Lake Brewster is a large inland water storage in the Lachlan River system that is managed for irrigation and environmental flows. The lake experiences regular cyanobacterial blooms, limiting the safe use of its water because releases may pose environmental risks to the downstream river. This project investigated the factors that lead to cyanobacteria blooms and its dynamics in the lake.
Being a shallow lake - with maximum water depths of 3.7 meters - thermal stratification is not persistent, and if formed is quickly broken down by wind. This suggests that the lake is highly polymictic, with the water column stratifying and remixing many times during the hotter months. Although stratification can lead to reducing conditions in the lower layer of the lake our data indicate that nutrient releases from sediments is not the major driver of cyanobacteria biovolume in the lake. Instead, the elevated turbidity levels throughout the study period most likely caused by sediment resuspension from high winds and bioturbation from common carp are promoting a cyanobacteria biovolume increase.
Cyanobacteria have a competitive advantage in low light conditions and during times the lake is stratified and can dominate the phytoplankton community. Long-term data on the phytoplankton community across sites shows a seasonal pattern and a clear increase in cyanobacteria biovolume as water moves through the Lake Brewster system.
While a seasonal increase of cyanobacteria during warmer months can be expected, understanding this pattern provides water managers with an indication of what times in the year water from Lake Brewster can be used without environmental risks for the river.
Theses insights can provide valuable input and reasoning into management decisions to increase water levels in the lake and delay or bring forward water release which could place considerable strain on the flexibility of irrigation and environmental flow deliveries.