ÐÓ°ÉÔ­´´

TWO CENTURIES of acid rain have taken their toll on the fresh waters
of many countries. Lakes and rivers have become acidic and the number of
fish that live in them has dwindled. In the early 1970s, some scientists
suggested that a good dose of lime would neutralise the acid and so solve
the problem. Since then, the practice of liming by organisations such as
angling clubs and water authorities and by landowners has become increasingly
common. At the same time, researchers have looked more closely at how liming
affects the freshwater biosystem and the habitats around it. They now realise
that liming is not the simple panacea it was once made out to be.

The source of the problem of acidified lakes, acid rain, is well documented.
In Britain, acid rain forms principally when sulphur dioxide and other emissions
from industry and vehicles dissolve in water in the atmosphere. The water
falls as acid rain, which in turn acidifies water on the ground. Acidified
waters have a low pH, a low concentration of calcium and a high concentration
of soluble aluminium leached from the soil. This combination is toxic to
fish. Acid water disrupts the mechanisms by which fish maintain their balance
of fluids, and they lose body salts, especially sodium. Newly hatched and
very young fish are extremely sensitive. Fish lay fewer eggs in acid water
and many of these eggs die. Aluminium in acid water damages the gills of
fish; they become covered in mucus, and the fish suffocate. Dissolving limestone
(calcium carbonate) in the water increases its pH and the concentration
of calcium, while lowering its concentration of aluminium at the same time.

Treatment usually involves dumping powdered limestone into a lake from
a lorry, boat or helicopter. The method is crude, and it is difficult to
calculate how much lime to apply and how often. The amount of lime needed,
and how often to apply it, depend on three things – the volume of water
in the lake, its chemistry and its turnover time (the time it takes for
all the water in the lake to be replenished from its natural sources). Some
lakes, including many Scottish lochs, have a turnover time of only a few
months. To have any noticeable effect, liming would have to be repeated
every fourth turnover, sometimes sooner, which simply is not practical.

The treatment has other drawbacks. For instance, adding lime to the
main body of a lake neutralises its outflow streams, but has no effect on
its inflow streams, which remain acidic. Inflow streams are often breeding
grounds for fish, and so despite treatments, the population of fish in the
lake eventually declines. Fish can also be poisoned by ‘pockets’ of acidic
water from inflow streams that have not mixed with otherwise limed water
in a lake. This happens in spring, when snow melts, and in summer, when
the water is stratified into layers at different temperatures which do not
mix. To overcome these problems, many people attempted to lime the inflow
streams.

This was not always successful. Limestone chippings and shells dumped
in streams to provide a slowly dissolving source of lime soon developed
a chemical crust and ceased to work. Streams need a constant supply of lime,
but the rate of flow of each stream varies, and with it acidity. The pH
falls dramatically after heavy rainfall, so the rate at which lime is added
has to vary with the flow, and this is technically very difficult – just
a few hours in acidified water is enough to kill fish. Many fish died in
the River Esk, in Cumbria, in June 1980 and September 1983, when the summer
storms that followed prolonged dry periods swelled the streams and reduced
their pH to below 5.

To solve some of these problems, researchers tried applying limestone
to catchments – land around lakes and rivers – rather than to the water
itself. The idea was that the soil would hold the lime and release it slowly
into the water, reducing acidity over many years and helping to prevent
aluminium from leaching from the soil into the water. But the price was
high: to achieve the same result, catchments need up to 100 times as much
lime as lake water. One way to reduce costs – and make the treatment more
effective – is to ‘target’ the lime on springs, flushes and bogs within
catchments. The dose of lime put on target areas is high, but by treating
only small areas within the catchment, the total amount of lime used is
reduced. Many organisations worldwide, including the Central Electricity
Generating Board (CEGB) and some water authorities in Britain, are now encouraging
‘target liming’ as a way of protecting fish.

Catchment liming has had some success. Following the CEGB’s experimental
liming of the catchment of Loch Fleet in Galloway, the water has become
safe enough to reintroduce fish. In Norway, water chemistry in Lake Tjonnstrond
was still acceptable five years after its catchment was limed, and scientists
there expect the water to remain good for many years. But the Tjonnstrond
catchment was in some ways ideal for liming, partly because of its thin
soils, and other trials have not been so encouraging. In 1988, David Brown,
then working at the CEGB Research Laboratories, reviewed the results of
recent experiments on catchment liming. He concluded that very few of these
have had a long-lasting, positive effect on the acidity of water. Only four
sites out of 39 throughout northern Europe and North America had water whose
pH stayed above 6 for longer than a year. Seventeen experiments resulted
in ‘some short-term positive effect’ on water quality and 18 had no effect
at all.

Researchers from the Catchment Research Group at the University of Wales
came up against a typical example of the problems of catchment liming at
Llyn Brianne in mid-Wales. They targeted water source areas, spreading between
15 and 25 tonnes of limestone per hectare over 15 per cent of the catchment
area. Although the pH of inflow streams rose immediately after treatment
from 5 to 6.5 and remained high for several months, it dropped back to about
5.5 during dry weather. During dry periods, most of the water feeding the
stream was bypassing the limed soil. Very heavy rain or a flush of melted
snow can also bypass lime, running straight over the surface of the catchment
rather than penetrating the soil.

Many areas affected by acid rain have sport fisheries worth millions
of pounds a year. According to one estimate in 1979, the loss of trout from
acidification in New York State costs $1 billion a year in lost tourism.
Understandably, the success or failure of liming is usually assessed in
terms of how it increases the survival of fish. But fish are only one part
of the freshwater ecosystem and there is evidence that liming might be damaging
other species. Acid rain damages streams and lakes only in areas where the
underlying bedrock weathers very slowly and does not contain lime. Water
in these areas is naturally slightly acidic and contains very little calcium
– it is sometimes called ‘soft’ water. Adding lime to such streams or lakes
when they have become acidified does not restore their natural chemical
balance. Rather, it overcompensates for the extra acidity, increasing the
pH and calcium concentration above natural levels. This changes the composition
of communities of plankton, invertebrates and plants.

The worry is that while acidification changes the character of lakes
and streams, liming changes it further. Steve Ormerod and his colleagues
at the University of Wales, Cardiff, confirmed this by looking at how liming
affects invertebrates. They compared the invertebrates in limed streams
with those in a computer model in which the deposition of acid was reduced
and the streams recovered naturally, without lime. As expected, the invertebrate
communities in both systems were different from those that live in acidified
streams. For example, the limed stream had more mayflies and fewer stoneflies.
But more importantly, the communities also differed from each other: the
invertebrates in streams that Ormerod treated with lime were typical of
those found in water of neutral pH, and not of soft water streams.

Catchment liming also has implications for the plants and animals that
live in the catchment. The plants and animals of many upland areas are adapted
to growing or living in soils which are naturally acidic and low in calcium.
One plant that suffers from an overdose of lime is the bog moss, Sphagnum,
which makes up a large part of moorland and peat bog vegetation. Bog moss
is important in the natural cycle of water through catchments: it acts like
a giant sponge, absorbing a huge volume of water and releasing it gradually,
so that it helps to even out the flow of streams and rivers. Destroying
this regulatory mechanism by liming can cause spates and flash-flooding
a long way downstream.

The evidence is that Sphagnum is extremely sensitive to calcium. At
Loch Fleet in Scotland, liming bleached and killed 90 per cent of the bog
moss in a treated area of 3.2 hectares around the lake. The moss still had
not recovered a year later. In another catchment at Bravattenbacken in Sweden,
all the bog moss in the limed area died within six years of treatment. When
Susan Mackenzie, of the University of Manchester, limed plots within the
Llyn Conwy catchment in North Wales – the source of the River Conwy – to
see how it would affect the vegetation there, Sphagnum bleached within a
few weeks, losing its growing tips and most of its physiological activity.
The treatment also killed the lichen Cladonia and damaged another moss,
Polytrichum. More than a year later the lime was still lying on the relatively
flat surface of the moor and Sphagnum showed little sign of recovery.

In flushes – where water emerges from the peat and flows over the surface
of the ground – bog moss did not fare so badly. It later recovered, so that
after a year only small patches of bleached moss remained. Ironically, the
moss survived because the liming treatment had not worked. The water had
washed away the lime relatively quickly and a year later the pH and calcium
content of water in both limed and unlimed flushes was the same. K K No
one knows what will happen once bog moss completely disappears – experiments
with liming catchments have not been running long enough to say. But bog
moss is the raw material of peat, and the consequences of peat erosion should
be considered before upland moors and bogs are limed. Upland bogs are built
on layer upon layer of ancient moss. Once the surface of living moss disappears,
it is highly likely that the surface of the peat will begin to erode. Researchers
at Loch Fleet are watching carefully for the first signs. Apart from anything
else, large quantities of eroded peat particles in streams and the build-up
of a sediment of eroded peat in lakes could harm the very fish that the
lime was supposed to protect.

Mackenzie also studied the effects on invertebrates in the Llyn Conwy
catchment by placing pitfall traps in limed and control plots. She trapped
fewer invertebrates in the limed plots than in the controls. Small, soil-dwelling
animals such as mites and springtails seemed worst hit, presumably because
lime persisted in the surface layer of peat. Liming had less effect on larger
flying insects. These insects spend more of their time near the tops of
plants, where rain washes away the lime. Mackenzie also found fewer small
mammals in her traps in limed areas. Shrews in particular avoided limed
areas, perhaps as the invertebrates they eat were in short supply.

All catchment liming affects plants and animals, but target liming may
be particularly harmful. The ‘wet’ areas within catchments – flushes, bogs
and springs – are particularly rich in wildlife . They support a wide variety
of plants and invertebrates, some of them rare, and are an important source
of food for birds. In target liming, it is these areas that are treated
selectively with a large amount of lime, a practice which could have far-reaching
effects on many species.

Trying to balance the costs and benefits of any method of liming is
a tricky exercise. Calculating the financial benefit of an improved sport
fishery – assuming it is achieved – and the cost of liming, is fairly straightforward.
It is much more difficult to assess the cost of ecological damage. Putting
a value on a species, whether it is a soil mite or an upland bird, is almost
impossible.

However much catchment liming may benefit fish, it almost inevitably
causes ecological damage. In some situations it may not be harmful, for
instance where agriculture has already greatly changed the ecological balance.
There are instances, when the object is to save a single species, such as
a rare fish, in which liming may benefit conservation – there are plans
to use lime at Loch Doon to protect the last population of Arctic charr
in southwest Scotland. But although liming can help a single species, it
is not appropriate for conserving whole habitats. In the end, liming only
treats the symptoms – the cure is to reduce emissions of acid at source.

* * *

Acid lakes and the mountains of lime

SOME 16,000 of Sweden’s 85,000 lakes and about 100,000 kilometres of
its rivers and streams have been acidified. The Swedes cannot combat their
acid rain by reducing emissions from power stations or industry – most of
it comes from abroad. Instead, they spend around Pounds sterling 14 million
a year on liming. Since 1976 they have treated over 4000 lakes. The Swedes
view liming as a ‘holding operation’ until other countries reduce their
acid emissions.

Lakes in Norway within an area of 33 000 square kilometres have decreased
fish stocks, and in 40 per cent of that area the lakes are nearly devoid
of fish. The Norwegians introduced liming in fish hatcheries as early as
the 1920s, but swift rivers and short lake turnover times make treatment
very difficult in most Norwegian waters. Liming all Norway’s acid waters
would cost about Pounds sterling 30 million annually; they spend about Pounds
sterling 1.3 million annually.

In Canada, acidification is too widespread for extensive liming, although
individual projects are under way. In Ontario, a combination of liming and
emission reductions has restored lakes that had been acidified by smelter
emissions. In Nova Scotia, where 12 rivers no longer support salmon breeding,
the Department of Fisheries and Oceans limes snow around rivers to counteract
acidification of the rivers when the snow melts.

In the US, liming projects are in progress in several areas, including
the Adirondack lakes in New York State and acidified streams in West Virginia.
As well as the State and Federal support for such projects, the electricity
generating industry funds many of them through the non-profit making organisation
‘Living Lakes Inc.’

A few lakes in Britain have been limed to protect fish. In 1985, the
Welsh Water Authority treated Llyn Hir, for example. Anglers have occasionally
tried to improve the fisheries of popular lakes such as Llyn Gamallt in
Gwynedd by adding lime. The North West Water Authority, the Welsh Water
Authority, and the Electricity Generating Boards together with British Coal
are studying the effects of catchment liming on the River Esk in Cumbria,
Llyn Brianne in Wales and Loch Fleet in Galloway, respectively.

* * *

Jewels in the catchment

VIVID emerald patches on heather-covered slopes; lines of rushes running
down hillsides; dense collections of flowering asphodels in an open bog;
these are just three examples of the vegetation of ‘wet patches’ within
upland catchments. Flushes, springs and bogs are characterised by luxuriant
growth of mosses, and in addition each has its own distinctive groups of
higher plants.

The balance of species in these wet patches depends on several things,
including water chemistry and especially pH. But it is water that gives
rise to vegetation which is diverse and different from that around. Flushes
may support as many as 26 species in a square metre, while the drier area
around may have only 15 species.

Wet areas, with their soft soils and diverse vegetation, provide a wide
range of niches for invertebrate species and so are richer in invertebrates
than the land around them. Ground and water beetles, spiders, craneflies,
dragonflies and shorebugs, including some rare species, live in the wettest
parts of catchments. The variety of nectar-producing flowers in these areas
provides vital refuelling points for many insects, and as wetter areas often
form depressions in the ground they provide shelter from the wind for flying
insects.

Britain’s uplands are home to a rich variety of birds, many of which
depend on wet areas for food. Red grouse, which are unique to the British
Isles, live on heather moorland. Before they lay eggs they prefer to feed
on the highly nutritious buds of cotton grass which grows in the wet, boggy
areas of the moor. Grouse chicks thrive on the many insects that live in
flushes and pools – Peter Hudson of the Game Conservancy found that Sphagnum
pools and flushes have 5 to 18 times as many insects, such as sawfly larvae
and click beetles, as the surrounding heather.

Derek Ratcliffe, until recently Chief ÐÓ°ÉÔ­´´ of the Nature Conservancy
Council, observed that golden plovers move their newly hatched chicks to
wetter areas of the moor. Further work by the NCC has shown that dunlin,
snipe, lapwing, golden plover and curlew all choose to nest on moorland
with small pools and flushes, where their chicks can find insect food more
easily. At high altitude, dotterel and ptarmigan often feed in flushes.

The bogs, springs and flushes within upland catchments are valuable
islands of biological diversity and resource. Widespread target liming could
selectively destroy them by killing vegetation, making conditions less favourable
for invertebrates and so reducing food supplies for birds.

Sarah Woodin is the Atmospheric Pollution Specialist of the Nature Conservancy
Council. Ute Skiba also works for the Nature Conservancy Council. She is
researching freshwater acidification on statutory conservation sites.