The impacts of metalliferous drainage on aquatic communities in streams and rivers - Ecology of Industrial Pollution - page 78

UK streams showed that there was a lack of significant change in community
structure in relation to metals, but the acid-base status of the river was much
more important (Hirst et al. 2002 ). However, under acidic conditions the tox-
icity of metalliferous drainage can be enhanced. A study by Gerhardt et al.
( 2005 ) showed that the effects of acid mine drainage were found to be more
noticeable when pH values were less than 5.
In response to low pH, there is generally a reduction in taxonomic diversity
and a change in community structure (Winterbourn et al. 2000 ; Liu et al. 2003 ).
Ephemeroptera are particularly affected by pH with reports of many taxa
within this group being removed at pH 6.3 (Gu´rold et al. 2000 ). Crustaceans

and molluscs are also strongly affected by low pH (Albers & Camardese 1993 ). It
is evident that many community responses to both metals and acidity are
similar, and therefore, it is difficult to separate them out in rivers receiving
acidic metalliferous discharges.
There is limited information on the physiological responses of macroinverte-
brates to acidity but pH is thought to affect organisms directly due to an
increase in cell membrane permeability at low pH (Camargo 1995 ; Havas &
Advokaat 1995 ), which can cause problems with osmoregulation (Cameron &
Iwana 1989 ). Harvas and Advokaat ( 1995 ) noted that, in acidic environments,
insects with less permeable cell membranes, such as Corixa punctata (Hemiptera,
water bug), were more tolerant. In fish, it is also reported that Ca levels can be
dramatically reduced within tissues which interfere with their reproductive
cycle (EPA 1980 ). Necrosis of the gill epithelium of fish has also been reported,
although this may be related to increased aluminium availability rather than a
direct effect of the acidity (Cronan & Schofield 1979 ).
Although the bioavailability of metals is thought to increase at low pH,
uptake of metals may actually be reduced in acidic conditions. Gerhardt et al.
( 2005 ), for example, reported that metal body burdens in mayflies were shown
to decrease at low pH. There have been a number of explanations proposed for
this pattern but the most viable are:

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1. The presence of hydrogen ions at membrane receptors could effectively
compete with metal ions, therefore reducing uptake (e.g., Campbell &
Stokes 1985 ; Yan et al. 1990 ; Gerhardt 1993 ).
2. Metals that are present at the cellular surface of organisms could be
desorbed at lower pH (Krantzberg & Stokes 1988 ).

Some organisms have been shown to be adapted to low pH environments
and mayflies have been reported to occur at pH as low as 4.5 (Kelly 1991 ).
Acidophilic organisms often maintain their cytoplasm at the same pH as
neutrophilic relatives through the use of mechanisms such as proton uptake,
cell wall permeability and internal buffering (e.g., Pick 1999 ). Experiments
using the mayfly Choroterpes picteti have shown that sub-lethal behavioural