What is nutrient enrichment?
Nutrients are a necessary and natural requirement for the growth of flora and fauna in and around waterways. The levels of nutrients will determine the extent to which living organisms can survive and multiply. Anyone paying attention to their lawn or a vegetable garden will understand that Nitrogen (N) and Phosphorus (P) are two vital nutrients that are necessary for growth. The N and P are available to plants when they are bound in chemical substances as organic or inorganic substances. Pure P does not occur naturally because it is very reactive, but N is the main component of the air we breathe and is much less reactive. Nutrient enrichment does not refer to a healthy aquatic environment but one that is overloaded with nutrients.
Why does it happen?
Nutrients enter waterways from a variety of sources. For example they can be attached to sediment eroded from the topsoil or from stream banks. They may be washed directly from paddocks after being applied as fertiliser. They may be recycled from mud and sediment in the bottom of pools and lakes, when the conditions are right. They may even blow in on the wind. Electrical storms are known to generate nitrates that come down in thunder showers.
Sources may be diffuse, ie only small amounts come from any one location but over a broad area they may produce a large cumulative effect. An example is fertilisers generally dissolving in the rainfall and percolating through the soil over the full length of a stream. There may also be 'point sources' which are much more localised and concentrated. For example a piggery effluent pond overflow. During heavy rains a large quantity of effluent may be discharged into a creek in a short period of time. A cattle crossing on a stream may be a significant point source if stock spend much time there.
When the levels of nutrients in a waterway rise above the point at which the naturally diverse range of species of living things can coexist, we say that nutrient enrichment is occurring. The term enrichment may suggest that things get better but the context here is that enrichment will cause a balanced system to break down and produce unwanted degradation of the waterway and the water quality. In these conditions various less favourable organisms begin to dominate to the exclusion of others. The resulting conditions may lead to an algal bloom, a rampant growth of one or more organisms that may become toxic to other creatures and even to the aquatic vegetation. Blue-green algae blooms are a well known example of this process.
The relationship between sediment and phosphorus input is well known. Reducing the erosion of sediment from the land surface will also reduce the risk of nutrient enrichment occurring.
Another term used is the 'limiting' capacity of a nutrient. If one particular substance controls the production levels more than others the system is said to be limited by that substance. Phosphorus has been shown to be a limiting factor in the growth of some algae while in some systems Nitrogen may be the key limiting nutrient. In agricultural production the growth enhancing effect of small amounts of trace elements has been recognised for many decades.
In ecological systems a large number of factors interact to produce complex changes and processes. A specific system may be able to tolerate a certain shift in the nutrient supply without drastically alterating, but serious outcomes may occur when a threshold is passed or if many factors are simultaneously changed.
What is being done about it?
Up until the 1960's people expressing concerns about the state of the waterways were considered 'doom and gloom merchants', but with the growing degradation of our environment and its unfavourable economic impact there is increasing doom and gloom in many places.
Land and water care have become major issues over the past two decades and the actions taken by the government and the community will prove to be pivotal to the health of the Australian landscape in the 21st century.
How can the community help?
It is community groups and individual landowners who will ultimately determine the workability of water resource management. Government agencies can coordinate a broad approach to the issues and conduct specialised investigations. Environmental degradation does not recognise property or district boundaries and communities are closest to the practical on ground issues and the economics involved in rectifying problems.
The community can help by recognising that the approach to water resources preservation and management, used in past years, will not solve the problems that have been created. New and innovative thinking is required. A partnership between the community, specialists and water resource managers dedicated to positive ongoing communication will provide the best springboard for good outcomes.