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temperatures fell between 2 Cand9 Catvarious
locations along 150 km of river downstream from
Willington (Environment Agency, unpublished
data) between 1964 and 1992. At the power station
intake located at Ratcliffe-on-Soar, upstream from
Nottingham, there were daily fluctuations of up to
8-10 C in the 1970s but less than 0.5 C in 2002
(Southampton University, Historic Data Archive).
At all four study sites an improvement in
dissolved oxygen concentrations began in the early
1960s, but in each case there was a delay in
the rate of recovery of the invertebrate fauna.
The cause of this delay is not certain, but
studies in other polluted rivers suggest that delays
are influenced by factors such as the presence
and mobility of potential colonizing species in
the catchment, chronic low-level pollution and
episodic incidents (Milner, 1996; Langford et al.,
2009, 2010). Limiting factors such as ammonia,
BOD or heavy metals will in many cases also have
more influence on the rate of recolonization than
dissolved oxygen (Langford et al. , 2009, 2010).
We plan to test this hypothesis in relation to
historic recovery rates. For example, Langford
et al. (2009) showed that ammonia concentrations
may be the best predictor of ecological status
in streams and that dissolved oxygen or BOD
may be less relevant as a biological surrogate.
Our results suggest that oxygen concentrations
may be a poor predictive measure, but this needs
to be investigated further. For example, severe
toxic pollution by cyanides or acids from metal
finishing industries can kill all living organisms,
including bacteria, without triggering severe de-
oxygenation. This is simply because no organisms
survive to process the organic matter which causes
de-oxygenation (Hawkes, 1956). Between 1990
and 2010 changes in oxygen and other water
quality variables were relatively small, but the
invertebrate fauna continued to recover (Figure
21.3).
Water temperatures declined in the middle and
lower reaches of the Trent during a similar period
to chemical improvement, as a result of the closure
of older, direct-cooled power stations. Today there
are fewer, large power stations fitted with cooling
towers, so raised temperatures can barely be
detected more than 100 m downstream of outfalls
(Langford, 1990). The biological response to this
change has not yet been quantified for the whole
of the River Trent.
The River Trent provides an excellent case study
for examining variation in river water quality,
temperature and biology over a long timescale.
The data reveal a complex pattern of recovery
Discussion
The chemical and ecological quality of the
River Trent has improved greatly since the
1960s. The very poor water quality in the
upper reaches during the 1920s and the whole
length of the river in the 1950s and early
1960s was followed by improvements in water
quality in the mid-1960s (Figure 21.2). There
was also a steady increase in the number of
benthic macroinvertebrate taxa, although the
rate of biological improvement was slower than
improvements in oxygen concentrations (Figure
21.3).
Evidence from this research and previous studies
(Spicer, 1950; Lester, 1975) confirms that pollution
in the main River Trent was concentrated around
the Potteries area until the 1940s, principally
caused by domestic sewage. The polluting effects
of the River Tame and also those tributaries
draining the East Midlands were evident but not
as significant. By the 1950s, the condition of
the whole river had deteriorated as measured
by dissolved oxygen concentrations (Lester,
1975). The most likely causes were industrial
expansion, lack of investment in pollution control
infrastructure and also possibly damage to sewers,
sewage works and factories by air raids during the
Second World War. During this period, pollution
of the Tame had become so severe that the Trent
below the confluence contained few invertebrate
taxaandnofishformorethan30km(Langford,
1972; Lester, 1975). This situation continued for
many years, with the lower reaches of the Tame
itself containing mainly oligochaetes and a few
other pollution-tolerant groups until
the early
1970s (Langford et al. , 2010).
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