Eutrophication: bloom and burst

Agriculture has been very successful in growing food for an ever-increasing population. This success is largely the result of widespread use of synthetic fertilisers.

In Western Australia, very little geological activity has taken place for millions of years. The soils of Australia are ancient compared with those of other areas of the world. They are the product of millions of years of erosion and leaching and are, as a consequence, low in fertility. It has been essential for farmers to apply phosphorus to the soil to obtain profitable crop yields; in fact, to obtain any yield at all on some soil types. Some of the phosphorus, along with other nutrients, is leached out of the soil and finds its way into rivers and estuaries.

The term eutrophication refers to the ecological changes that result from the overloading of water bodies with nutrients. Nitrogen and phosphorous are the most important of the nutrients causing eutrophication. Algal blooms need phosphorous and nitrogen to occur. Phosphorous is the main nutrient causing algal blooms in most inland waters in the south west of WA. Generally, sandy soils do not hold nutrients and allow direct leaching of dissolved nutrients into watercourses. Sandy soils that have been artificially enriched with phosphorous can contribute to the eutrophication of a wetland or estuary. In other areas where soils have high clay content the nutrients are bound to the clay particles. Soil erosion plays a leading role in eutrophication when the nutrient enriched clay particles are washed into watercourses as sediment and release the attached nutrients.

Many estuaries, rivers, reservoirs and lakes around Australia suffer from eutrophication. The Select Committee into Land Conservation (Legislative Assembly, 1991a) reported that in the south west of Western Australia only seven estuaries out of twenty two had low nutrient levels. The effects of eutrophication have increased since the early 1970's, including:

• The fouling of beaches with macroalgae;
• The loss of important habitat;


• Algal blooms in river and wetland systems and a decline in water quality; and


• Significant losses of wild life, including large fish kills, mortality of entire ecosystem components and loss of some ecosystem functions.

The Peel-Harvey estuary, Cockburn Sound, Whitford's lagoon, Denmark's Wilson Inlet and Albany's Princess Royal and Oyster Harbours are all examples of waters in Western Australia that are affected eutrophication.

The Peel-Harvey estuary

The Peel-Harvey estuary is one of the largest along the Western Australian coastline. It stretches from the Harvey River delta to the Mandurah Ocean Channel and holds up to 150 million cubic metres of water. Although the estuary has a 133 square kilometre surface area it is quite shallow and rarely reaches over 2 metres in depth. It receives water from Serpentine, Murray and Harvey rivers. In addition 2000 kilometres of drains from the surrounding catchment area discharge into the estuary. In a wet winter, a million cubic metres (a million tonnes) of water flows into the estuary. In a dry winter the flow is around a quarter of a million cubic metres, which is still a significant amount of water.

The estuary is separated from the ocean by a series of dunes and limestone. White sandy beaches stretch along the coastline. Offshore, the seabed is a combination of reef and sand supporting a variety of marine algae, fish and small invertebrates such as crustaceans and worms.

Since the 1950s there has been a six-fold increase in the amount of phosphorus reaching the estuary. This extra quantity of an otherwise scarce element causes an algal bloom: the rapid growth of one type of alga. Decay processes involving respiration take place when algae populations crash and die. Respiration removes dissolved oxygen from the water and results in the death of aquatic organisms. It also produced the offensive odours familiar to people living in Mandurah.

What can we do about eutrophication?

Phytoplankton blooms and excessive growth of macroalgae which have been observed since the 1960's have had detrimental effects on the estuarine ecology and human use of the estuary. A channel was cut between the estuary and the ocean to improve flushing and water circulation within the Peel-Harvey estuary. Improving the tidal exchange between the ocean and the estuary resulted in a more constant saline environment in the estuary, improved water quality and has made conditions unfavourable for the toxic phytoplankton Nodularia which flourishes in fresh to brackish water conditions.

Flushing nutrients to the ocean

The principal aim of constructing the Dawesville Channel was to increase the flushing of nutrients from the estuary into the ocean. Once the channel was open (April 1994), it was necessary to monitor the movement of the estuary water into the ocean. The Department of Transport's nutrient monitoring program shows that water leaves the estuary and forms a clearly defined plume, extending approximately 2 kilometres offshore and 3 kilometres north of the channel.

In the first year following the opening of the channel, 100 to 120 tonnes of total phosphorous and 900 to 1100 tonnes of total nitrogen were flushed from the catchment into the ocean via the Peel-Harvey estuary. This is approximately the amount of nutrient that would normally enter the ocean during average rainfall years, when the only entrance to the ocean was the Mandurah Channel. A monitoring program is being undertaken to determine what effects the higher amounts of nutrients will have on the ocean environment.

How is the beach changing?

Natural processes of marine sediment movement are quite complex. In winter, more marine sediment is moved offshore by the winter storm waves than is deposited onto the beaches. In summer the opposite is true, with beaches and nearshore banks building up sediment. In addition some marine sediment moves up and down the coastline depending on the direction of the currents in the form of littoral drift.

The challenge for the Peel-Harvey Catchment is to have only sustainable forms of land use. These will maintain production and profits but avoid the pollution of the water. Co-operation of all landholders is needed. Co-ordinating the use of land by all landholders is required and is called 'integrated catchment management'. This strategy is the only effective way to deal with eutrophication problems in the long term. The Dawesville Cut may have been a solution in the short term but sustainable land management practices should be adopted to prevent additional nutrients reaching the ocean. In the case of the Peel-Harvey estuary, part of the strategy involves modified use of fertilisers. Farmers are advised when and how much fertiliser to spread and they are encouraged to use a different, less-soluble phosphate fertiliser to match phosphorus 'release' with plant growth, thus reducing the amount of phosphorus reaching the estuary. The use of Tasmanian bluegum plantations has been proposed to lower water tables, reducing the volume of water reaching the estuary and intercepting more of the 'free' phosphorus.

The regulation of certain agricultural industries, such as piggeries and animal feedlots, can play a role to ensure that these industries are placed where they will not contribute to eutrophication. Horticultural landuse zones, where large amounts of fertiliser are required, needs to be carefully positioned within the catchment and to have proper nutrient and irrigation management. Urban development is accompanied by sewage systems that need careful monitoring. Land capability studies are carried out to determine where housing development can proceed without the risk of increased eutrophication. Land use planning legislation is then implemented to regulate use.

References:

Davies-Ward, Edwina and Finlayson, Rob (Ed.), (1997) Environment Western Australia 1997 Draft State of the Environment Report for Western Australia, Department of Environmental Protection: Perth, WA.

Legislative Assembly (1991a) Select Committee into Land Conservation Discussion Paper No. 1, South West Region of WA, Legislative Assembly: Perth, WA

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