The Lower Vasse is green over summer months due to algal blooms. The blooms are made up of millions of phytoplankton (tiny photosynthetic organisms) which discolour the water. Phytoplankton are an important part of all aquatic ecosystems, however in the Lower Vasse River blue-green algae (cyanobacteria) are often the dominant type of phytoplankton over summer months. Blue-green algae are particularly concerning because they can produce harmful toxins. Algal blooms in the Lower Vasse River have been common over many decades and recorded as early as 1931.
Algal blooms occur when there are a combination of suitable environmental conditions including warm weather, light, high nutrients and calm water. Nutrients are very high in the Lower Vasse River all year round, but algal blooms only occur in warmer months when temperatures are higher, there is little shading and river flows are low. Naturally the Lower Vasse River would not have flowed in summer and would have dried out like other rivers in the catchment. Installation of weir boards (now removed) near the Old Butter Factory in the 1930s and gradual landfill behind the boards has changed the river into a still pool over summer months, providing ideal conditions for algal blooms. Sediment that has built up in the river over many decades has also contributed to water quality problems.
As long as you don’t drink the water or come into contact with it during a bloom, the health risks from an algal bloom are very low. Some species of blue-green algae can be toxic to animals and humans if they are eaten, inhaled, or come in contact with the skin. Exposure to high levels of blue-green algae can cause diarrhoea, nausea or vomiting; skin, eye or throat irritation; and allergic reactions. The City of Busselton puts up signs in summer advising people to avoid contact with the river when there is an algal bloom. There have been media stories about the link between blue-green algae and neurological diseases, but there is no definitive evidence to support this link.
Nutrients enter the Lower Vasse River through surface water flows, groundwater and from nutrients released from the sediment on the river bed. Nutrients in surface water come from activities in the catchment and local groundwater. Modelling has shown that most of the nutrients entering the Lower Vasse River come from fertiliser (from farms and gardens) and septic tanks.
Only removing sediments will not stop algal blooms because nutrients can still enter the river from other sources. Removal of sediments in some places may, however, improve visual amenity where they are exposed in summer. In the past summer (2019-2020) visual amenity of the river was poor due to low winter and spring rainfall that lead to low water levels exposing sediments. Water quality was however similar to previous years. Management of sediments to protect natural, cultural and social values is an objective in the Lower Vasse River Water Management Plan.
Unfortunately flushing out the Lower Vasse River to improve water quality is not feasible. There are a number of reasons why we can’t flush algal blooms and sediments out of the river:
Options to increase flows were evaluated in the Reconnecting Rivers project. For more information visit this link.
The Vasse Diversion drain can’t be removed because it protects Busselton from flooding. Although average annual rainfall has reduced, flood events still happen and may even increase in the future. The Vasse Diversion drain was built in 1927 to reduce the risk of flooding to Busselton. It originally diverted around 90% of the upper Vasse and 60% of the Sabina Rivers directly to Geographe Bay. A culvert installed between the Vasse Diversion drain and Lower Vasse River diverts about 30% of flow in the drain back into the river. In a recent study scientists looked at further increasing flows into the river to improve water quality. As a result of this study a second culvert will be constructed in the Vasse Diversion Drain by the Water Corporation as part of their Vasse Diversion Drain Upgrade in 2020/21. Scientists are now looking at how to best manage flows through the new culverts to improve water quality. For more information see this link.
A lot of work has been done over the last 20 years to try and improve water quality in the river. The City of Busselton, Geographe Catchment Council (GeoCatch), Department of Water and Environmental Regulation and their partners have been working together to try and reduce algal blooms and improve water quality. Actions have focused on reducing nutrients off the catchment such as improving rural and urban fertiliser use, deep sewer connections to eliminate nutrient release from septic tanks, protection and revegetation of riverside vegetation and upgrading dairy effluent and stormwater infrastructure. Actions have also been trialled to reduce algal blooms in the river such as sediment removal, phosphorus-binding clays, oxygenation and reed rafts. Unfortunately due to the very high concentrations of nutrients in the river and modification of flow actions to date have not reduced algal blooms. For more information on what has been done to improve water quality in the Lower Vasse River, click here.
Water quality in the Lower Vasse River have neither improved nor got worse over the last 20 years. Water quality measures (phytoplankton and nutrient concentration) have been relatively stable over the last 20 years despite increasing urbanisation and intensification of agriculture in the catchment. This indicates that actions undertaken in the catchment to reduce nutrients have halted water quality decline. This is a positive step towards long-term water quality improvements in the river. View water quality data for the Lower Vasse River.
The Lower Vasse River supports a thriving ecology including waterbirds, water rats, native fish, crayfish and macroinvertebrates. A study looking at the health of the Lower Vasse River in 2018 showed that the river has significant ecological values. This included a high diversity of native fish and crayfish, with a low numbers of exotic species. Native species present in the river include gilgie, western pygmy perch, western minnow and Carter’s freshwater mussel. Results of the River Health Assessment can be found here.
It is hard to say how long the river will be green as it will depend on the management approach going forward. Improving water quality is very challenging in the Lower Vasse River as nutrients levels are very high and natural flows have been changed. Reducing nutrients entering the river will take decades, so managers are exploring options to make faster visual improvements. These include using living stream principles and in-situ water treatment options like phosphorus-binding clays.
The City of Busselton are leading the implementation of the Lower Vasse River Water Management Plan that will guide future management directions for the river.
Actively managing Mexican waterlily (Nymphaea Mexicana) is a priority for the City of Busselton. Mexican waterlily is an invasive aquatic weed that covers large areas of the Lower Vasse River, impacting on the ecological and amenity values of the river. They have however been found to improve water quality and reduce the severity of algal blooms in the river. The City of Busselton is managing further spread of Mexican waterlily through strategic herbicide control to gradually reclaim areas of open water, targeting important waterbird habitat, while minimising adverse impacts and preventing a return to algal blooms in these areas. For further information, click here.
Living streams are a way to manage stormwater by mimicking a natural stream to improve water quality or ecological values. Living Streams are one of the concepts the City of Busselton are considering for the Lower Vasse River. Aspects of Living Streams such as altering the morphology (shape of river channel), hydrology, shading, sediment removal, and planting vegetation are being considered for the Lower Vasse River.
Algal blooms in the Lower Vasse River are a symptom of the activities that we all do every day at home, in our gardens and on farms. We can all make small changes that will help reduce nutrients leaving our property through stormwater or groundwater. These include adopting low nutrient Bay OK gardening principles at home and best practice fertiliser management, dairy shed and waterway management techniques on our farms (visit www.geocatch.asn.au for details).