Environmental Engineering Reference
In-Depth Information
Figure 4.8
Aluminium concentration in relation to pH
for 20 lakes near Sudbury, Ontario
will be the gradual destruction of the biological
communities in the ecosystem.
Most of the investigations into the impact of
acid rain on aquatic communities have involved
fish populations. There is clear evidence, from
areas as far apart as New York State, Nova
Scotia, Norway and Sweden, that increased
surface water acidity has adverse effects on fish
(Baker and Schofield 1985). The processes
involved are complex and their effectiveness
varies from species to species (see Figure 4.9).
For example, direct exposure to acid water may
damage some species. Brook trout and rainbow
trout cannot tolerate pH levels much below 6.0
(Ontario: Ministry of the Environment 1980),
and at 5.5 smallmouth bass succumb
(LaBastille 1981). The
salmonid
group of fish is
much less tolerant than coarser fish such as pike
and perch (Ontario: Ministry of the
Environment 1980). Thus as lakes become
progressively more acid, the composition of the
fish population changes.
The stage of development of the organism is
also important (see Figure 4.10). Adult fish, for
example, may be able to survive relatively low
pH values, but newly hatched fry, or even the
spawn itself, may be much less tolerant (Ontario:
Ministry of the Environment 1980). As a result
the fish population in acid lakes is usually wiped
out by low reproductive rates even before the
pH reaches levels which would kill mature fish
(Jensen and Snekvik 1972).
Fish in acid lakes also succumb to toxic
concentrations of metals, such as aluminium,
mercury, manganese, zinc and lead, leached from
the surrounding rocks by the acids. Many acid
lakes, for example, have elevated concentrations
of aluminium (Brakke
et al.
1988), which has
been recognized as a particularly potent toxin
(Cronan and Schofield 1979). The toxic effects
of aluminium are complex, but in lakes where
the pH has fallen below 5.6 it is commonly
present in sufficient quantity to kill fish (Stokes
et al.
1989). The fish respond to the presence of
the aluminium by producing mucus which clogs
the gills, inhibiting breathing and causing a
breakdown of their salt regulation systems
Source:
From Harvey (1989)
Some uncertainties over the relationship
between industrial emissions and acid rain will
always remain, because of the complexity of the
environment, and the variety of its possible
responses to any input. There can be no doubt,
however, that acid rain does fall, and when it
does its effects on the environment are often
detrimental.
In addition to reduced pH values, acidic lakes
are characterized by low levels of calcium and
magnesium and elevated sulphate levels. They
also have above-normal concentrations of
potentially toxic metals such as aluminium
(Brakke
et al.
1988)(see Figure 4.8). The initial
effect of continued acid loading varies from lake
to lake, since waterbodies differ in their sensitivity
to such inputs. Harmful effects will begin to be
felt by most waterbodies when their pH falls to
5.3 (Henriksen and Brakke 1988), although
damage to aquatic ecosystems will occur in some
lakes before that level is reached, and some
authorities consider pH 6.0 as a more appropriate
value (Park 1987). Whatever the value, once the
critical pH level has been passed, the net effect
