ANABAENA, A TOXIC BLUE-GREEN ALGA CONTAMINATING LAKES IN EAST ANGLIA G. D. H EATHCOTE During the summer droughts of 1989 and 1990 several reservoirs and lakes in East Anglia were 'attacked' by a mysterious blue-green slime which was eventually shown to be growth of a toxic blue-green alga. It first caused trouble at Rutland Water and 18 sheep and 12 dogs died after drinking the water. Some soldiers who swam in affected water also became severely ill (pers. comm. Mr. Ian Hill of the National Rivers Authority). The public was assured that normal filtration was sufficient to render infected water harmless but it was necessary to prevent people from swimming or sailing on those waters and to keep animals away. The alga thrived in the hot weather, especially where there was a high phosphate content to the water from fertilizers and other agricultural chemicals. Anglian Water spent £2 million applying ferric sulphate to diminish the growth of the alga. My nearest affected stretch of water was a large flooded gravel pit at Flempton near Bury St. Edmunds, used by Flempton Sailing Club. On 20th August 1990 the Environmental Health Department of St. Edmundsbury Borough Council posted a notice informing the public that the National Rivers Authority had found the blue-green alga Anabaena in the lake and warning everybody to keep away from the water. The alga has been known to cause a skin rash, eye irritation, vomiting, diarrhoea, fever and pains in muscles and joints. Few would wish to swim in the lake after such a warning! Of course the Sailing Club closed the lake temporarily. When I visited the lake in September 1990 there was a band of blue-green scum a metre or so wide in patches round the edge of the water, and pale brown scum extending over the mud at the edge of the lake. The scum had the consistency of emulsion paint (thick enough to retain the footprints of the water birds for some time) and it had a foul smell, more like pig slurry than stagnant mud. It could be described as 'moss coloured', but with a trace of blue when it dried. Nearby waters were unaffected. It is not surprising that the identity of this unwelcome addition to the freshwater flora was not immediately obvious. The blue-green algae (Schizo- phyceae, or Cyanophyceae in an earlier classification) are a group of microscopic plants with uncertain affinities. They are today considered closer to the bacteria than to other green algae (Chlorophyceae). They can occur as individual roundish cells or linked together in chains of many cells. Individ- ual cells are very small but they can occur in quite large gelatinous masses. Peter Wanstall and I found Nostoc forming dark brown gelatinous masses the size of plums in fast-flowing streams in the Pyrenees. Blue-green algae are common on damp mud and the trunks of trees. Anabaena belongs to the Chroococceae group of blue-green algae, simple roundish cells consisting of a cell wall of cellulose and pectic substances containing a hollow, blue-green sphere (chromoplasm) with colourless tissue (centroplasm) inside. There are no chloroplasts, vacuoles or true nuclei. Trans. Suffolk Nat. Soc. 27 (1991)