In September of 1966, a general biological survey of Lake St. John was carried out to determine the pollution status of the lake. This information indicated that the lake was not grossly altered by waste discharges from Chemical Developments of Canada Limited; however, toxic conditions were evident at the sampling site most proximal to the chemical plant. A more intensive sampling programme was undertaken in September of 1967 in order to define the zone of toxic pollution. Both chemical and biological parameters indicated a graduation of water quality extending outward from the main discharge. The zone of toxic pollution established by biological parameters extended 600 feet east, 600 feet north-east and 900 feet north of the main effluent outlet. The blue-green algae Anabaena and Aphanizomenon were present in extremely high numbers in Lake St. John. Both these forms of algae are capable of collecting in large masses sufficient to form algae blooms. Nutrients present in the surface water of the lake were sufficient to foster high algae populations.