Restoration of freshwater lakes - Restoration Ecology - page 167

great. Therefore, acidification due to N compounds is
now a more important issue.

In the Netherlands, Lamers et al. (2002) and
Roelofs et al. (2002) have reviewed works on the
restoration of fens and macrophytic vegetation in
acidified and eutrophic wetlands. The desiccation of
the fens by drainage (infiltration), rather than direct
acid emissions, has caused a decrease in the acid-
neutralizing capacity of the fens. Vegetation in some
shallow soft-water bodies is strongly endangered due
to atmospheric deposits of SO 4 2− and NH 4 + estimated
at 44 -50 and 84 -103 mmol m −2 yr −1 , respectively.
During dry summers, the pH values range between
4.1 and 5.4 but decrease to c .3.7 due to oxidation of
sulphur compounds in the sediments. In a restoration
study (Roelofs et al. 2002) small amounts of alkaline,
nutrient-poor ground water extracted from deeper
aquifers was used to raise alkalinity of small acidi-
fied, soft-water lakes after the top sediment layer
had been removed by dredging. A rapid decline of
NH 4 + and CO 2 led to an increase in pH from 4 to 7.
In contrast to fens and bogs, the larger Dutch lakes
are highly buffered due to naturally high inherent
concentrations of calcium and bicarbonates, and
therefore not affected by atmospheric emissions
(Gulati & van Donk 2002).
A critical analysis of the effects of liming of
freshwaters is lacking. The effects were examined
in plankton, benthic fauna and fish in Sweden
(Henrikson & Brodin 1995). Sensitive species of fish
were reported to re-colonize and increase in densities
after liming but fish deaths were reported if aluminium
levels remained high (Leivestad et al. 1987, cited
by Mason 1996). Also, whole-catchment liming is
reported to cause terrestrial vegetation to die, espe-
cially Sphagnum mosses (e.g. Hindar et al. 1998). The
growth of Sphagnum and Juncus bulbosus , which are
especially stimulated by elevated levels of both CO 2 and
NH 4 + (Roelofs et al. 1994, Lucassen et al. 1999), should
become limited on liming and sediment removal.
Liming leads to an increase in acid-neutralizing
capacity of water and stimulates decomposition of the
accumulated organic matter, and thus enables growth
of macrophytes ( Myriophyllum spicatum and Elodea
canadensis ) using bicarbonates as a carbon source. For
more sustainable results, the liming of a catchment,
or watershed, rather than a waterbody itself, might
prove more effective (Mason 1996). Nevertheless, the
best way to prevent lake acidification is to control the
source of acidification (Schindler 1997).

Liming: methods, strategies and ecological effects

Liming is by far the most common in-lake tech-
nique for restoring acidified lakes, for example in
Scandinavian countries, the UK (Scotland and Wales)
and eastern Canada (Wright 1985, Schindler 1997).
Limestone is the most commonly used compound.
In most treatments it includes calcite (CaCO 3 ) and the
powdered dolomite that contains a relatively high pro-
portion of magnesium carbonate (CaMg(CO 3 ) 2 ). Lime
dissolves slowly depending on the grain size so that
it produces longer-lasting buffering effects. In the field
it seldom raises pH above 7.0, even on excess addi-
tion. Other forms of lime used for de-acidification
of lake water are quicklime (CaO) and slaked lime
(Ca(OH) 2 ), in addition to alum ((Al 2 SO 4 ) 3 ·14H 2 O;
Dickson & Brodin 1995). The following equation
describes the dissolution of limestone (CaCO 3 ) in acid
aqueous solution.

H + Ca 2+ HCO 3 −

CaCO 3 + → + (pH<6)

Ca 2+ HCO 3 − OH − CaCO 3 + H 2 O → + + (pH>6)

Liming is a temporary remedial measure in anti-
cipation of more enduring reductions of acidifying com-
pounds. Moreover, continued inputs of acidic water
from the drainage basin during the liming process can
nullify the effects of liming.
Both Sweden and Norway have chosen large-scale
liming as a national strategy for preserving species
threatened by acidification (Henrikson & Brodin 1995,
Svensson et al. 1995). In Sweden, between 7500 and
11,000 km of streams are limed repeatedly every year
(Svensson et al. 1995) to raise the pH above 6.0 for
the natural fauna and flora to survive. Between 80
and 90% of the acidified surface waters have thus been
restored. Lime treatment of River Tovdalselva (1885
km 2 ) is perhaps the largest integrated liming project
in the world (Hindar et al. 1998). Watershed liming,
as attempted in Germany and the USA, is expected
to have more long-lasting beneficial effects but is a
costly option. In south-west Scotland successful
liming of Loch Fleet, Galloway, facilitated the intro-
duction of a self-sustaining trout population (Dalziel
et al. 1994).