Cyanobacteria blooms are a problem in lakes worldwide. The underlying physical and chemical factors that drive blooms vary. Understanding these factors both broadly and on a lake specific basis is essential to managing blooms. Lake Tuggeranong in the ACT is a man-made lake that provides recreational, aesthetic and environmental benefits to the community. These benefits are affected by regular cyanobacteria blooms.
The period from September 2019 to March 2020 provided a rare opportunity to investigate how rainfall patterns influence the physical and chemical conditions that support cyanobacteria blooms. The period from early November until mid-January was extremely dry, with 2.6 mm of rain falling in 74 days. This contrasts to the period from mid-January to March when there were several large storm events. External nutrient inputs to the lake after mid-January caused large increases to in-lake nutrient loads. This included a doubling and six-fold increase of the of the in-lake total phosphorus and dissolved reactive phosphorus loads respectively. These changes in turn affected the biovolume and community composition of cyanobacteria in the lake. Up until mid-January Aphanocapsa delicatissima was the dominant species of cyanobacteria in the lake. After mid-January there was an increase in cyanobacteria biovolume in the lake and a mixture of heterocyst forming species including Sphaerospermopsis aphanizomenoides, Dolichospermum spiroides, Dolichosprermum planktonicum and Dolichospermum circinale dominated cyanobacteria biovolume in the lake. The changes in the lake demonstrate the importance of rainfall driven external nutrient loads in supporting cyanobacteria blooms in the lake. With future climate forecast predicting greater variability in weather, these findings have implications for future lake management.