Lake Trevallyn

Water quality and blue-green algae monitoring occur all year round on Lake Trevallyn. Additional monitoring is undertaken by Tamar Estuary and Esk Rivers (TEER) Program staff between December and April. This coincides with the warmer months in summer and autumn when algal blooms are more likely, and the peak recreational period. Monitoring provides an early warning to authorities and the public on when the lake is safe for use. Data is used to advance our understanding of the conditions that influence algal blooms, and to help water managers make timely and informed decisions and mitigation strategies. 

Focus

Water quality measurements and counts of blue-green algae are completed weekly at the Trevallyn boat ramp and at Blackstone Park beach. Water quality information such as temperature, dissolved oxygen, pH, and blue-green algae concentrations are also captured at a 15-minute interval by an in-situ buoy near the dam wall. This information is shared with the Lake Trevallyn Working Group, a collective of stakeholders who work collaboratively to manage Lake Trevallyn and keep the public informed about any changes to water quality that may affect recreational use.  

Although referred to as blue-green algae, the organisms that cause blooms are cyanobacteria, which occur naturally in water. Cyanobacteria can become harmful to humans when their growth accelerates and they accumulate into dense, visible patches at the water’s surface. In high concentrations, algal blooms can disrupt water chemistry and harm other aquatic life as well as humans. 

The Lake Trevallyn Algal Bloom Monitoring Program commenced in the summer of 2007/2008 in response to a significant bloom in the previous summer. Algal blooms occur when a range of environmental conditions align to promote growth such as consistently warm temperatures, low water flows, and high levels of available nutrients, particularly phosphorus. Contact with water containing high concentrations of cyanobacteria can cause skin rashes, eye irritation, earaches, itchiness, and swollen lips, as well as unpleasant taste and odour compounds in drinking water. 

Value

Lake Trevallyn is an important storage for Launceston’s drinking water supply. It is also a valuable asset that attracts wildlife and is popular with recreational users. Since 2007, the monitoring program has gathered seasonal data on water quality and the abundance of blue-green algae in Lake Trevallyn, providing one of the longest datasets for any water body in the state and increasing our understanding of conditions that lead to algal blooms in Lake Trevallyn. The Lake Trevallyn Working Group has demonstrated the importance of collaborative management, keeping the community informed and safe via early warmings when there are changes to water quality that affect recreational use. 

Delivery

The Lake Trevallyn Algal Bloom Monitoring Plan and Communications Plan outline how monitoring and communication should be undertaken when algal blooms occur. Both documents are informed by the  National Health and Medical Research Council Guidelines For Managing Risk In Recreational Water (NHMRC, 2008). NHMRC 2008 specifies the thresholds or ‘biovolumes’ of blue-green algae which trigger expanded monitoring and testing, and public notifications. If algal concentrations increase beyond a specified threshold (Surveillance), the TEER Program initiates expanded and more frequent monitoring to determine the location and severity of the bloom, and further testing for toxicants is undertaken. Beyond a higher threshold (Alert), the Lake Trevallyn Working Group coordinates clear and timely messaging to inform the public of any risks. A dedicated webpage with weekly Lake Trevallyn and First Basin status updates is maintained by the TEER Program for the duration of a bloom. 

2025/26 summary

The 2025/26 Lake Trevallyn Blue-Green Algae (BGA) monitoring season was relatively uneventful despite very low rainfall during early summer, which increased the likelihood of a bloom developing. No significant BGA concentrations were observed, and biovolumes remained well below the National Health and Medical Research Council (NHMRC) health thresholds that trigger additional monitoring and management responses. As in the previous season, BGA concentrations peaked between early February and March, shortly after peak surface water temperatures, with Microcystis aeruginosa remaining the dominant species for the second consecutive year.

Average water temperatures throughout the water column were almost 2°C cooler than the previous season, while consistently windy conditions during the first half of summer helped maintain a well-mixed water column, further reducing the likelihood of bloom development.

The TEER Program's telemetry buoy continues to play an important role in monitoring lake conditions in near real-time, transmitting more than 250,000 data points across 17 monitoring channels between November 2025 and April 2026.

TEER Program staff serviced and redeployed the telemetry buoy for the winter period in late May and will begin preparations for the 2026/27 monitoring season in November.

 

Quick Q&A

  • Algal blooms are not common in Lake Trevallyn. Only three significant algal blooms have occurred over 16 years of monitoring: 2007/08, 2008/09, and 2023/24. However, algal blooms are possible every summer, with reduced inflows, increased temperatures, and elevated nutrient levels creating favourable conditions for algae to grow. In most years, blue-green algae concentrations increase towards the end of summer.

  • Lower temperatures, greater mixing through the water column, and reduced nutrient supply are all factors that may decrease the chances of a bloom or dissipate a bloom once it has formed. However, these factors are not guaranteed to prevent a bloom, as different species respond differently to environmental conditions. This is why ongoing monitoring and data collection are important.

  • Algal blooms persist when conditions are warm, the water column is stable, and there is sufficient food, i.e. nutrients. During these conditions, the water column often stratifies, with the upper layer becoming much warmer than the underlying layers. This promotes rapid algal growth and accumulation of algae at the surface.

  • The Lake Trevallyn Working Group is committed to providing clear, coordinated, and timely messaging to the public in the event of a bloom. The community will be notified through media releases, social media messaging, and signage at Lake Trevallyn.

  • For more information on algal blooms, check out the TEER Program’s Algal Bloom Factsheet.