (Barbus fluviatilis) or dace (Leuciscus leuciscus)
[37]. In this respect, the case of the European bullhead (C. perifretum) is particularly
remarkable: considered typical of the upstream zones, this oxyphilic species has
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upstream urban discharges and experienced a very comparable water quality trajectory. In the 1970s, the oxygen deficit was particularly severe as far as the estuary;
only the area immediately downstream of navigation dams provided satisfactory
oxygenation conditions in very local areas. The oxygen deficit then gradually
decreased, reaching levels compatible with the survival of most fish species in the
2000s (Fig. 6).
Long-term variations in IPR scores across sectors are consistent with changes
in water quality in the Seine River (Fig. 7, Table 1). Thus, the sectors that have
experienced the greatest improvements in their water quality (sectors 4 and 3 and to
a lesser extent sector 2) also showed the most substantial signs of recovery in their
fish communities. In the upstream sector, IPR scores also show a significant decrease
over time, but this improvement trend remains slight (only four out of seven metrics
show improvement trends and one metric – density of omnivorous species – even
shows the opposite trend) but without any comparison with the magnitude of the
changes observed further downstream. The temporal trend curves (Fig. 7) suggest
that in the upstream urban sector (sector 2), a considerable community improvement
would have occurred in the early 1990s, while further downstream (sectors 3 and 4),
the improvement would have been more pronounced later, in the early 2000s. This
is consistent with the fact that organic pollution problems were solved earlier
upstream of the urban area than farther downstream. However, this statement must
be addressed with some caution given that the sites sampled may have changed over
time, which may bias temporal trends.
Despite the general improvement in the fish community composition, fish tissues
are still heavily contaminated by various pollutants such as mercury, PCBs and some
pesticides [5]. This widespread contamination has led to regulations prohibiting fish
consumption on a large part of the Seine River axis and its main tributaries. Despite
a significant reduction in organic pollution and eutrophication in recent decades,
pollution by various metal or organic contaminants remains a major problem in the
basin [34–36].
Beyond questions regarding the integrity of fish communities, a trend towards
increasing species richness has emerged in recent decades. First noted on the Seine
River in the Paris metropolitan area [5], this trend is actually more widespread and
also affects the sectors located upstream and downstream of the urban area. It implies
the establishment of new species or the extension of populations of species that were
initially much more restricted in their distributions. For example, the wels catfish
(S. glanis), a species introduced quite recently, started to be sampled occasionally in
the 2000s, mainly in the mid-Seine River. It is now commonly found along the entire
river course. Other species such as bitterling (Rhodeus amarus) and pumpkinseed
(Lepomis gibbosus), initially mainly located upstream of the urban part of the Seine,
gradually extended their distribution downstream over the recent decades as the
water quality improved. A similar pattern was also observed for some lithophilic and
rheophilic species such as barbel (Barbus fluviatilis) or dace (Leuciscus leuciscus)
[37]. In this respect, the case of the European bullhead (C. perifretum) is particularly
remarkable: considered typical of the upstream zones, this oxyphilic species has
314
J. Belliard et al.