Gardening practices have an important influence on the quality of water in our streams, creeks and bays. Australian soils and waterways are generally low in nutrient content, and consequently the organisms living in our waterways have adapted to low nutrient conditions. If nutrient levels increase above normal, Australian aquatic plants and animals can be affected in several ways.


Sources of nutrients in our waterways related to gardening can come from:

  • Plant matter (cuttings, leaves, grass clippings)
  • Garden fertilisers
  • Ash from fires

When it rains a certain amount of water is absorbed into your garden, but a significant amount ends up in our drains and waterways as stormwater runoff. As it moves through your garden, runoff picks up fertilisers and ash and carries plant matter into our waterways. Anything that you sweep into the gutter: leaves, grass clippings etc. washes directly into our waterways via drains. The plant matter rots to release nutrients and utilises oxygen at the same time. Fertilisers sprayed and sprinkled around your garden are picked up in a dissolved state and are readily absorbed by aquatic plants, potentially leading to excess growth and algal blooms.

Too many nutrients in a waterway, particularly nitrogen and phosphorus, can lead to:

1) Excessive Plant Growth: which can lead to the choking of waterbodies. Fluctuations in dissolved oxygen levels can affect oxygen-consuming fish. Reduction of light penetration can affect other plants. If the plants die, this can threaten the survival of planteating fish and other organisms.

2) Blue-green Algal Blooms: although this type of algae (cyanobacteria) occurs naturally in our waterways, excessive nutrients can be a major contributing factor to an outbreak. Many blue-green algae release toxins that can be fatal to wildlife, stock and domestic animals and affect human health.


Toxins are substances, which at certain concentrations are poisonous to living things. Some toxins can accumulate hundreds or thousands of times in fish, shellfish and fish-eating birds, even though the concentration in water is not directly toxic or detectable. Toxicants can kill organisms, weaken an organism’s ability to fight disease or interfere with life cycle development and reproduction.

Sources of toxicants in stormwater related to gardening can come from:

  • Garden pesticides
  • Garden insecticides
  • Garden herbicides
  • Garden fertilisers
  • Treated timbers


Pathogens are microscopic organisms and include viruses, bacterium, fungi and parasites. They are common and widespread with some occurring naturally in soil and water. Pathogens can cause disease in plants, animals and humans. To gauge water quality we measure levels of the bacteria E.coli as an indicator of the presence of human gut pathogens in water.

Sources of E. coli relating to gardening include:

  • Garden fertilisers
  • Garden manure

Sustainable Gardening Australia asks you to consider chemical alternatives or use low environmental impact chemicals, slow release fertilisers and use only the amount recommended by the manufacturer. Try not to apply these chemicals before heavy rain or on windy days.