Environmental Engineering Reference
In-Depth Information
BICARBONATE
LAKES
TRANSITION
LAKES
ACID LAKES
Emissions of
S and N
7
Atmospheric
transport and
deposition
6
Terrestrial
ecosystem
5
2
Large pH-
fluctuations
during the
year, kills fish
1
Bicarbonate
prevents
acidification
Atmospheric
transport and
deposition
3
pH low and stable,
no fish
Runoff
4
Aquatic
ecosystem
SO 4 2- < HCO 3 -
SO 4 2-
HCO 3 -
SO 4 2- > HCO 3 -
Fig. 12.3 The main steps in the acidification process
of lakes. The process is related to the decrease in
pH caused by a lowering of the buffering capacity
of water due to a shift in the dominant anions, from
bicarbonates (HCO ions) to sulphates (SO 2− ). From
Mason (1996). Reproduced by permission of Pearson
Education, Inc.
Fig. 12.2 The steps linking emissions of SO 2 and NO x
and deposition, both direct and indirect, to aquatic
ecosystems. Note that the catchment area (terrestrial
ecosystem) and the runoff from here into lakes is a
more important cause of lake acidification than direct,
atmospheric deposition alone. From Steinberg and
Wright (1994). Reproduced by permission of John
Wiley and Sons Ltd.
when acidified, but mercury and vanadium become less
soluble. Many of the adverse effects on organisms are
attributed to the increased solubility of aluminium and
its shift to the toxic Al 3+ form. Increased mobilization
of Al 3+ ions in lakes also causes precipitation of P and
humic substances and such acid lakes tend to become
oligotrophic and thus more transparent. Reduced rates
of organic-matter decomposition and mineralization
and O 2 consumption by micro-organisms lead to
decreased availability of nutrients such as PO 4 -P so
that phytoplankton production decreases. In contrast,
the development of algal mats at the lake bottom may
increase due to improved light climate.
Increased solubility of metals at lower pH will
impose physiological stresses on zooplankton: both H +
and Al 3+ ions interfere with the sodium balance of most
crustacean zooplankton; for example, larger species
of Daphnia and calanoid copepods disappear below
pH 6.0, whereas Bosmina longispina still occurs at pH
values of <4.1 (Brett 1989, Steinberg & Wright 1994).
A survey of c .1500 Norwegian lakes showed that snails
and bivalves, with calcareous shells, largely disappeared
below pH 6. The crustaceans Lepidurus arcticus and
Gammarus lacustris , important food items for fish,
are sensitive to acidity and their decreases adversely
influence species richness and the structure of the
intensive agriculture and animal husbandry, nitrate
that is formed from the large amounts of ammonium
emitted (NH 4 + → NO 3 + 2H + ) from the farms acidifies
the watercourses. In addition, groundwater and forest
soils are affected via the runoff in the catchment.
Limestone in a drainage basin may help prevent
acidification considerably; regions with a calcareous
geology are not sensitive to acidification (Henriksen
et al. 1989). Figure 12.3 illustrates the pH decreases
that occur during the acidification of lakes and the
role of bicarbonate ions in buffering of lake water to
prevent acidification before sulphate concentrations
increase further.
It is not clear if the marked biotic changes in com-
munities of lake organisms caused by acidification are
direct, physiological effects of pH decrease (tolerance)
or are indirect effects of changes in biotic interactions.
For example, community structure could be altered by
shifts in the competitive relationships of the algae or
by the disappearance of keystone species (Eriksson et
al. 1980). Altered solubility and speciation of many
metals due to a decrease in pH can cause important
biological effects. Aluminium, iron, copper, zinc,
nickel, lead and cadmium become more soluble in water
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