Geography Reference
In-Depth Information
Figure 3.3 The processes involved in the formation and deposition of acid pollution.
Source: Based on Mannion 1997.
produce compounds that are readily washed or
leached out of the system. With time, soils and
peats become progressively impoverished as the
cation stock is diminished. The lack of nutrients
will limit vegetation growth, leaving soils and peats
vulnerable to erosion.
In lakes, the accumulation of hydrogen ions
reduces pH. This can have direct and indirect
impacts in relation to fish stocks; populations and
ranges of species diminish with increasing
acidification, which reduces their reproductive
success. Moreover, fish kills may be caused when
surges of acidified water occasioned by cloud
bursts or rapid melts of winter snowfall occur. The
indirect effects of acid precipitation are due to its
impact on other aspects of environmental
chemistry. In particular, aluminium is more soluble
at low rather than high pH and is, consequently,
removed from soils or peats if they are in receipt
of acidic precipitation. Drainage from such soils
may eventually enter lake basins, where high
concentrations of aluminium will adversely affect
fish populations. A combination of pH in the
range 5.0-5.5 and high concentrations of
aluminium in calcium-deficient water causes
physiological changes in fish, particularly in the
immature fry, and can cause death. For example, at
concentrations of less than 100 µg l-1 the ability
of fish to regulate their salt and water content is
impaired, and at concentrations above 100 µg l -1
aluminium hydroxide [Al(OH 3 )] is formed as a
gelatinous precipitate on fish gills. Eventually, this
causes death through the impairment of
respiration (see Wellburn 1994 for details).
According to Gleick (1993), fish do not survive at
a pH of less than 3.5; salmonids cannot tolerate
pH 3.5 to 4.0, although tench, roach, pike and
perch can survive, and even at pH 4.0 to 4.5 adult
salmonids are likely to be adversely affected, while
their fry and eggs will continue to be impaired at
pH 4.5 to 5.6. This pH range is suitable for tench,
roach and carp, etc., but only at pH above 5 are
there no adverse impacts on fish populations.
Aluminium in lake waters may also combine
with phosphorus to form chemical complexes,
effectively removing phosphorus for uptake by
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