Lake Trevallyn

Water quality monitoring in Lake Trevallyn occurs over the summer months, recording the presence and abundance of blue-green algae during the peak recreational period. This provides an early warning to water authorities and the public on when the lake is safe for use. The data is used to understand the conditions that influence outbreaks and to inform management and mitigation strategies.

Focus

Pathogen (enterococci) and blue-green algae assessments are completed weekly at the Trevallyn boat ramp and at the Blackstone Park beach. Water quality data (pH, electrical conductivity, turbidity, chlorophyll-a, Phycocyanin and water temperature) are collected at 15 minute intervals near the dam wall and water temperature is collected to a depth of 10 m at the same location.

Value

Since 2007, the collection of data through the monitoring program has increased our understanding of the bloom processes in Lake Trevallyn,  an important storage of drinking water in Launceston, and important to recreational users.

Although referred to as algae, the organisms that cause a bloom are a type of blue-green bacteria called cyanobacteria that occur naturally in water. This bacterium can become harmful to humans when it grows very fast and accumulates into dense visible patches at the surface of the water creating algal blooms. In these conditions, algal blooms can become a serious public health and environmental problem.

The blue-green algae were first discovered in Lake Trevallyn in bloom proportions in January 2007. Algal blooms can occur because of a range of environmental conditions such as low water flows and high levels of available nutrients,  which promote its growth. Contact with an algal bloom can potentially cause human health problems including skin rashes, eye irritation, earaches, itchiness, and swollen lips, and can also cause unpleasant taste and odour compounds in drinking water.

Delivery

If an algal bloom is suspected based on the weekly assessments of water quality, the Tamar Estuary and Esk Rivers (TEER) Program will initiate boat-based monitoring to determine the location, severity, and potential causes of the bloom.

If a bloom is confirmed, a clear and coordinated message from program partners will be communicated to the public to advise on the safety of recreating in Lake Trevallyn.

2022/23 summary

The 2022/23 monitoring season concluded at the end of April 2023, with no bloom detected, marking 16 years since monitoring commenced. The monitoring program continues to gather important information on parameters that influence algal concentrations, such as water temperature and stratification (where there is a difference in water temperature between the surface and at a depth of 10 metres). Water temperatures above 20 degrees Celsius are known to stimulate algal growth, a temperature threshold that was exceeded fairly consistently from late December 2022 to late January 2023. However, water temperature stratification didn’t occur during this period, and very few algal cells were observed.

Algal cell concentrations peaked at 686 cells/mL in April 2023, well below the alert level of 2,000 cells/mL that would signal a potential bloom event. The peak cell count occurred uncharacteristically late in the season when the water temperature had cooled to approximately 14 degrees Celsius. The dominant algal species observed was Dilochospermum planctonicum, in contrast to Dilochospermum circinalis which has historically been the dominant species. These unexpected differences observed between the current and previous monitoring seasons highlight the value of ongoing data collection relating to factors that potentially contribute to algal bloom formation.

 

Quick Q&A

  • The first recorded bloom in Lake Trevallyn occurred in the summer of 2007. During the 2008/09 summer, a small bloom occurred over a two-week period. Most years, blue-green algae numbers increase towards the end of summer.

  • Water flows are a major controlling factor of blue-green algal blooms. High water flows during the preceding spring and summer may prevent the blooms from becoming established, as seen in the 2009/2010 summer.

  • Algal blooms persist when water temperature continues to rise, promoting cell growth and surface concentration.

  • All partners of the Lake Trevallyn algal bloom monitoring program are committed to a coordinated and clear communication approach in the event of an algal bloom in Lake Trevallyn. The community will be notified of any bloom event through the media as well as signage at Lake Trevallyn.