Algae of various types that occur naturally in ponds are an important part of a pond ecosystem, as they provide food for animals at the bottom of the food chain. However, growth can easily become excessive, covering the pond surface and shading out submerged plants beneath. A mat of algae also prevents oxygen mixing into the water, harming aquatic life, and decaying algae further deoxygenate the water. Amass of algae looks unattractive, and blocks inflow and outflow pipes. Some blue-green algae give off harmful toxins.
Algae can be categorised into three main types, as follows:
Filamentous algae or blanket weed may occur in mineral- enriched (eutrophic) water, and in clear conditions, germinate in early spring on the pond bottom, and can spread to quickly dominate the pond. As the plants mature they become detached and float to the surface, suspended by trapped oxygen bubbles, producing unpleasant-smelling mats of algae.
Single-celled planktonic algae are usually mobile, swimming to light areas of the pond. They prefer muddy, enriched waters, such as duck ponds. Growth tends to be in brief periods or ‘blooms’, usually following sudden enrichment of the water. Blooms can harm other aquatic life.
Blue-green algae can occur in blooms that look like spilt paint on the pond surface, or as slimy encrustations on rocks, and despite their name, may also be yellow, red, violet or brown. Some species produce toxins than can be fatal to small animals and cause serious illness to humans.
For a healthy, well-balanced pond, with algae kept in check, follow the guidelines below:
- Artificially high levels of nitrogen and phosphates should be reduced before they reach the pond. This is most likely to be a problem with ponds that are fed by run-off from farmyards or fertilised farmland. Curing the problem at source will require major changes to farming practice and land drainage, which may not be possible. Either divert the run-off away from the pond and use water from another source, or divert the run-off into a reedbed and then into the pond. The reedbed will take up a large proportion of the nutrients, as well as various pollutants.
- Existing high nutrient levels in the pond can be reduced by cleaning out accumulated silt in autumn. Any disturbance will initially release nutrients and possibly cause a brief bloom, but levels will drop as the pond settles.
- Ensure there is a good species mix and number of oxygenating plants, which compete with the algae for nutrients. About one third of the pond surface should be shaded by floating leaved plants or by trees. Emergent vegetation provides a habitat for algae-grazers such as water fleas (Daphnia).
- Avoid attracting ducks to the pond, and don’t feed any existing ducks or fish in the pond. Fish eat daphnia, thus reducing a control agent of algae.
- If blooms do occur, algae can be raked out as a temporary measure. Leave the algae beside the pond for a day or so, to allow mobile organisms to return to the pond. Then compost the algae well away from the pond, to avoid run-off washing nutrients back into the pond as the algae decays.
- Aerate the pond to increase the levels of oxygen.
Fountains and waterfalls are the method for garden ponds. In large ponds and lakes, aerators linked to air compressors can be installed in the bed of the lake. These bring the water from the bottom to the surface, increasing the level of dissolved oxygen and allowing the aerobic breakdown of organic matter from the lake bed. Where fish are present, oxygenated water is vital for their survival, especially in warm weather. The London Lakes Project (report to be published in1997) has installed 16 aerators in the 6 hectare (14 acre) Battersea Park Lake, resulting in an improvement in the water quality.
Barley straw
As a further preventative measure, the use of barley straw to control algae is a well-established technique, developed by the Aquatic Weeds Research Unit and others. It is effective in flowing and still water bodies, large and small, including reservoirs, lakes, ponds, rivers and streams.
Waterlogged, decomposing barley straw gives off a substance which inhibits algal growth and reduces its spread, but does not damage other pond organisms. The anti-algal activity requires warm temperatures, a high level of dissolved oxygen in the water, and sufficient sunlight on the water surface. The anti-algal effect starts about one month after the straw has been put in the water, and lasts about six months. There is no evidence of adverse effects, except where excessive doses of at least 100 times recommended levels have caused deoxygenation of the water. If the straw starts to smell there is too much straw in too little water and the straw should be removed.
The submerged straw attracts and supports invertebrates, which may in turn support greater populations of tadpoles, fish and other organisms, but because of increased vigour of plants and animals, appears to keep the ecosystem in balance. Suppression of the algae allows emergent and submerged plants to recolonise, which in turn suppress the algae so the need for straw treatment declines.
Application rate
An application rate of about 10g per m2 of water is suggested. For many garden and wildlife ponds this will only amount to a few handfuls. A pile of loose straw about 300 x 300 x 100mm (1’ x 1’ x 4”) weighs about 100g (4oz). The anti-algal effect is strongest if the straw is floating, with plenty of oxygenated water around it, and not touching the mud. Put the straw loosely into a plastic mesh sack, such as an onion sack, with a plastic bottle or ball in it to act as a float, and then anchor it by a string and weight so that it floats just below the surface. Place it near the inflow where water movement is higher. On ponds over about 50 m2, make two or more nets and anchor them at regular intervals around the pond, avoiding the most sheltered and still areas of the pond.
On larger ponds, the rate is equivalent to five normal-size 25 kg bales per hectare of water surface. The straw can be retained behind a screen forming a loose dam at the point of inflow, so that oxygenated water is continually flowing through it.
Timing
Straw should be applied in late autumn or very early spring, so that it is activated by the time algal growth starts in late spring. Further applications should be made at six-monthly intervals in autumn or spring. Algal growth can recover quickly once the straw has lost its effect at about six months, or if it rots away completely. It’s therefore important to leave the old straw in place for at least a month after the new straw is put in, and to immediately add more if the straw rots away before six months. The following timetable is suggested:
February-April 1st application
Sept-Nov 2nd application
February-April 3rd application and remove 1st application
Sept-Nov 4th application and remove 2nd application, etc.
If dense mats of algae already exist, the anti-algal activity will only be effective on the area closest to the straw. Using more straw at this time may worsen the problem. The advice is to rake off as much algae as possible, apply straw at the normal rate, and then rake off again after about six weeks. In the autumn start a regular programme of prevention. Information sheets giving further advice are available from the Centre for Aquatic Plant Management.
