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Algal blooms

What are algae?

Algae range from microscopic (microalgae) to large seaweeds (macroalgae). They are usually thought of as simple aquatic plants which do not have roots, stems or leaves and have primitive methods of reproduction. Algae are a natural part of the rivers and estuary and an important part of the ecosystem. They provide habitat and a primary source of food to complex foodwebs, and oxygenate the water during daylight hours. Algae may attach to a substrate (e.g. rocks, shells, seagrass) or be free floating. Different types of algae occur in different parts of the system and at different times. Macroalgae tend to be most abundant in the shallows of the lower parts of the estuary where there is more light throughout the water column.

Algal blooms

Like most plants, algae need warmth, light and nutrients to grow, but when increased nutrients or other factors alter the balance of the ecosystem, algae can become abundant and algal blooms occur. Phytoplankton blooms can be seen as discoloured water, including the possible formation of surface scums.

Most algal blooms are harmless, but they can cause problems. Macroalgae tend to accumulate in thick mats in shallow water and wash up on to beaches where they rot and give off hydrogen sulphide, or rotten-egg gas, which has an unpleasant odour. Decomposition of micro or macro-algae in the water, or night-time respiration of dense accumulations, can cause low oxygen conditions which can result in nutrient release from the river sediments. These conditions may favour harmful algal blooms.

Harmful algae

Some species of algae are considered harmful because they can pose a health risk to humans or other animals. In the Swan Canning Riverpark, there are two common pathways of risk to humans:

  • Direct - through contact with or swallowing water affected by harmful algae. This may lead to skin, ear, respiratory or intestinal complaints
  • Indirect - through eating shellfish (e.g. mussels) that have accumulated biotoxins in their edible tissue through filtering harmful algae. The Department of Health warns against the consumption of wild shellfish from the rivers and estuary because it cannot guarantee the quality of the shellfish

Monitoring and reporting on algal blooms

Parks and Wildlife keeps a watch on conditions in the river, including microalgae and the abundance of harmful algal species, through an extensive water quality monitoring program. Weekly microalgae activity reports keep the public informed of algal levels in the Swan Canning Riverpark.

The Department of Health will close off areas to swimming or fishing if it is deemed appropriate based on information provided to them by Parks and Wildlife or their own public health monitoring program.

If you notice any unusual discolouration or scum in the Swan or Canning rivers or significant numbers of dead fish, please report it to Parks and Wildlife on 9219 9000 or after hours on 0419 192 845.