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Approved-By: Jacky Foo <[EMAIL PROTECTED]>
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Date: Thu, 8 Jul 1999 15:30:21 +0200
Reply-To: "IBSnet: General Forum on Integrated Bio-Systems"
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From: Jacky Foo <[EMAIL PROTECTED]>
Subject: [IBS-GEN] straw to control algal booms
To: [EMAIL PROTECTED]
------forwarded
Frm: Jacky Foo <[EMAIL PROTECTED]>
Questions on the use of straw to control algal booms have been asked from
time to time and here is a response with concrete information. I also hope
that the information will stimulate some discussion.
regards
jacky
----------------------------------------------------------------------
Source: [EMAIL PROTECTED]
>Has any one here ever heard of using barley straw as a
>means to decrease algal growth in ponds or small
>lakes? The lakes in question are in central Ontario.
The Centre for Aquatic Plant Management (previously the Aquatic Weeds
Research Unit) publishes an information sheet on the practical application
of straw. I am posting the text of it below. If anyone wants the version
with pictures please email me and I can send you a WordPerfect 6 version of
the file (or any other format you want). We have been involved in the
research on straw for about 15 years now and can recommend as a good
treatment. There are still some unknowns about dose rate and why it doesn't
work in all ponds, but in most cases it gives good, long-lasting control.
We have also published scientific papers on the process which I can also
send out by post if anyone is interested.
Dr Jonathan Newman
Senior Research Scientist
IACR Centre for Aquatic Plant Management
Broadmoor Lane, Sonning,
Reading, Berkshire, RG4 0TH, UK
[EMAIL PROTECTED]
Tel: +44 (0) 118 969 0072
Fax: +44 (0) 118 944 1730
+++++
IACR-CENTRE FOR AQUATIC PLANT MANAGEMENT INFORMATION SHEET 3: CONTROL OF
ALGAE WITH STRAW INTRODUCTION
Algae cause a number of problems in water. They impede flow in drainage
systems, block pumps and sluices, interfere with navigation, fishing and
other forms of recreation,cause taint and odour problems in potable waters,
block filters and, in some instances, create a health hazard to humans,
livestock and wildlife. These problems seem to be increasing, probably
because nutrient levels in water are rising as a result of human activity
and natural processes. At the same time there is a growing worldwide demand
for improvement in water quality. Thus, the need to control algae is
increasing for environmental, recreational and public health reasons.
Because of their small size and rapid growth rates, algae are difficult to
control by methods used for other aquatic plants. Cutting and other forms
of mechanical control can help to reduce problems with filamentous algae
but are of very limited use. Many algae are susceptible to appropriate
herbicides but this approach is unpopular in some waters on environmental
and public health grounds. Furthermore, herbicides which control algae also
kill higher plants so that, although the water is cleared temporarily of
all plants, once the herbicide has gone from the water, the regrowth of
algae is not restricted by competition from the higher plants and the
problem can get worse in subsequent years.
A new method of controlling algae has been developed by the Centre for
Aquatic Plant Management which overcomes many of these problems. This
involves the application of barley straw to water and has been tested in a
wide range of situations and in many countries throughout the world and has
proved to be very successful in most situations with no known undesirable
side-effects. It offers a cheap and environmentally acceptable way of
controlling algae in water bodies ranging from garden ponds to large
reservoirs, streams, rivers and lakes.
Despite the simplicity of the idea, experience has shown that there are a
number of basic rules which must be followed to ensure that the straw works
successfully. The purpose of this leaflet is to provide practical advice on
the optimum ways of using straw.
HOW STRAW WORKS
In order to use straw effectively, it is necessary to understand something
of how the process works. When barley straw is put into water, it starts to
rot and during this process a chemical is released which inhibits the
growth of algae. Rotting is a microbial process and is temperature
dependent, being faster in summer than in winter. As a rough guide, it may
take 6-8 weeks for straw to become active when water temperatures are below
10oC but only 1-2 weeks when the water is above 20oC. During this period,
algal growth will continue unchecked. Once the straw has started to release
the chemical it will remain active until it has almost completely
decomposed. The duration of this period varies with the temperature and the
form in which the straw is applied and this will be discussed in more
detail later.
However, as a generalisation, straw is likely to remain active for
approximately six months, after which its activity gradually decreases.
Although the exact mechanism by which straw controls algae has not been
fully proven we believe that the process may occur as follows. When straw
rots, chemicals in the cell walls decompose at different rates. Lignins are
very persistent and are likely to remain and be released into the water as
the other components decay. If there is plenty of oxygen available in the
water, lignins can be oxidised to humic acids and other humic substances.
These humic substances occur naturally in many waters and it has been shown
that, when sunlight shines onto water which contains dissolved oxygen, in
the presence of humic substances, hydrogen peroxide is formed. Low levels
of peroxide are known to inhibit the growth of algae and experiments have
shown that sustained low concentrations of hydrogen peroxide can have a
very similar effect on algae to that of straw. Peroxides are very reactive
molecules and will only last in water for a short time. However, when humic
substances are present, peroxides will be continuously generated whenever
there is sufficient sunlight. The slow decomposition of the straw ensures
that humic substances are always present to catalyse this reaction.
There are various factors which affect the performance of straw and which
support this hypothesis. It is important to take these factors into account
to ensure successful treatment of algal problems with straw.
1: Type of Straw
Barley straw works more effectively and for longer periods than wheat or
other straws and should always be used in preference to other straws. If
barley is unavailable, other straws, including wheat, linseed, oil seed
rape, lavender stalks and maize can be used as a substitute. The
information in this leaflet describes the use of barley straw. If other
straws are used, it is likely that the quantities applied and frequency of
application may have to be increased.
We have tested a range of barley straw varieties, including some grown
organically; all these were active at the same level. Hay and green plant
materials should not be used because they can release nutrients which may
increase algal growth. Also they rot very rapidly and may cause
deoxygenation of the water.
2: The anti-algal chemical
The chemical released by the straw does not kill algal cells already
present but it prevents the growth of new algal cells. Thus algae which die
will not be replaced when the straw is present and so the algal problem is
controlled.
3: Speed of effect
Once the straw has become active, the time taken for control to become
effective varies with the type of alga. Small, unicellular species which
make the water appear green and turbid, usually disappear within 6-8 weeks
of straw application. The larger filamentous algae, often known as blanket
weeds, can survive for longer periods and may not be controlled adequately
in the first season if the straw is added too late in the growing season
when algal growth is dense. It is, therefore, preferable to add the straw
very early in the spring before algal growth starts.
4: Production of the anti-algal activity
Activity is only produced if the straw is rotting under well oxygenated
conditions. Usually, there is adequate dissolved oxygen in water to ensure
that the chemical is produced by the straw. However, if the straw is
applied in large compact masses such as bales, or to very sheltered and
isolated areas of water, there will be insufficient water movement through
the straw, which will progressively become anaerobic (without oxygen).
Under these conditions, only the surface layers of the straw will produce
the chemical and so the majority of the straw will have no useful effect.
5: Absorption and inactivation of the chemical
The chemical is very quickly absorbed by algae and is inactivated by mud.
Therefore, in waters which have high algal populations and are turbid with
suspended mud, it is necessary to add more straw than in clear waters.
6: Selective effect on algae
The chemical does not appear to have any effect on higher plants. In our
experiments, we have seen that the suppression of dense algal growth has
allowed flowering plants (macrophytes) to recolonise waters which were
previously dominated by algae. In several shallow lakes where straw was
used, algae were replaced by higher plants which suppressed the subsequent
growth of algae, so eliminating the need for further straw treatments.
7: Effects on invertebrate animals and fish
There are no reports of harmful effects on invertebrates or fish except in
a few instances where excessive amounts of straw were applied to small
ponds and the water became deoxygenated. These excessive doses were at
least 100 times the doses recommended in this leaflet. In most instances,
invertebrate populations increase substantially around the straw so
providing a useful food source for fish. There is anecdotal evidence that,
in fish farms and fisheries, straw treatments may be associated with
improved gill function and fish health and vigour.
HOW MUCH STRAW TO APPLY?
In ponds, lakes and other still water bodies. We have found that the most
important measurement in calculating the quantity of straw required is the
surface area of the water. Surprisingly, the volume of the water does not
appear to affect the performance of the straw as might be expected. This
may be because the majority of algal growth takes place in the surface
layers of the water and so it is not necessary to measure the depth of the
water or volume of the lake when calculating the quantity of straw
required.
In still waters such as lakes, ponds and reservoirs, the minimum quantity
of straw needed to control algae is about 10g straw m-2 of water surface.
However, when a water body with a history of severe algal problems is first
treated, a higher dose is preferable (25 g m-2) and quantities up to 100 g
m-2 have been used. Once the algal problem has been controlled, and further
additions of straw are being made to prevent a recurrence of the problem,
the dose can be reduced.
In turbid or muddy waters, it will always be necessary to add more straw
than in clear, mud-free waters. It is clear from numerous trials in
different types of water body that the quantity of straw needed can vary
considerably and it is better to apply too much initially and then to
reduce the quantity gradually each time straw is added until the dose has
been reduced to 10g m-2 or until algal growth starts to increase again when
the dose should be increased to a previously effective level.
There is a theoretical level at which straw could cause problems by
deoxygenating the water. This is caused by the microorganisms which
colonise the straw and absorb oxygen from the water and by chemical oxygen
demand of the rotting process. However, straw decomposes slowly and the
oxygen demand of these microorganisms is unlikely to cause any problems
unless excessive amounts of straw (more than 100g m-2) are applied.
Deoxygenation can occur as the result of natural processes especially in
prolonged hot weather when the solubility of oxygen in water is reduced and
biological oxygen demand increased. This deoxygenation is often caused by
algal blooms and so the presence of straw, which prevents the formation of
these blooms, can reduce the risk of deoxygenation. However, straw should
not be applied during prolonged periods of hot weather to waters containing
dense algal blooms as the combined oxygen demand from the algal bloom and
the straw could temporarily increase the risk of deoxygenation.
In flowing waters such as streams and rivers. We do not yet have sufficient
information on the properties of straw to express a quantity of straw
required in relation to the surface area or volume of water flowing down
the stream. However, straw has been used effectively in these situations by
placing quantities of straw at intervals along either bank of the river.
The distance between straw masses has usually been between 30-50m and the
size of each straw mass was chosen, for convenience, as about one bale
(20kg).
The risk of causing deoxygenation in flowing waters is very small as the
continuous supply of fresh oxygenated water will prevent any local
deoxygenation around the straw.
HOW TO APPLY STRAW
The best way of applying straw varies with the size and type of water body.
Suggestions as to the most appropriate methods for different types of water
body are given below.
Fast flowing rivers and streams. Straw can be applied in the form of bales
because the flow of water will keep oxygen levels high enough to prevent
the straw from becoming anaerobic. Only small bales (approximately 20kg)
should be used. Bales can break up under the forces produced by fast
flowing water and they should normally be wrapped with netting or chicken
wire and securely anchored to the bank or posts driven into the river bed.
Another way of applying straw which has worked effectively in flowing water
is to place the straw in gabions. These are wire mesh boxes (usually filled
with stone for bank protection) but they work equally effectively as cages
for straw. They have the additional advantage that they can be refilled as
the straw rots away. Nets and loose woven sacks (e.g. Onion sacks) filled
with straw can also be used. In all instances, it is essential to ensure
that the straw container is well anchored to the bank or to stakes in the
bottom which will hold it in place during periods of high flow.
Slow flowing rivers. Straw should be applied in a loose form, either in
gabions or as straw sausages. This increases the diffusion of oxygen to the
site of decomposition and speeds up the process in this type of
environment.
Ponds, lakes and reservoirs. In still or very slow flowing water, bales
should not normally be used as they are too tightly packed and do not allow
adequate water movement through the straw. It is preferable to apply the
straw in a loose form retained in some form of netting or cage.
In small garden ponds where only a few grams of straw are needed, the straw
can be put into a net bag, nylon stocking or simply tied into a bundle with
string. This can be attached to an anchor made of a stone or brick and
dropped into the pond. However, as the straw becomes waterlogged, the net
will gradually sink to the bottom. In this position, it will not work as
effectively as it does near the surface and it is advisable to include some
form of float in the net. Floats can be made of corks, polystyrene or small
plastic bottles with well-fitting screw tops. Once the straw has rotted,
the net, complete with float and anchor can be removed and used again.
Some garden centres supply small packets of straw for use in ponds. They
will work best if anchored and attached to a float as described above.
In larger ponds, lakes and reservoirs, where larger quantities of straw are
needed, bales should be broken up on the bank and the loose straw wrapped
in some form of netting or wire. One of the simpler ways of wrapping large
quantities of loose straw is to use one of the various forms of tubular
netting normally sold for wrapping Christmas trees, constructing onion
sacks and for other agricultural purposes.
When used in conjunction with a tree wrapping machine they can be used to
construct straw sausages which can be made up to about 20m long and contain
some 50kg of straw. The length and size of each sausage is determined by
the size and shape of the water body (described later). It is advisable to
incorporate some floats within the netting to keep the straw near the
surface when it becomes waterlogged. When first constructed, these sausages
float well and can be towed behind a boat to the required position and
anchored by rope to concrete blocks or sacks of gravel. It is preferable to
anchor these straw sausages at only one end so that they can swing round to
offer minimum resistance to wind or currents. Straw sausages can interfere
with angling and boat traffic and their positioning needs to be carefully
considered so as to have the minimum adverse effect on water users. Floats
or buoys can be attached as markers to warn boat traffic or anglers of the
position of the straw.
WHERE TO APPLY STRAW
It is always preferable to apply several small quantities of straw to a
water body rather than one large one. This improves the distribution of the
active factors throughout the water body. Straw works best if it is held
near to the surface where water movement is greatest. This keeps the straw
well oxygenated and helps to distribute the anti-algal chemical. In
addition this ensures that the chemical is produced close to where the
majority of the algae are growing and away from the bottom mud which will
inactivate the chemical. The following aspects should be considered when
deciding where to place the straw withina water body.
Small ponds In small ponds where only a single net of straw is required,
this should be placed in the centre of the pond. However, if there is an
incoming flow of water, either as a stream or fountain, the straw net
should be placed where there is a continuous flow of water over and through
the straw. This will help to keep the straw oxygenated and spread the
chemical throughout the pond.
Lakes and Reservoirs In any body of still water, it can be assumed that the
anti-algal chemical will diffuse outwards in all directions from each net
of straw gradually being absorbed by algae and inactivated by mud until the
concentration becomes too low to be effective. Beyond this distance, algal
growth will continue unchecked and these algae will gradually drift back
into the treated areas giving the impression that the straw is not working.
In order to ensure that there are no areas within the water body unaffected
by the straw, it is necessary to calculate how much straw is needed, how
many nets should be employed and how far apart each net should be. Nets or
sausages of straw should then be placed so that each net is roughly
equidistant from its neighbours and from the bank. The steps involved in
this calculation are explained overleaf with an example:
In rivers and streams From the point of view of getting maximum benefit
from straw, it would be preferable to place the straw as a barrier across
the flow of water. However, this is seldom possible because the force of
the water would tend to wash the straw away and the straw would impede
water movement and boat traffic. Therefore, bales, straw nets or gabions
should either be placed opposite each other in pairs or alternately along
both banks. In fast flowing streams where there is little mud to absorb the
chemical, the space between straw nets can be as much as 100 m (50 m if
placed alternately) but in slow-flowing muddy watercourses, this space
should be reduced to no more than 30 m. In very narrow streams, it may be
necessary to place the straw close to the bank so as not to impede flow but
in larger watercourses the straw should be as far out from the bank as
possible. This makes it less subject to vandalism and damage from livestock
and ensures that there is a good flow of water around and through the
straw.
Always ensure that the straw is well secured to the bank or to stakes in
the bottom so that it does not get washed away during floods. It is usually
necessary to consult the local water authority before applying straw to
flowing water because they have the responsibility of ensuring that there
is no danger to water supplies or other riparian users caused by partial
obstruction to the flow.
Marine situations There has been very little research with straw in
seawater and any treatments in these conditions should be regarded as
experimental. Results from a very limited number of trials in salt water
lagoons and artificial pools suggest that straw can work in salt as well as
fresh water. However, it is very unlikely that it would have any effect on
the large marine algae, normally found on rocky shores or on kelp beds in
the seas because of the problems of short persistence time and exposure. It
is alsounlikely that sufficient straw could be placed and held for long
periods in the open sea.
Table 1. Method for estimating amount of straw required.
1 Estimate the surface area of the lake e.g. 1.5ha (15,000 m2)
2 Decide on the dose rate of straw required. This will range from 10g/m2 in
a clear lake with little algae or mud to 50 g/m2 in a heavily infested lake
with muddy water e.g. 25 g/m2
3 Multiply the area of the lake (in m2) by the quantity of straw required
per m2 to obtain the total quantity required e.g 15,000 x 25 = 375,000 g
375,000?1000 = 375kg
4 To obtain the number of bales to be purchased, divide the total weight of
straw by the weight of bales (small rectangular bales normally weigh about
20kg). Weights should be checked on other sizes and shapes of bales. e.g.
375?20 = 19 bales
5 Decide on the weight of straw to be placed in each net. (Bear in mind
that the smaller the quantity in each net, the more nets there are and so
the better the distribution of the chemical. Against this, the time and
labour to construct the nets and the interference that they may cause to
the lake functions will limit the numbers). Nets should normally contain
between 1kg (in small lakes) to 40kg (in very large lakes). e.g 25 kg
6 Calculate the number of nets which will have to be constructed. Divide
the total quantity of straw required (3) by the weight in each net (5).
e.g. 375 ?25 = 15 nets
7 Calculate the area of water which will be treated by each net at the dose
rate decided in 2 (above). e.g. 25kg?25g/m2 = 1,000 m2
8 Calculate the radius of a circle with an area of the size calculated in 6
(above) using r2. e.g. r2 = 1,000 r = 1,000?3.142 r = 17.85m
9 The diameter of a circle of 1,000 m2 is = 35.7 m
10 Decide on the most appropriate placement of the nets of straw in the
lake so that each one is approximately 35m from its neighbour and 17m from
the bank. e.g. Usually a regular square grid pattern with centres at 35 m
The spacing of nets does not need to be exact. Practical considerations may
influence the number of nets and their local placement. For example, it may
be necessary to leave a wider corridor between some sets of nets to allow
for adequate boat passage or angling purposes. Where possible any enlarged
gaps between straw nets should be compensated for by decreasing the gaps
between adjacent nets. If there are any inflowing streams, it is advisable
to increase the number of net near the inlet to that water flows through
the straw and distributes the chemical into the lake. It is possible to
compensate for this local concentration of straw nets near the inlet by
reducing the numbers of nets near any outlet as the chemical released from
these may be washed out of the lake.
In an irregularly shaped water body, the preferred place for some of the
nets is opposite any promontories or points where the nets will be exposed
to maximum wind and wave action. The remainder should be spaced between
these, using the method of calculating the gaps shown above.
WHEN TO APPLY STRAW
Although straw can be applied at any time of year, it is much more
effective if applied before algal growth takes place. This is because the
anti-algal agents released by the straw are more effective in preventing
algal growth than in killing algae already present. Therefore, straw is
best applied in the autumn, winter or very early spring when the water
temperature is low. The straw will usually become active within one month
and will continue to inhibit algal growth for about 6 months. However,
rapid algal growth can take place once the straw has rotted away and so
further applications should be made each 6 months.
It is important to note that the rate at which straw rots varies
considerably and regular observations should be kept on the straw so that
fresh straw can be added before the end of the 6 month period if necessary.
It is not always possible to predict that an algal problem will occur and
so it is sometimes necessary to treat an algal problem which has already
developed. Some algae, mainly the small unicellular species and the
cyanobacteria (blue-green algae), can be controlled by adding straw to
existing blooms.
The time taken for the algae to be controlled depends on a number of
factors, of which water temperature is probably the most important. At
water temperatures above 20oC straw has been effective in controlling algal
blooms within 4-5 weeks, sometimes even faster. Avoid applying straw during
prolonged periods of hot weather as the combined effect of the dying algae
and the rotting straw may increase the risk of deoxygenation. At lower
temperatures, the process is slower and it may take 8 - 10 weeks to control
the algae but the risk of deoxygenation is then minimal.
When filamentous algae are the main problem, straw applied to dense
floating mats will have very little useful effect unless combined with
other treatments which will be described later. After the initial straw
treatment, further additions will be required to prevent the return of the
algae. Although a period of 6 months is suggested as the likely interval
between straw applications, more frequent treatments may be necessary. It
is inadvisable to wait until all the straw has rotted before making a
second application as there will then be an interval when no chemical is
being produced and rapid algal growth can take place. For the same reason,
the old straw should not be removed for at least one month after the
addition of the new straw. This allows time for the new straw to become
active.
THE USE OF STRAW IN COMBINATION WITH OTHER CONTROL METHODS
Filamentous algae are not easily controlled by straw once they have formed
floating mats. However, they can be controlled by other methods. In some
situations, filamentous algae can be raked out. However, many fragments
will remain in the water and rapid regrowth is likely. To prevent this
straw should be added about one month before the alga is raked out.
In other situations, herbicides (diquat or terbutryn) have been used in
combination with straw. The herbicides control the algae but their effects
may not persist for long once the herbicide has decayed or been otherwise
dissipated from the water. By adding straw at the same time, or soon after
the herbicide has been applied and maintaining a straw treatment regime as
outlined above, the straw helps to prevent the return of the algae.
OTHER EFFECTS OF STRAW
During the numerous field trials in which straw has been applied in a
number of forms and in a range of water bodies, various effects in
additional to the control of algae have been noted. While these have not
been investigated in any detail, they have occurred sufficiently frequently
to be worth noting as possible consequences of using straw.
1 Effects on other aquatic plants. No direct effect of straw on aquatic
vascular plants has been found in either laboratory or field experiments.
However, in several trials where straw has successfully controlled algae,
there has been a noticeable increase in the growth of submerged vascular
plants. It is likely that this is a result of the loss of competition from
the algae which has allowed the vascular plants to recolonise water in
which previously they were unable to compete with the algae. In some
instances, the recovery of the vascular plants has been so marked that
they, in turn, caused problems to water users and also required some form
of management. However, they are generally easier to control and less
troublesome than the algae and so are more acceptable in most waters. In
some instances the recovery of the vascular plants has been so strong that
they replaced the algal growth as the dominant plant form so that
subsequent treatment with straw was no longer needed.
2 Effects on invertebrates It has been observed frequently that loose
masses of well oxygenated straw provide a good habitat for some of the
aquatic invertebrate animals such as the Water Shrimp (Gammarus spp.).
These invertebrates, mostly detritus eaters, breed and grow rapidly in the
safe environment created by the straw and their numbers can increase by
several orders of magnitude within a few months. As the straw gradually
rots away and the numbers of invertebrates increases, individuals leave the
safety of the straw and become prey to fish and waterfowl. Invertebrate
animals are generally beneficial to water bodies as they help to decompose
organic matter in the bottom; some of them graze on algae and aquatic
plants and they form an important part of the food chain.
3 Effects on fish and waterfowl There have been a number of observations of
improved growth, vigour and health of fish in waters treated by straw. One
reason for this is likely to be the increased food supply in the form of
invertebrate animals. Fish may also find it easy to find food in water
which is not densely colonised by unicellular or filamentous algae.
However, another possible explanation is that, by controlling the algae,
the straw allows better lightpenetration to occur to deeper levels in the
water so that photosynthesis can occur in a greater volume of the water
body and so provide an improved environment for the fish. It has also been
noted by the Game Conservancy that young ducklings require a diet which
consists mainly of invertebrate animals. They found that adding straw to
gravel pits significantly increased the survival of young ducklings.
In a number of water bodies, ducks and other waterfowl have been observed
to nest and roost on floating masses of straw. This has been particularly
beneficial to these birds in waters subject to high levels of human
interference and terrestrial predators as the floating straw masses are
usually inaccessible from the bank.
There have been a number of anecdotal reports that incidents of some fish
diseases and parasites appear to have been reduced in fisheries and fish
farms in which straw has been used.
4 Effects of straw in flowing waters When straw has been applied in flowing
waters, either in the form of bales or in gabions, it has been noted that
water is deflected around the straw and the accelerated flow caused silt
and fine gravel to be washed away from the vicinity of the straw. In a
small stream which had a very uniform depth, pairs of gabions containing
straw were placed opposite each other and angled downstream so as to create
a rapid flow between them. This caused the gravelly bed of the stream to
wash out and so scour holes were formed. These were immediately colonised
by trout which were the dominant fish species. The overall effect created
by three pairs of gabions placed at approximately 100 m intervals was to
create a pool and riffle environment which is usually considered to be
preferable to a uniform channel for fish and aquatic life generally. In
small streams, it is likely that careful placement of straw bales or
gabions could be used to manipulate the location of silt deposits ensuring
that an open channel is maintained and that silt beds are allowed to
develop only in acceptable locations.
SUMMARY
1 When algal problems occur in water bodies ranging from garden ponds to
large reservoirs, lakes and rivers, barley straw offers an environmentally
acceptable and cost-effective form of control.
2 It should be applied twice each year, preferably in early spring before
algal growth starts and in autumn.
3 Particularly in static waters, the straw should be in a loose form
through which water can pass easily and should be held in nets, cages or
bags. 4 The minimum effective quantity of barley straw in still or very
slow flowing water is about 2.5 g m-2 but higher doses of up to 50 g m-2
should be used in densely infested waters and muddy waters.
5 In rivers, masses of straw (bales or nets) should be spaced along the
sides at intervals not more than 100m apart.
6 Straw should be supported by floats so that it does not sink to more than
one metre below the surface, even when waterlogged.
7 If the straw starts to smell then it is not working and should be
removed.
This is caused by too much straw in too little water.
Copyright IACR-Centre for Aquatic Plant Management, 1997
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