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The Control of
Blanket Weed Algae in Ponds |
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by Jack Cox |
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Green
Algae are primitive aquatic plants that differ from
other plants in that they have no true stems, leaves
or roots. They have a place in the overall food
chain as they are able to photosynthesise, converting the energy of the sun into
food for other
aquatic life. In the presence of sunlight, they
produce oxygen, thus helping to increase the level
of dissolved
oxygen in the water. However, like all green plants,
they use oxygen at night. Oxygen is also taken
out of the water when they die and decay. |
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There are over 20,000 different
species of green algae in the world but the one we
are concerned with here is Blanket weed (Cladophora glomerata).
This is the one which causes most problems in
freshwater systems around the world although there
are actually over 150 different species of
Cladophora. |
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In natural ecosystems blanketweed has
its place and knows its place. However it
loves Nitrate and Phosphate and when these nutrients
are present in the water in excessive
concentrations, blanketweed takes advantage of it
and just grows and grows. Phosphate is really the
limiting factor for plant growth, rather than
Nitrate. Of course your fish also require a certain
amount of phosphate, so their formulated feeds
contain quite a lot of it and any excess is released
into the water to feed your algae. |
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Remember that food enters the fish at
one end and something else comes out of the other
end. It then decomposes in the pond releasing
ammonia which is broken down to
nitrite and then to nitrate through the activity of
bacteria. Both nitrite and nitrate are good
sources of nitrogen for plants including algae. |
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As a responsible finca owner I am
sure you manage your land organically but maybe your
neighbours do not. In wet weather fertiliser
can run off your neighbour's land onto yours or seep
into the ground water. From there it easily
gets into your pond or lake, especially if you fill
it from a well. Runoff from animal pens and
horse stables can also get into your pond. |
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Blanket Weed is a type of Filamentous Algae.
It forms as thread-like structures that can
be floating or attached to submerged items such as
the pond walls or aquatic plants. The common name
“blanket weed" is an excellent description as it can
quickly form a thick blanket over the surface of
your pond, shutting out sunlight. In very high light
levels, such as here in Spain, and with high levels
of nutrients, blanket weed can grow at over 2 metres
a day. |
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Other similar algae which may cause problems in
ponds include species of Spirogyra, Rhizoclonium and
Enteromorpha, indeed there are over 400
different species of the genus Spirogyra
around the world. |
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Spirogyra tends to appear in
ponds as a tangled pond scum. It is also called
“water silk”, “silk weed” and “mermaid tresses”. On
sunny days, the mats of spirogyra filaments usually
float on the surface of the water. They are kept
afloat by tiny bubbles of oxygen which the plant
gives off during
photosynthesis. |
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These algal mats then sink when the
sun goes down and the process reverses as
photosynthesis stops for the night. As a result, the
strands of Spirogyra consume oxygen for cellular
respiration. Carbon dioxide is then produced as a
waste product. Where there are such thick algal
mats, large fluctuations in the dissolved carbon
dioxide and oxygen levels in the water can occur.
This can lead to rapid changes in the pH of the
water that in turn can cause stress and even death
to other organisms such as fish. |
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Enteromorpha intestinalis is another form of blanket
weed that occurs in ponds. It is tubular in form and
seldom grows longer than about 40cm. There are over
50 different species of the genus Enteromorpha.
And lets not forget the 20 or more species of the filamentous genus
Rhizoclonium. |
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So there are a lot of different
potential algal blooms which cause blanket, 'pea soup' or
floating scum conditions. As well as looking
unsightly, excessive algal growth, encouraged by
surplus nutrients, can lead to de-oxygenation of
water with a consequent detrimental effect on
aquatic life. Of course the presence of algae in ponds need not necessarily
cause a problem but in most cases it does sooner or
later and its probably a good idea to remove it
periodically. |
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As with most things, prevention is
better than cure. Here are some useful guidelines: |
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Depth of water:
Ensure a minimum pool depth of 75cm as shallow pools
warm more rapidly, especially under the hot Spanish
sun. You can use black liners to minimise heat
reflecting into the water. |
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Minimise nutrients: Remove any
soil from the pool before filling it, and only
immerse plants in containers of aquatic compost,
surfaced with shingle. Apply fertilisers to nearby
lawns and vegetable beds carefully, as run-off can
occur after heavy rain. |
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Barley straw: Submerge mesh bags of barley straw or lavender clippings just
below the water's surface in early spring. Replace them when they turn black. The straw
encourages phytoplankton grazing organisms. Also as the barley straw rots, through the action of
bacteria and fungi, it gives off certain phenolic phytotoxic compounds, chemicals which prevents the
algae from growing. The bacteria and fungi which produce these special chemicals will only thrive in
very well aerated water, so the straw needs to be placed near a water fall, or close to the surface of
the pond. If the water is not well aerated, the straw simply decomposes, adding further pollution to
the pond without the release of this phytotoxic chemicals. Where barley straw is used correctly, it
can be very effective. Roughly 50g of straw per cubic metre of water seems about right, depending on the severity of the problem,
although it can be a bit hit and miss. You can also get liquid extract of barley straw to make dosing
much simpler. |
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Plant up the pond: It can help
to add plants to the pond to compete with the algae
for available nutrients. Water lilies are most
peoples' favourites. Watercress (Nasturtium
officinale or N. microphyllum) is also
popular and will readily compete well for available
nutrients. However, this is also quite an
invasive plant and needs to be controlled by regular
pruning to prevent it from also becoming a nuisance.
Make sure that at least one third of the pool's
surface area is covered by the leaves of oxygenating
plants or other aquatic plants. Avoid over-stocking
with fish, as they produce nutrient-rich waste. |
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Leaves and debris: Cover the
pond with netting in autumn to prevent leaves
falling in as they are a source of nutrients. |
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Changing the water: Avoid
changing the water as this upsets the natural
ecological balance. Remove blanket weed with a rake;
duckweed can be skimmed off or removed using a net.
Remove sunken debris every few years. Clean ponds in
early summer, after aquatic life has bred. Divide
aquatic plants at this time. |
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Pond filters:
Pond filters can be installed to clean the water and
remove algae and nutrients through mechanical and /
or biological methods. There are several
different kinds available. |
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UV Light: Planktonic algae
(but not blanketweed) can be controlled in ponds by
installing ultra violet lighting. Ultra-violet is a
wavelength of light which has unique properties, we
are very conscious of its strength as it tans our
skin. Ultra-violet light damages the algae cells
which prevents them from converting energy from
sunlight into sugars and in the absence of a food
resource, they die. The light unit is left to run
continually and once the algae have died it will
keep the water clear, by preventing further blooms.
The ultra-violet light needs to have an electrical
source and to be connected to a pump in order to
circulate the water through the unit. There is a
range of filter units which have ultra-violet units
fitted as an integral part of the system but there
is also a wide range of manufactured lights which
can be installed separately. But this article is not about Planktonic algae
and UV lights have no effect on filamentous or macrophytic algae
so finca owners must find other control methods. |
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Ultrasound:
Trials at the
Centre for Aquatic Plant Management found
promising results in the use of ultrasound to
control pond algae. A device left permanently in the
water emits ultrasound waves and works by shaking
apart the algae cell wall, leading to algae death
within 7-21 days. Currently it appears to be most
effective on blanket weed. There appears to be no
adverse effect on pond wildlife but more research is
being conducted in this area. The equipment is not
cheap to buy, but may be worth considering for
larger ponds or hired for shorter periods where
other methods of control have failed. |
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Crayfish:
Crayfish will eat blanket weed as well as your other
plants and marginals. However throughout Europe
there are very stringent regulations governing the
introduction of any crayfish to any open water,
including garden ponds. There are very good
reasons for these new rules. There has been a
huge problem caused by the introduction of exotic
species of crayfish, which are rapidly taking over
the habitat of our three native European species (Astacus
astacus , A. leptodactylus and A.
pachypus). |
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Furthermore our
native crayfish are being decimated by 'crayfish
plague' (Aphanomyces astaci), a water mould
brought over by ships and also carried by the invading species especially the
Signal Crayfish (Pacifastacus leniusculus).
Once infected our European Astacus crayfish
die within a few weeks. When experimentally tested,
species from Australia, New Guinea and Japan were
also found susceptible to the infection. It is
believed that all species of crayfish world wide are
susceptible to one degree or another. |
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The mould first arrived in
Italy in 1860 with ballast waters from a North
American ship and quickly spread through Europe.
From there it spread quickly through Europe and was
discovered in Sweden in 1907, in Norway in 1971,
here in Spain in 1972, in the United Kingdom in
1981, in Turkey in 1984 and in Ireland in 1987. |
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This American
mould has now wiped out
large populations of our native European Crayfish. Unfortunately in the 1950s and the 1960s,
the Swedes tried to find a replacement crayfish that
could resist the mould. They decided on the Signal crayfish (Pacifastacus
leniusculus. As this is an American species
indigenous to the western United States it does have a
high level of immunity to the plague. But the
Signal Crayfish is also a carrier of the plague.
Efforts to reintroduce the original European Crayfish have been quite unsuccessful because of
subsequent large importations of signal crayfish,
most of them done illegally on private initiative.
So please do not be tempted to put exotic crayfish
into your pond in an attempt to control blanketweed.
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Cures for Blanket Weed:
When prevention fails there are several cures on the
market, physical, chemical and
microbial. If you have been reading my other
articles you will be in no doubt about what I think
about chemical methods and you will not be surprised
when I suggest you try some natural methods first. |
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It is possible to
physically remove blanket weed from the pond and
there is an assortment of
tools available to help twist the blanket weed and
lift it out of the water. Where there is only a
little blanket weed growth, these tools are quite
useful in controlling this algae. |
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Although I have not tried them yet,
one reader has suggested the use of Viresco™ microbial
products. Apparently these reduce the
phosphate and completely remove the nitrate from the
water and the blanketweed just starves to death. If
you want to give it a go, there website is:
http://www.viresco-uk.com . Another good
eco friendly product is Hydra POND CLEAR™ from
http://www.pondclear.eu/ |
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Please do
write in and let us all know how you get or, or
indeed to tell us of any other methods you have
tried. |
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One Parting
Thought: How about considering Blanketweed
as a useful resource, a crop to be grown and
harvested. It is high in plant nutrients, rots
down quickly on the compost heap, and makes a great
weed suppressing mulch to use between your rows of
vegetables. And best of all ITS FREE !!! |
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