Geoscience Reference
In-Depth Information
D). At both Willington and Gunthorpe, lower
dissolved oxygen levels during the 1950s and early
1960s were followed by a rise during the mid-1960s
and relatively high (7-8 mg L
−
1
), stable levels after
the 1970s (Figure 21.3C, D).
At Hanford, dissolved oxygen concentrations
were typically much lower than at the other
three sites between 1923 and 1962 (Figure 21.3).
However, the recovery phase during the middle
1960s was broadly synchronous, with minimum
oxygen concentrations at all four sites generally
above 5 mg L
−
1
by the early 1970s. From the mid-
1970s, minimum dissolved oxygen concentrations
at all four sites generally exceeded 5 mg L
−
1
.
The 4-year low flow (1988-1992) conditions were
reflected in lower oxygen concentrations. Oxygen
data from 1982 to 1987 are not available in the
records, but the apparent fall in concentrations
from 1982 to 1987 at Hanford, Yoxall and
Willington did not occur at Gunthorpe (Figure
21.3).
and 1990s. By 2000, taxon richness was mostly
between 18 and 30 per sample compared with five
or less in the 1960s and 3-10 in the 1970s (Figure
21.3C).
At Gunthorpe, downstream of Nottingham
sewage treatment works, the rise in taxon richness
reflected more closely the trend in dissolved
oxygen, with increases following within 2 years of
the increase in oxygen (Figure 21.3D). The major
biological improvement was after 1994-1995,
even though the minimum dissolved oxygen
concentration had usually been more than 5 mg
L
−
1
for almost 20 years. Taxon richness rose from
5-6 in the 1960s and 1970s to 18-28 after 1995
(Figure 21.3D).
At all sites, the changes in oxygen and taxon
richness did not correspond after 2004 (Figure
21.3). Although minimum oxygen concentrations
remained relatively stable, there were clear declines
in taxon richness by between 6 and 16 taxa at all
sites during 2004-2005 to 2010. The reasons for
this are not yet clear, though episodic pollution
(
The Ecologist
, 15 October 2009) and low flows may
have been influential. The decline at Gunthorpe
was less marked than at the other sites, while most
recent surveys by the Environment Agency have
shown improvements from the lowest 2004-2010
values.
Ecological recovery from the 1950s to
2010
In the Hanford reach just downstream of the
Potteries (Figure 21.1) the rapid recovery in
dissolved oxygen concentrations in the mid-1960s
was not matched by an immediate recovery in
the invertebrate fauna (Figure 21.3A). Instead
taxon richness increased slowly during the 1970s
but more rapidly after 1995, following a period
of relatively stable and high dissolved oxygen
levels from 1975. By 2008, 17-24 benthic
macroinvertebrate taxa were recorded per sample
compared with fewer than five taxa during the
1950s to 1970s.
At Yoxall, which was generally a reach of
recovery from the pollution from the Potteries, the
increase in dissolved oxygen in the mid-1960s was
followed by a gradual increase in taxon richness,
rising from five to 11 in the 1950s-1960s, about 20
in the 1980s and 25-30 since the year 2000 (Figure
21.3B).
At Willington taxon richness began to increase
about 5 years after the oxygen levels rose in the
early 1960s; however, numbers rose more rapidly
after 1980, a trend that continued during the 1980s
Other chemical changes and water
temperatures
There have been reductions in the majority of
polluting substances in the River Trent since
the 1950s, most notably the concentrations
of ammonia (as N), hard detergents, 5-day
biochemical oxygen demand (BOD) and heavy
metals (Jarvie
et al,
2000; Environment Agency,
unpublished data). As examples, reductions at
Nottingham (7 km upstream of Gunthorpe),
included:BOD14to3mgL
−
1
(1963-95); ammonia
3.5to0.2mgL
−
1
(1963-93); and dissolved copper
45 to 7
gL
−
1
(1974-2004) (Environment Agency,
unpublished data).
Water temperatures in the middle and lower
Trent have fallen as older power stations have
closed and been replaced by stations with cooling
towers
(Langford, 1990). Mean annual water
