Geoscience Reference
In-Depth Information
Results
'worst case' situation. This parameter also has the
longest record of measurements. A figure of 5 mg
L
−
1
of dissolved oxygen is used as a threshold,
because it represents about 50% saturation at 16
◦
C
and is considered the minimum level for the long-
term survival of salmonid fish (Tebbutt, 1983).
Benthic macroinvertebrate data, derived from
hand-net surveys, were compared by using taxon
richness, which is approximately equivalent to
the Biological Monitoring Working Party (BMWP)
index, (N
taxa
)(Wright
et al.,
2000). Taxon richness
(N
taxa
) was preferred as a measure because it is
an un-weighted indicator of change to compare
with the oxygen trends (Clarke
et al.
, 2002). Other
indices such as Average Score Per Taxon (ASPT)
and BMWP (Wright
et al.
, 2000), are both partial
surrogates for chemical conditions because they
depend on tolerance to chemical variables and
are not therefore independent of chemistry. Taxon
richness does not represent composition of the
fauna but the assumption is that low numbers
of taxa indicate the presence of a few pollution-
tolerant taxa, whereas higher numbers include
the appearance of less tolerant, cleaner-water taxa
(Wright
et al.,
2000; Clarke
et al.,
2002).
Trends in data on dissolved oxygen and
invertebrate taxon richness were analysed at four
key sites chosen to represent perceived different
pollution zones related to specific tributaries,
conurbations or effluents (Figure 21.1). The sites
are:
Hanford
: located about 4 km downstream from
the Potteries and Fowlea Brook, the most polluted
tributary in that reach (Lester, 1975);
Yoxall
: tuatedinareachcon ideredto
be recovering from pollution about 75 km
downstream from the Potteries and 3-5 km
upstream from the confluence with the River
Tame;
Willington
: located about 14 km downstream
from the River Tame confluence, (the most
polluted tributary in the middle reaches of the
Trent) and 5 km downstream from the breweries
of Burton-on-Trent;
Gunthorpe
: it td 5km ow t am
from Nottingham and 5 km downstream from
Nottingham's main sewage treatment works.
Longitudinal dissolved oxygen
profiles
In the 1920s and 1930s the major pollution
problems were associated with Stoke-on-Trent
and the Potteries, near the source of the river
(Figure 21.1). Although there were some effects
caused by the River Tame entering from the
Black Country industrial conurbation, minimum
oxygen concentrations below 5 mg L
−
1
were
only recorded over short distances (Figure 21.2A,
B). Dissolved oxygen concentrations were also
reduced downstream from the East Midlands and
Nottingham sewage treatment works, but these
remained mostly above 5 mg L
−
1
.
In 1953, minimum oxygen concentrations below
5mgL
−
1
were recorded for almost the whole of the
Trent from the Potteries downstream, including the
tidal reaches (Figure 21.2C). This corresponded to
the effects of an increase in munitions and other
manufacturing during the Second World War. By
1972, minimum oxygen concentrations in the river
had risen to over 5 mg L
−
1
for almost the whole
of the river downstream from the Potteries (Figure
21.2D), a situation that still prevailed in 2008
(Figure 21.2E).
Temporal changes at specific sites
The oxygen profiles from the 1920s to 2010 differ
among the four selected individual sites (Figure
21.3). At Hanford, gross pollution is indicated
by low dissolved oxygen concentrations from the
1920s to 1966, after which a marked recovery
took place (Figure 21.3A). At Yoxall, oxygen
levels were generally higher than at Hanford
but concentrations declined locally in the 1940s
and 1950s, followed by a rise during the mid-
1960s (Figure 21.3B). By the mid 1970s minimum
oxygen concentrations were generally more than
5mgL
−
1
, albeit with lower levels during the late
1980s and early 1990s, which coincided with a 4-
year period of low rainfall and low flows. Dissolved
oxygen concentrations at Willington (middle Trent)
and at Gunthorpe (lower Trent) were similar and
matched those of the upper reaches (Figure 21.3C,
