White chalk, green water

The day started with a heavy band of rain - not very promising when we were planning to go on a field visit to Houghton Regis Chalk Pit, to check the accuracy of the habitat map I'd drawn (and to have a day away from the computer!). As we had to drop into St. Neot's on the way, to collect Ben's IGCSE certificate, we set off while it was still raining, hoping that it would have stopped by the time we arrived. And we were lucky!!

Although there was a chilly breeze, it stayed dry for our two hour trek round, and the sun even came out. The water levels in the pit were much higher than during our summer visits, and a whole new shallow lake had appeared. But my photograph today is of the permanent marl lake in the southern part of the pit, which had the most amazing coloured, crystal clear water.

Marl lakes are one of the rarest types of water body found in the UK. In southern England they are now confined to man-made sites such as chalk and brick pits, although historically there were very large marl lakes in the fens, such as Whiitlesey Mere. The main characteristics of marl lakes are the very high calcium content of the water. Marl is a chalky deposit produced mainly as a result of the removal of carbon dioxide from the water by the photosynthetic activity of aquatic plants, both algae and macrophytes, which results in the shifting of the bicarbonate/carbonate equilibrium towards carbonate. As calcium carbonate is very insoluble it is precipitated in these lakes.

The precipitation of marl also has other effects on the ecology of the lake; phosphorus is precipitated in the form of various insoluble compounds and becomes unavailable to phytoplankton. The low levels of dissolved carbon dioxide may also limit the phytoplankton production and organic matter in suspension becomes bound to the marl and is sedimented out. As a result the water is the clearest of any category of lake in Britain and often has a very characteristic blue tint, though today the water looked more green than blue.

PS Thanks for all the kind thoughts about Henny-Penny, our house chicken. She hasn't returned, so I'm afraid our worst fears have been realised. But she was a very happy chicken (particularly in the last six weeks or so) who enjoyed her freedom, even if, in the end, she paid the ultimate price.