Environmental Engineering Reference
In-Depth Information
Taxon recolonisation and successions
Recovery of rivers from catastrophic disturbances is a function of both habitat
conditions such as water chemistry and to a lesser extent channel geomor-
phology and the proximity of species for recolonisation (Milner
1996
;
Langford & Frissell
2009
). Despite chemical controlling factors, the fauna of
streams can be heavily dependent on the fauna of neighbouring streams
(Sanderson et al.
2005
). Four potential recolonisation mechanisms have been
identified for river invertebrates, namely downstream migration (drift),
upstream migration, aerial migration (mainly by insects) and upward migra-
tion from deeper substrates (Williams & Hynes
1976
; Williams
1981
; Milner
1996
). Migrations from upstream and downstream are the only natural
recolonisation mechanisms for fish, though often fish are introduced by
anglers or fishery managers once conditions are judged satisfactory. Plants
might use the first three mechanisms, with wind or birds as an aerial agent.
In many rivers, severe pollution or disturbance to the source has destroyed
the fauna and therefore the major upstream (and downstream) sources of
recolonisation. Further, many feeders and tributaries are similarly polluted
so there may be no nearby sources of aerial colonisers other than the pollution-
tolerant taxa such as chironomids. The Tame tributaries in the Black Country
are classic examples (Harkness
1982
).
The general patterns of taxonomic recolonisation and succession in rivers
recovering from pollution have been recorded in many places over many years
(e.g., Richardson
1921
; Butcher
1946
; Hawkes
1956
,
1975
; Hynes
1960
; Milner
1996
) but the variations in relationships between absolute chemical and bio-
logical parameters are less well-known. The onset of taxonomic succession in
the Tame was similar at the three sites with predictable increases in taxon
richness as ammonia concentrations fell, indicating general chemical and
biological improvement (
Figs. 13.5
,
13.6
). The presence of specific taxa is still
difficult to relate to absolute values of chemical determinands because of the
use of annual averages and restricted numbers of biological collections, but
there is clearly a basis for predicting communities from ranges of chemical
quality. At both Chetwynd Bridge and Eagle Lane, average ammonia concen-
trations exceeding 5mg L
1
were associated with Oligochaeta (sludge worms),
Asellidae (hog-lice), Chironomidae (bloodworms and fly-larvae) and Erpobdelli-
dae (leeches) (
Figs. 13.5
and
13.7
). However, at Lea Marston, average ammonia
concentrations around 5mg L
1
were associated with a greater number of taxa,
including those above plus moderately tolerant groups such as Planariidae
(flatworms), Corixidae (water-boatmen), Baetidae (mayflies) and Gammaridae
(freshwater shrimps) (
Fig. 13.6
). At Chetwynd Bridge clean-water taxa (Wright
et al.
2000
) such as free-living caddis (Psychomyiidae), damsel flies (Caloptery-
gidae) and cased caddis (Molannidae, Leptoceridae) colonised as ammonia
concentrations fell below an average of around 2.5mg L
1
. At Lea Marston,
