Environmental Engineering Reference
In-Depth Information
Therefore, the use of the Chironomus genera as an indicator of metal pollution
may be limited by the extent of contamination. However, the alternative genera
Procladius may be a viable alternative as it is more tolerant to industrial con-
tamination and tends to show morphological response to contaminant levels
where Chironomus is highly affected or has been eliminated (Diggins & Stewart
1993
). Fluctuating asymmetry, which is a disturbance in the form of a bilaterally
symmetrical organism (such as Chironomus), has also been reported to occur as a
result of metal contamination, although this is seen as a manifestation of stress
at a genetic level (Groenendijk et al.
1998
;Ilyashuket al.
2003
).
The presence of some macroinvertebrate species in areas with high concen-
trations of metals is thought to be related to inherited metal tolerance. Some
species that have been found in acidic metal-contaminated streams are not
normally known for their tolerance to metals and/or acidity, and it has been
proposed that this is the result of evolutionary exposure to naturally acidic
streams in the area (Winterbourn et al.
2000
). Adaptation to metals has been
implicated in the lack of differences observed between uncontaminated and
contaminated samples within moderately polluted rivers (De Jonge et al.
2008
).
Metal adapted genes have been found in populations of Chironomus riparius
exposed to Zn and Cu (Groenindjik et al.
2002
). Consistent patterns in adaptation
in field conditions have, however, not been found, and this is thought to be due
to the influx of non-adapted genes from local non-tolerant populations via
invertebrate drift from unpolluted sites (Groenindjik et al.
2002
). The mechanism
of tolerance to metals is not fully understood, although some tolerant species
such as Chironomus riparius have been shown to lose metals such as Cd and Zn
during metamorphosis (Groenendijk et al.
1999
). In addition, adapted midge
populations have been shown to have an increased storage capability in the
guts and increased excretion efficiency (Postma et al.
1996
). An increase in gill
ventilation in response to acid mine drainage could suggest that organisms
use this as a way of removing toxic substances from this sensitive region
(Gerhardt et al.
2005
). As noted earlier the stage of life cycle of the organism
is important with earlier stages tending to be more sensitive than later stages as
observed in Chironomus tetans and Corbicula manitensis (Wang
1987
).
Resistance to metals by some species and a decrease in interspecific competi-
tion because of low variety of benthic organisms in polluted zones could
explain the proliferation of certain species within impacted communities
(Mori et al.
1999
).
Importance of pH
The speciation and bioavailability of metals is strongly dependent upon pH.
Metals respond in different ways to pH, but in general, as pH decreases, the
availability of metals increases. In many cases the effect of metals at circum-
neutral pH has been shown to be minimal; for example, studies in southwest
