Ecologists at the wildlife charity are particularly interested by the fact that there are only two other records in the UK, from Surrey and Kent. Another first for the county.

The small 30 metres pond is set amidst the oak, birch and rowan-dominated woodland, which means that much of the lake is in almost permanent shade and, sheltered from the wind - ideal conditions for the development of surface films.

The surface film was greyish-brown in colour, and covered the entire surface. When stirred it with a twig, it broke up, quickly closing up again as the water settled.

Duncan Hutt, Head of Conservation and Living Landscapes says: “We have to confess that this was beyond the knowledge of us at the Trust and we had to get specialist help with this one. We are relieved that the cause of the incident was natural but we will have to continue to assess the pond to see what long term effect it might have."

“The apparent rarity of Chromulina ferrea may be to do with the very few people who can actually recognise it. It may well be an everyday part of the pond but that this year conditions favoured its rapid growth to cause the effect that it did.”