Blue-green algae (cyanobacteria) blooms have become a problem in urban lakes in many parts of the world. The abundance and community composition of cyanobacteria within a lake are controlled by various physical and chemical factors. Lake Tuggeranong in the ACT is a man-made lake that provides recreational, aesthetic and environmental benefits. In recent summers Lake Tuggeranong has been closed for use because of Blue-green algae blooms. In Lake Tuggeranong, during the warmer months, the stratification of the lake water into layers of different temperatures and dissolved oxygen concentrations provides conditions conducive to the formation of algal blooms. After stratification there is a period of rapid cyanobacteria population growth in the lake until the pool of bioavailable nutrients becomes exhausted. After this time the abundance and composition of the cyanobacteria in the lake is determined by the availability and ratio of nutrients. The nitrogen to phosphorus (N:P) ratio in Lake Tuggeranong ranges from 5 to 17 during the bloom. This N:P ratio is indicative of a system that is nitrogen limited. Low concentrations of dissolved forms of nitrogen in the lake during the bloom period support the hypothesis that cyanobacteria blooms are nitrogen limited in Lake Tuggeranong. This presentation will discuss the progression of the algal bloom and how chemical and physical changes affect the composition of the cyanobacteria community as well as measures that could be taken to control the bloom.