Pond by Ross Hoddinott
Whilst unattractive and potentially harmful, algae are very important members of a healthy, well-balanced pond ecosystem, providing food for species at the lower end of the food-chain. To deal with algae effectively is not to eradicate it, but to prevent, or control, its excessive growth.
There are many different types and species of algae that tend to present seasonal problems, depending on the pond type. The major factors influencing algal communities within ponds are: light; temperature; nutrient type; presence of animal grazers; and pathogens. Few ponds have exactly the same habitat and conditions, so different algae species will thrive in different ponds.
Identifying algae
Many algae species can only be found at specific times of year, whilst others are usually present but in greatly fluctuating numbers. Some species occur in such high numbers that they cause the water to turn green or brown for a short time. Some thrive in cold spring conditions, but decline as the water warms up. While the greatest abundance and diversity of species occurs in late spring and summer, even autumn and winter suits some species.
Although an expert eye is needed for detailed identification of species, the average pond observer can broadly identify three main algae types:
| Filamentous algae (multi-cellular ‘blanket weed’) | These need clear, mineral-enriched waters with good light penetration to the depths where they germinate, anchored to the pond floor. They germinate in February and become established and dominate before other species can get a hold. As the algae plants mature they float to the surface forming unpleasant smelling algal mats of decaying algae. |
| Planktonic algae (single-celled) | The algae are often mobile and can move to the area which most suits them. They tend to prefer organic, enriched turbid waters – such as those inhabited by duck or fish. Planktonic species tend to be very seasonal, lasting a week or two often as ‘water blooms’, with a succession of different species waxing and waning throughout the year. |
| Blue-green alga (single or multi-celled) | These are, in fact, cyanaophyte bacteria that have characteristics of bacteria and algae. They can fix their own nitrogen so simply rely on phosphates. They can occur in large colonies, resembling spilt green paint on the pond surface, or as soft and slimy, various-coloured encrustations on rocks. |
Take care not to confuse small floating plants, such as the reddish-tinged Azolla water fern or green Lemmna duckweed, with algae. These are found in sheltered, often deeply silted, ponds where water quality may be good but deep silt prevents rooted aquatic plants from growing.
Harmful effects of algae
Algal blooms and mats can harm a pond by shading out and preventing other submerged or floating-leaved plants from getting established. Algal mats may prevent oxygen mixing into the water, thus lowering available oxygen for invertebrates, fish and other pond life. Decaying algae further deoxygenates the water and algal mats can block up inflows and outflows. Blue-green algae produce toxins that can poison wildlife.
Control of algae
Where algae is a nuisance, consider controlling it by the following methods:
• Reduce high nutrient levels (nitrogen and phosphates) in source water supplying the pond – ideally by buffering pond inflows laden with nutrients (such as farm ditches with fertiliser or farmyard run-off) with grass margins, or diverting them away from the pond.
• Remove high nutrient levels in the pond by de-silting and spreading the spoil well away where it will not leach into the pond again.
• Make sure the pond has a high number, and good mix, of submerged (oxygenating) plants to compete with algae for nutrients. Ensure that up to a quarter of a pond’s surface is shaded – either by floating leaves of aquatic plants or a tree at a pond edge.
• Check the pond has some deeper areas of cooler water to provide relative stability of temperature without the water warming up greatly throughout the whole pond in the spring.
• Avoid encouraging duck or feeding them as they will remain longer than they would normally and start to enrich the pond and upset the balanced ecology.
• Do not feed fish, even if they have naturally colonised the pond. In addition to predating priority species such as great crested newt and dragonfly species, having too many fish can result in an imbalance in the whole food chain – the fish eat all the daphnia which cannot then consume the algae.
• Ensure that there are plenty of shallow areas with emergent vegetation for algae-grazers such as daphnia to live in, relatively protected from higher predators such as fish.
Barley straw treatment
Waterlogged, decomposing barley straw produces a substance which inhibits algal growth and when applied as loose straw at a very low rate (10-50gm per m2 of pond surface area) is thought to prevent algal growth in spring.
But incorrectly applied, barley straw can exacerbate an algae problem.
• Straw should be applied twice each year, preferably in early spring, before algal growth starts, and in autumn.
• Straw should be loose, through which water can pass easily and should be held in nets, cages or bags. Straw should be supported by floats so that it does not sink to more than one metre below the surface, even when waterlogged.
• In small garden ponds where only a few grams of straw are needed, straw can be put into a net bag, nylon stocking or simply tied into a bundle with string and attached to an anchor made of a stone or brick and dropped into the pond.
• 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.
Filamentous algae are not easily controlled by straw once they have formed floating mats. In some situations, filamentous algae can be raked out but rapid re-growth of the remaining fragments is likely.
To prevent this, straw should be added about one month before the algae is raked out. Raking algae out of a pond in spring and early summer does damage pond wildlife, for example very small great crested newt larvae with only gills for breathing will hide amongst algae and are unable to crawl back into the pond even if it is left by the side of the pond for a day or two.
More detailed management techniques are available at the Centre for Ecology and Hydrology.