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Page last updated at 11:45 GMT, Monday, 11 January 2010
Big freeze and ice is 'good for pond life'
By Matt Walker
Editor, Earth News

Frozen garden pond
Under the ice, wildlife is flourishing

The big freeze, which has led to many of the UK's ponds freezing over, may benefit animals living beneath the surface.

Received wisdom says that pond owners should break a hole in the ice to allow oxygen to reach the water.

But new research by conservation charity Pond Conservation has shown the opposite is true.

Oxygen levels can actually rise in a frozen-over pond, benefiting the animals and plants living beneath.

The recent Arctic conditions and adverse weather has had a significant impact on wildlife, which will be detailed by Snow Watch, a special BBC programme broadcast at GMT2000 on Wednesday 13th January on BBC Two.

To safeguard pond wildlife during freezing weather, standard advice has always been to make a hole in the ice to allow oxygen into the pond, says Dr Jeremy Biggs, director of policy and research at Pond Conservation.

But new research undertaken by Pond Conservation suggests that most garden ponds and their wildlife will survive during the big freeze if left to their own devices.

Frog in a pond
There are estimated to be around three million garden ponds in the UK, and half a million wild ponds
Ponds support more endangered freshwater plants and animals than either rivers or lakes
Ponds provide habitat for a range of species including dragonflies, damselflies, mayflies, water beetles, hibernating frogs and others

Ponds freeze from the surface downwards and very rarely freeze completely, leaving large volumes of freshwater under the ice.

Making a hole in the ice makes very little difference to the amount of oxygen in this water, says Dr Biggs.

This is because oxygen diffuses so slowly into still water, moving through about 2mm a day.

So in a pond 50cm deep, it takes over 8 months for oxygen to diffuse to the bottom.

Even under ice, plants continue to photosynthesise, producing oxygen.

With a covering of ice the oxygen is trapped in the pond and, if the ice cover lasts for long enough, oxygen levels will rise.

So if garden ponds have lots of underwater plants or algae, oxygen levels can nearly double in the coldest weather.

The only time that pond owners should intervene is if they own fish, or the bottom of their ponds are full of silt and dead leaves.

Then it is worth stirring up the water, mixing oxygenated and deoxygenated water, preferably using a pump or fountain, says Dr Biggs.

Also any snow should be brushed from a frozen pond's surface, as it blocks the light and will stop underwater plants from producing oxygen.

Under such conditions, oxygen levels can fall significantly.

Health warning

However, Dr Biggs cautions that pond owners should take great care around frozen ponds, and ensure they never step onto the ice which could easily break.

According to Pond Conservation, ponds are among the most threatened UK habitats, with government data showing that 80% of ponds in England and Wales are in poor or very poor condition.

To maintain a healthy garden pond in the long term, says Dr Biggs, make sure that the pond has plenty of underwater plants and is shallow.

"Shallow ponds are better lit than deep dark ponds so can produce more oxygen for their volume," he says.

"Ideally, also keep the pond water as clean and unpolluted as you can to help the submerged plants flourish."

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