Canberra’s urban lakes play an essential role, adding to the aesthetic and recreational values of the city and delivering environmental benefits. The consistent occurrence of blue-green algal blooms in Lake Tuggeranong over the summer months has a negative impact on those values and benefits. As part of ACT Healthy Waterways, a project funded by the Australian and ACT governments, an investigation into causes of the blooms in the Lake is being undertaken. The first stage of the project aims to provide an understanding of the conditions and underlying processes in the Lake that lead to algal blooms. The algal density and composition of the algal community in the Lake were studied. To understand the drivers of change in the algal community concentrations of nutrients, suspended solids and organic carbon were measured in the Lake and at major inflows to the Lake. The chemical form of nutrients was analysed to gain an understanding of the chemical processes in the Lake. Temperature and dissolved oxygen profiles were taken within the Lake to provide an understanding of stratification and water movement through the Lake over the warmer months. Median values of total phosphorus and nitrogen at the inflows were 0.1 and 2.1 mg/L respectively. The median phosphorus concentration at the inflows during high flow events was 2.5 mg/L. Elevated concentrations of both phosphorus and nitrogen were present below the oxycline when the Lake was stratified. These results indicate that both external and internal sources of nutrients are contributing to algal blooms. This presentation will discuss the chemical and physical processes leading to algal blooms in the Lake, focusing on the relative contribution of nutrients from external sources compared with the internal load of nutrients from within the Lake.