Environmental Engineering Reference
In-Depth Information
vegetation or weeds (Armitage et al.
2001
). Also, where riffle-pool sequences and
instream-vegetation stands have been physically destroyed or removed there
would be some suppression of invertebrate diversity even under clean-water
conditions. However, the physical condition of the channel is unlikely to
prevent colonisation by 'mid-stream' clean-water, lotic taxa such as species of
Ephemeroptera, Plecoptera and Trichoptera if they were available to colonise.
Therefore, water quality variations combined with the lack of animals for
recolonisation remain the major constraints on recovery in the upper Tame.
Morphological constraints are relatively small. The effects of biological impro-
vement in relation to further chemical improvement are therefore difficult to
predict in view of the poor availability of clean-water colonisers. Colonisation
by fish may be more dependent on physical heterogeneity, particularly at high
discharges (Booker
2003
).
From this and other studies of the long-term recolonisation of rivers
polluted to the source (e.g., Langford et al.
2009
), it is clear that the problems
of ecological recovery in many previously polluted streams can be a combin-
ation of residual pollution problems and biological processes such as the
proximity and mobility of relevant taxa. Whilst it may be possible to improve
chemical quality over a relatively short time, the dispersal of colonisers from
clean streams and rivers may take much longer, possibly more than 30 years in
the upper Tame (Langford et al.
2009
). This has cost implications for effluent
and water quality control in that to achieve biological recovery it may not be
necessary to aim for continuous chemical improvement once a certain thresh-
old has been reached, though the threshold quality must be maintained.
Further, ecological quality improvement will not be directly related to che-
mical improvement but may be significantly delayed. Thus, though fish may
recolonise quickly, invertebrate recolonisation may take many years and this
may be very relevant for the upper Tame and rivers in similar condition in
other industrial conurbations.
There are also implications for monitoring in that, though the chemistry
of the water may indicate improvement, the fauna does not. Whether this is a
result of other chemical variables not used in this analysis or of the lack of
colonisers is not yet fully investigated. However, if the problem is biological
rather than water quality, the validity of the biological assessment as a monitor
of water quality is called into doubt. This may also have implications for
classifications determined by the Water Framework Directive, which we hope
further analysis of the long-term datasets will clarify.
Acknowledgements
The authors are indebted to Jean Langford for copy editing and for collating
and checking the bibliography. Tom Worthington of Southampton University
