Harvesting aquatic vegetation

Freshwater lakes are dynamic environments with plants growing more rapidly in some years than in others. The presence of aquatic plants is entirely natural in these lakes, unlike the tidal lake and canal systems found throughout the Gold Coast. It is neither practical nor desirable to maintain our non-tidal waterways in a weed-free condition.

The role of aquatic plants

Aquatic plants play an essential role in removing nutrients from the water column and lakebed sediment. They are the primary users of nutrients – nitrogen and phosphorous – in the lake systems. They also provide a valuable habitat for native fauna. This includes zooplankton and other microscopic life that graze primarily on algae.

Any overzealous removal of aquatic plants can lead to an increase in the volume of waterborne nutrients. This tends to increase the rate of algal growth to the extent where blooms occur. This includes the potentially toxic cyanobacteria blooms (blue-green algae). Systems dominated by algae are significantly more difficult to manage. Under these conditions, fish die, oxygen is depleted, birds die and other adverse impacts become far more common.

When & why harvest

To combat any unbalanced nutrient levels and to provide a more cost-effective, long-term solution to managing water quality in the lakes, we no longer totally remove vegetation. Instead, we do manual inspections of the lakes on a regular basis to assess the growth of the plants and identify areas that may require necessary harvesting. This has led to aquatic plants growing largely unimpeded throughout the spring and summer months. When we harvest, we tend to take out the nutrient-rich 'crop' each autumn, before the plants die off. This maximises the amount of nutrient removed from the lake, helping to prevent an outbreak of algae.

A buffer zone of aquatic vegetation is maintained around the perimeters of the lakes for their long-term wellbeing. This is to ensure that there is sufficient plant life in place to support fish and colonies of microorganisms necessary for control of algal growth. Keeping these buffer strips assists in reducing algal outbreaks by providing a refuge for native fauna during harvesting operations. The buffer zone plants also absorb nutrients from the sediment and water column on a year-round basis. Its retention assists in the gradual improvement of water quality in the lake system.

In general, the vegetation of the buffer strips in the larger lake systems are comprised of a 2- to 5-metre-wide band of ribbon weed and snow flake lily. Other species present include azolla (native floating fern), salvinia and Mexican lily. The latter 2 species are both exotic pest species, which are subject to additional controls.

To find out more about the City's harvesting program, contact our Maintenance Services team on 07 5667 5974.