Macroalgae (or seaweeds) are aquatic plants that form underwater gardens and can be seen by the naked eye.
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Macroalgae, like seagrasses, typically occur in shallow water where there is suitable light for growth in freshwater, estuarine and marine environments.

Macroalgae generally attach themselves to structures such as rock, wood or other plants. Some species, particularly filamentous species, are free-floating and move around with the currents.

Despite being known as weeds, macroalgae are important in aquatic ecosystems as a food source and habitat to invertebrates, fish and birds. 

Macroalgae grow rapidly in response to nutrient enrichment. Land clearing and increased drainage for agriculture have been among the main causes of pollution in the drains and streams that flow into estuaries. Fertilised lawns and septic tanks are also a problem.

We generally notice more macroalgae in summer when there is more light, longer days and warmer temperatures, which are ideal conditions for macroalgae and other plants to grow. Summer storms can result in excessive macroalgal growth as more nutrients than normal wash into the estuary.

Macroalgal blooms

Macroalgal growth, when excessive, is known as a macroalgal bloom. These events are considered a nuisance as they can disrupt the growth of other plants in the estuary, such as seagrass.

Shading of seagrass can result in fewer flowers being produced (lower reproductive success), slower growth rates and ultimately the death of whole plants. The loss of seagrass and macroalgae is followed by decomposition, which consumes oxygen from the water column. Over a wider area this is undesirable for fish and invertebrates, and in extreme cases can result in fish kill events.

A good example is the Peel-Harvey estuary which, in the 1980s, became choked by dense growths of green algae. The bloom was persistent, and so severe and unpleasant, that a 'weed' harvesting scheme had to be introduced to remove accumulations which were rotting on the shore.

As part of our estuary management role, we are working to better understand the timing and triggers of these nuisance macroalgal blooms, and their impact on the affected systems as a whole.

The frequency and severity of macroalgal blooms is an indication of estuary ecological health, and an important consideration in the development of our estuary management plans and water quality improvement plans.

Read more about macroalgae on our Healthy Estuaries WA website.