Studland’s landlocked Little Sea, its best kept secret, is one of the most pristine lakes in lowland England.

Set in heathland, within a National Nature Reserve and owned by the National Trust (NT), it has extremely important plant and animal life, including many rarities.

But it is being attacked by invasive plants which trust volunteers regularly do battle with.

Defra has launched the second phase of its Be Plant Wise campaign, to raise awareness of the damage caused by invasive aquatic plants at a time when pond owners may be thinking of tidying their ponds before winter.

The release of a tiny fragment of plant can establish a population that can choke an entire waterway.

Crassula helmsii was discovered in the neighbouring Eastern Lake and the NT has joined forces with Natural England and volunteers to try and control it.

But it was found in Little Sea about three years ago, along with large quantities of Nuttall’s water weed.

The trust has carried out detailed canoe surveys, installing black plastic mats and digging it out, but it continues to spread.

It is now becoming semi-terrestrial and going in to mires and marshes and smothering rare plants.

It is growing among reeds and willow where it is very difficult to manage.

In a bid to see it off, the trust is funding a PhD student to look at ways of managing it and buying a boat to gain access to the areas.

It is not known how Crassula and Nuttall’s water weed got into Little Sea but it could be deliberate dumping of water weeds or even brought in on duck’s feet or by Sika deer. Flooding and wind can then spread it further.

“Making sure that ponds and rivers are cleared properly has real benefits to our natural environment,” said Environment Minister Richard Benyon.

“We must prevent the spread of these plants so that our rivers and wildlife can thrive and remain a safe and enjoyable place to visit.”