Common duckweed (scientific name Lemna Minor) is a native perennial marine plant that is most obvious throughout spring and summer on water bodies across the country (and indeed large parts of the wider globe). It tends to build up in slow moving water and ponds. It is green in colour and has small rounded leaves with a short root that extends roughly a centimetre into the water. The leaves allow for individual plants float on the surface of water bodies and ‘raft’ together to cover areas of water in a green ‘lawn’.
Duckweed is not a form of algae. The distinction is one of scientific semantics but the major distinguishing features are that algae does not have a defined root, stem or leaf system. Duckweed is often mistaken for algae, but it is a plant in its own right owing to it’s root, stem and leaf structure. Duckweed and algae are often confused owing to their prevalence in slow moving water and ponds. (Whilst on the topic, most algae is perfectly harmless, if a little unsightly).
Rafts of duckweed alone are rarely overly damaging to a water body. There is potential for these rafts to block out light to the lower levels of a water body with harmful effects of organisms that live there. These rafts can also use up the oxygen and nutrients available to other organisms.
Duckweed rafts can also encourage other weeds and algae to form just under the duckweed which can worsen the effects of light and oxygen loss. However, it should be made clear that the circumstances when serious loss of light/oxygen/nutrients leads to negative impacts on an ecosystem are rare and blooms of duckweed are generally not a major concern for a water course.
Duckweed is not harmful to people.
Duckweed’s prevalence is down to its incredible ability to multiply by division. In nutrient rich waters and with sufficient sunlight, a raft is able to double in size over the course of a few days. Where there is a flow on a slow moving stream or river, the plant is likely to be transported downstream by that flow. It is also resilient enough to survive being transplanted by vectors such as people or waterfowl (people may carry it on their shoes; ducks, for example - which feed on the plant - can fly between watercourses and carry it on their feathers).
Duckweed is a source of food for lots of waterfowl (including ducks, geese and swans). It is also good cover for frog, toad and newt species.
If duckweed is in your garden pond, you can remove it periodically with a net or rake if it starts to dominate the surface.
If you see it building up on a watercourse then it is very likely to be under observation and monitoring from local Council and Environment Agency operatives who can measure the levels of sunlight, oxygen and temperature to assess the health of the waterbody. When you see a build up on a flowing watercourse, have a look downstream to see if something is blocking the flow (it may be as simple as a fallen log or it may be a permanent feature such as a weir) but as the flow increases with heavy rain, it is likely the duckweed rafts will be dispersed.
If the build-up is on a public pond or lake then, as mentioned, it is likely to already be under observation by local officials.
Council officers are in regular contact with Environment Agency officials to monitor any serious blooms of duckweed.
The health of our ecosystems is paramount to our operations so we are keen to keep our watercourses as healthy as possible. A build up of duckweed upstream of Middle Mill weir and East Bay weir in Castle Park is common during the summer months. Park Rangers have operated the Middle Mill weir gate in order to increase flow rates where appropriate and break up larger rafts of duckweed.
Park Rangers have also collaborated with the Environment Agency in physically clearing duckweed from the pond in Lexden Park.
Yes, this is not an isolated issue.
Admittedly, large blooms of duckweed can be unsightly compared to a shimmering water surface on a sunny day. British water ways often suffer from the effects of eutrophication (which is the run-off of excess nutrients from land - often agricultural land). This effect can cause an increase in duckweed (and algal) blooms as the excess nutrients used to grow crops and animals are washed into the waterways, absorbed by marine plants and extra growth is encouraged.
When this is coupled with the increased hours and intensity of sunshine that we are already seeing because of climate change, it is likely that duckweed is something we need to learn to live with for the foreseeable future.
This information in this article is compiled from the Colchester Borough Council Ranger teams, Environment Agency, Wildlife Trusts and Royal Horticultural Society. If you would like to know more, these organisations who may be able to offer further information.