There is no single factor which causes an algal bloom. A combination of optimum factors such as the presence of good nutrients, warm temperatures and lots of light all encourage the natural increase in numbers of blue-green algae in our waterways. Nature mostly takes care of the temperature and light, but the increased presence of nutrients such as phosphorous is largely due to poor farming practices such as high use of fertilizers and presence of livestock near water supplies, as well as effluent and run-off from towns and cities near waterways. The ponding of water and reducing river flow rates tends to improve the light and sometimes the nutrient environment for algal growth making water turbulance a major factor in bloom develpment. Pesticides and other chemicals may affect the natural grazers which would otherwise control algal growth and their presence increases the risk of blooms.

A blue-green algal bloom in a billabong near Gundagai. The billabong was heavily degraded by stock and devoid of aquatic plants, both contributing to the problem. Fencing off the area from stock is essential for the well-being of the stock, and is an important management strategy for this billabong. (Photo courtesy of James Maguire, DLWC)



NUTRIENTS - Presence of excessive nutrients (eutrophication) is the basis for the rapid growth of blue-green algae. Phosphate is the key nutrient but nitrogen is also important. Nutrient presence is enhanced by human activities:

  • Farms - run-off from agricultural areas,
  • Deforestation - contributes to erosion which increases run-off of nutrients and top soils.
  • Urban - point sources of pollution (sewerage, septic systems, stormwater drains, industrial effluent outfalls etc.)

Reduce the nutrient in-flow into water sources.

  • Farms - avoid run-off from fertilizers, pesticides and stock. Reduce the amount of fertilizer used, particularly phosphate. Fence dams, creeks and rivers off from stock where possible.
  • Deforestation - plant trees and bushes. Increase vegetation around farm dams and river banks. Fence off dams and creeks to allow grass and vegetation to re-grow.
  • Urban - Reduce sewerage in-flow through use of water treatment programs e.g. sedimentation ponds and wetlands. The Urban Stormwater program has been implemented by the NSW EPA to reduce waterways pollution through improving urban stormwater management.

LIGHT - Algae need light for photosynthesis in order to produce the energy they need to grow.

It may be possible to cover a small water body such as a pond or dam to block out the light. Mixing the water increases the turbidity and thus reduces the light available to the algae. Use a pump or outboard motor to mix water. This prevents algae collecting and growing at the surface where it is warm and they have light . Trees around a dam can shade the dam to some degree. It may not always be practical to change the light regime.

TEMPERATURE - Algae grow best at warmer temperatures which is why most blooms occur in summer.

Mix the water to prevent the top layer heating up too much. Adding cooler water from a creek or river may lower the temperature (Beware -do not let contaminated water out of the dam into a creek or river).

FLOW CONTROL - Changed flow regimes are the cause of algal blooms in many rivers and creeks. Changed flows allow less water for dilution of nutrients, reduced turbidity and longer standing (retention) times which contribute to more favourable conditions for algal growth (stratification) which sees the layering of the water with a warmer layer at the top, in which algae are able to grow close to the light.

Environmental flow releases have been incorporated into the water sharing plans under the Water Management Act 2000. At the catchment level, environmental flows are permitted in order to maintain the normal ecosystem while at the same time not causing adverse effects to water users.

In a water body containing high nutrient levels (not blue-green algae); if water of low phosphorous concentrations is available in sufficient quantities it can be used to flush the dam/lake to reduce the nutrient levels.

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? 2002. NSW Murray Regional Algal Coordinating Committee, MRACC. Unless otherwise specified, maps and images are copyrighted to Department of Land and Water Conservation, NSW.