Information and Advisory Note Number 3 Back to menu
In lochs, reservoirs, rivers, canals and drainage ditches, freshwater macrophytes form an integral and often important part of the freshwater ecosystem. Normally there is no need to carry out active management of these plants. It is only when the growth of freshwater macrophytes starts to interfere with the functioning of the system that control measures are required. In Scotland it is fairly rare for any single species to be the cause of an aquatic weed problem (although exceptions do occur - for example recent problems caused by Canadian Waterweed in Orkney). More commonly, it is a group of species, growing in nuisance quantity, which causes the problem. In flowing waters aquatic weeds may interfere with water flow (increasing flood risk or impeding drainage), and increase siltation. They may also interfere with navigation or recreational use of the water, and cause public health problems.
Macrophytes fall into three groups. It is important to know these because they
cause different problems and are susceptible to different weed control
approaches.
2.1 Submerged plants have their foliage below the surface (although leaves or
flowers may emerge above the surface occasionally). Examples: Spiked
Water-milfoil (Myriophyllum spicatum)', Canadian Waterweed (Elodea canadensis)
2.2 Floating plants have their foliage and flowers lying on the water surface.
They may be free-floating (roots dangling free in the water), or rooted in the
sediment.
Examples: Duckweed (Lemna minor) - free-floating; White Water-lily (Nymphaea
alba) -rooted.
2.3 Emergent plants have their foliage and flowers above the surface.
Narrow-leaved and broad-leaved emergents are usually distinguished. Examples:
Common Reed (Phragmites australis) - narrow-leaved emergent; Water Plantain (Alisma
plantago-aquatica) - broad-leaved emergent.
2.4 In addition to these plants there are the macrophytic filamentous algae (e.g.
Cladophora, Enteromorpha) which, when present in large quantities in fresh
waters, may also cause severe weed problems.
"British Water Plants" (by S. Haslam, CA Sinker & P.A. Wolseley). Field studies
Council, Shrewsbury (1975). Contains useful illustrated keys and monochrome
drawings.
"Aquatic Plants - a guide to recognition" (by D. Spencer-Jones & P.M. Wade).
ICI, Farnham. 169 pp (1986). A colour guide available from ICI, produced in
collaboration with Loughborough University. Contains good pictures and details
of selected species of problem-causing macrophytes.
Controlling weeds in watercourses and lakes must be approached with caution. It
is important to ensure that professional advice is sought, whatever control
method is being considered. Care must be taken to ensure that the proposed
treatments will not contribute to contamination of drinking water sources. Where
feasible, it is usually best to clear only
part of the water body at a time, to minimise the risk of damage or disturbance
to wildlife. For the same reason it is also desirable to avoid sensitive periods
of the season (e.g. bird breeding periods) when carrying out clearance
operations. There are four main sets of options, based upon use of mechanical,
biological, chemical and environmental control measures (see Table 1).
4.1 Mechanical control
Uses either simple hand-cutting or raking techniques, or machinery to clear
aquatic plants from the system. Purpose-built weed-cutting boats, as well as
bank-mounted machines using equipment such as Bradshaw buckets, are available to
clear weeds from freshwater systems. Whatever method is used it is important to
ensure that cut weed is removed from the water, to minimise risks of
deoxygenation associated with the decay of large quantities of cut plant
material within the system. Regrowth of plants is usually fairly rapid (days to
weeks, depending on conditions and season), except where intensive dredging is
used to remove both plants and sediments. Regrowth after dredging usually occurs
within months to one year of treatment.
4.2 Biological control
Uses grass carp (Ctenopharyngodon idella) to graze aquatic plants, and reduce
the problem to acceptable proportions. Stocking rates of fish in the region of
50-100 kg/ha are usually recommended. Grass carp are considered highly unlikely
to be able to breed naturally in British waters, so their use has few of the
risks usually associated with introduction of an alien species. In Scotland they
have been used successfully in some waters, but they are not a panacea for all
aquatic weed problems. They do not feed at temperatures <10°C, so their success
tends to be lower the further north they are used. Their feeding preferences
also preclude their use against some types of weeds. In many circumstances it is
likely that grass carp may give good control over several years with minimal
damage to the functioning of the target system. They should not be used in a
Site of Special Scientific Interest (SSSI), which has been designated for its
aquatic plants, or in an adjacent water-body unless their retention can be
assured. On other SSSIs individual circumstances must be considered prior to an
introduction being sanctioned. Read the National Rivers Authority grass carp
booklet before trying to use this control option (see Further Reading).
4.3 Environmental control
Shading by trees is a possibility for providing long-term control in smaller
watercourses. However, trees interfere with access to the channel, which may be
a problem. Manipulation of water levels can provide reasonably good control of
aquatic weeds, but is obviously impractical outwith systems where water levels
are easily controllable. The use of barley straw to control filamentous algal
problems is still under investigation, but looks promising. Decomposing straw
appears to release an algistatic substance, which prevents the development of
dense growths of filamentous algae. Exactly how this works is not yet known, but
there is sufficient evidence available to suggest that the technique is at least
worth trying if algae are a problem in a target water body. Depending on
conditions, straw is usually introduced at between 10 -100g per cubic metre of
water. It should be noted that this technique only tackles the symptoms of
nutrient enrichment and should always be used as part of an integrated programme
aimed at reducing nutrient loss from the catchment.
4.4 Chemical control
Several herbicide products are currently marketed which provide a complete
spectrum of control options for dealing with aquatic weeds (see
Table 2). Because the use of herbicides involves applying a toxin to the water
there are strict controls on their use (see The Legal Framework" below).
Information on the toxicological risk to fish, invertebrates and other
non-target organisms is provided by the label information on aquatic herbicide
products. The label must be read and complied with before using any aquatic
herbicide. Aquatic herbicide use carries a risk of indirect effects on the
aquatic environment. These may include deoxygenation and other problems, so
before using herbicides it is recommended that alternative options should first
be investigated.
Where a water body has been designated as an SSSI partly or wholly on the basis
of its macrophyte flora there should be a presumption against herbicide use,
unless the vegetation of interest is known not to be susceptible to the proposed
treatment (see Table 2 and product label information). Some herbicides are
suitable for use in "partial control", i.e. clearing only part of the system of
aquatic weeds. This is easy where the weed
problem is caused by emergent forms, much less so for submerged ones: only two
herbicides (diquat-alginate and dichlobenil) can be used in this way for
submerged weeds.
The use of herbicides for aquatic weed control is covered under the Control of
Pesticides Regulations 1986, enforced under the Food and Environment Protection
Act 1985. Herbicide spraying is covered by the Health and Safety at Work Act and
suitable certification must be held by anyone intending to apply herbicides to
water. Training involves a foundation module, followed up by a range of
appropriate modules for the application equipment to be used (e.g. powered
granule applicator; knapsack sprayer; tractor sprayer; boat-mounted applicator
modules).
5.1 Scottish Natural Heritage should be consulted before contemplating any aquatic
weed control on SSSIs. Prior to releasing grass carp a licence must be obtained
under the Wildlife and Countryside Act 1981, for which application should be
made to the Scottish Office, Agriculture, Environment and Fisheries Department,
Fisheries Division.
5.2 The Scottish Environment Protection Agency (SEPA), as of 1/4/96, must be
consulted if the intention is to use any herbicide to control aquatic weeds, and
its agreement is needed before herbicides can be applied in or near
watercourses.
6.1 What type of weed is causing the problem? (there may be more than one!)
6.2 What options are available to deal with this type of weed problem?
6.3 Can the risk of potential environmental side-effects, including potential
impacts on non-target organisms, be evaluated for each possible option?
6.4 For what period of time will the clearance operation be effective?
6.5 What type of weed is likely to regrow after this period? What are the
implications of dealing with any regrowth? (For example, will the same clearance
operation be needed or a different one?)
6.6 Can a cost::benefit exercise be undertaken before treatment is approved?
(Ideally this should include the costs of any environmental side-effects)?
6.7 What are the legal and safety implications of the proposed clearance
operation?
Anon. 1985. Guidelines for the use of herbicides on weeds in or near
watercourses and lakes. Ministry of Agriculture, Fisheries and Food. (MAFF
Booklet 2078). (Note: the standard guidelines, now somewhat out of date
Available from HMSO 0171 873 9090).
Eaton, J.W. 1992. Grass Carp for Aquatic Weed Control: A Users Manual. Bristol,
National Rivers Authority. Research and Development Note 53. (Note: a detailed
operational guide to this biological option for aquatic weed control. Price £10
00. Available from MailCom pic, 1-5 Alston Rd., Pattinson N. Industrial Estate,
Washington NE38 8QA).
Lassiere, 0.1996. Botanical Survey of Scottish Freshwater Lochs. Perth, Scottish
Natural Heritage, Information and Advisory Note 4. ISSN: 1358 5843
Maitland, P. 1996. Freshwater Fisheries Management Perth, Scottish Natural
Heritage, Information and Advisory Note 26. ISSN: 1358 5843.
Pieterse, A.H. & Murphy, A.H. (eds.) 1990 Aquatic Weeds. Oxford, Oxford
University Press. (Note: standard text on the ecology and management of nuisance
aquatic vegetation. Paperback edition available from 1994. Price £35 00)
Roberts, J. & Wood-Gee, V. 1996. Waterside Margins and their Management Perth,
Scottish Natural Heritage, Information and Advisory Note 14. ISSN-1358 5843.
Dr. Kevin Murphy
Institute of Biomedical and Life Sciences
Division of Environmental and
Evolutionary Biology,
University of Glasgow
Glasgow,
G12 8QQ
Aquatic Environments Branch
Research and Advisory Services Directorate,
2/5 Anderson Place
Edinburgh EH6 5NP
Tel: 0131-447 4784