In Nov. 1970 the acidity of Calthorpe broad in Norfolk and some adjacent ditches changed abruptly from near-neutrality to just above pH 3. The entire fish community of the broad perished as did freshwater mussels and most aquatic macrophytes. There was a spectacular invasion by the acidophilous alga Tribonema minus. After a period of acute acidity the pH of free water returned to neutrality but similar fluctuations occurred in most years between 1971 and 1979. These events were paralleled by improved drainage in surrounding farmland that left a perched water table in the area around the broad. This situation was maintained by dams during the winter but evaporation and seepage caused extreme falls in water levels during the summer. As a consequence, soil, which was rich in pyrites, was subjected to alternate drying and saturation. In agricultural contexts these conditions lead to the oxidation of pyrite and the formation of sulfuric acid. Treatment by the addition of lime in 1978 was successful in maintaining neutral water in the broad although the acidity of the water in nearby ditches continued to fluctuate. A number of plants and animals recolonized the broad in a fashion consistent with their dispersal mechanisms.