Environmental Engineering Reference
In-Depth Information
Figure 4.10
The age-class composition of samples of
the white sucker populations in lakes Skidway (pH
5.0) and Bentshoe (pH 5.9)
lakes have established that 50 per cent of lakes
with a pH <5 are likely to contain no fish (Mason
1990). More than 300 lakes in Ontario, Canada,
mainly in the vicinity of Sudbury, are fishless and
many others have experienced a reduction in the
variety of species they contain (Ontario: Ministry
of the Environment 1980; Park 1991); several
rivers in Nova Scotia, once famous for their
Atlantic salmon, are now too acidic to support
that fish (Israelson 1987). In the Adirondack
Mountains of New York State between 200 and
400 lakes have lost some or all of their fish species
(Harvey 1989). Damage to fish populations has
occurred in an area of 33,000 sq km in southern
Norway, with brown trout particularly hard hit
(Baker and Schofield 1985). In nearby Sweden,
100 lakes sampled in the mid-1970s had already
lost 43 per cent of their minnow, 32 per cent of
their roach, 10 per cent of their arctic char and
14 per cent of their brown trout populations as
a result of acidification (Almer
et al.
1974), and
by the end of the decade it was estimated that
some 4,000 lakes in Sweden were fishless
(LaBastille 1981). In Britain, no obvious
problems emerged until the 1970s, when rivers
and lakes in southwest Scotland, the English Lake
District and Wales showed the first signs of
decline in the numbers of trout, salmon and other
game fish (Park 1987). Since then a number of
studies have shown that the problem is
particularly acute in those streams draining
forested catchment areas, which are commonly
more acidic. Many of these streams are
completely devoid of fish, or support only a few
species in their lower reaches, where the acidity
is usually less (Elwood and Mulholland 1989).
Waterbodies which have lost, or are in the
process of losing their fish populations are often
described as 'dead' or 'dying'. This is not strictly
correct. All aquatic flora and fauna will decline
in number and variety during progressive
acidification, but, even at pH 3.5, water boatmen
and whirligig beetles survive and multiply
(LaBastille 1981), and species of protozoans are
found at pH levels as low as 2.0 (Hendrey 1985).
Phytoplankton will disappear when pH falls
below 5.8 (Almer
et al.
1974), but acid-tolerant
Source:
From Harvey (1989)
waterways, until the spring melt occurs. At that
time they are flushed into the system in
concentrations many times higher than normal.
Measurements in some Ontario lakes have shown
a reduction in pH values of more than two units
in a matter of a few days although decreases of
the order of one pH unit are more common
(Ontario: Ministry of the Environment 1980;
Jeffries 1990). This augmented level of acidity
may last for several weeks, and, unfortunately,
it often coincides with the beginnings of the
annual hatch. The recently hatched fry cannot
survive the shock, and fish populations in acidic
lakes often have reduced or missing age groups
which reflect this high mortality (Baker and
Schofield 1985). The aquatic ecosystems of lakes
which are normally well-buffered are not immune
to major damage if the melt is rapid and highly
acidic (Ontario: Ministry of the Environment
1980).
Fish populations in many rivers and lakes in
eastern North America, Britain and Scandinavia
have declined noticeably in the last two to three
decades, as a consequence of the effects of acid
rain. Surveys of European and North American
