Water quality was one of the first environmental pressures relating to sustainability to affect farmers. It is a long time ago since we began to hear about eutrophication and other matters relating to the loss of nitrogen and phosphorus to water. And we can see the consequences of eutrophication for ourselves.

Clean water is essential for life and for our environment. And clean generally implies largely free from nutrients, pesticides, bacteria, etc.

Eutrophication is predominantly caused by human actions and agriculture is just one cause. Loss of nitrogen and phosphorous to water also occurs from the use of fertilisers on lawns, golf courses and the disposal of sewage into water bodies.

We often see it affecting stretches of local streams and lakes but I have seen the sea turn green from this problem.

Pesticides

Water quality is not just confined to surplus nutrients and we have seen frequent reporting of the presence of some pesticides, predominantly the herbicides MCPA and 2,4-D, in surface water. Again, farming is being blamed for this and it is a contributor, given the obligation being placed on farmers to control rushes on grassland to keep land eligible for aid. While agriculture is attempting to address the problem through ensuring that application equipment meets the required standards and that sprayer operators are trained, there is a growing realisation that a proportion of this contamination is arising from non-professional use of these actives in gardens and other amenity areas.

Other pesticides more commonly used in cropping are occasionally found in water at low levels. However, our varied cropping structure and its distribution within grassland helps dilute the implications. Obligatory buffer zones are used for each pesticide product and the use of buffer strips by watercourses is also helping to keep more pesticides away from water.

Eutrophication

Eutrophication presents a serious ecological problem in open water bodies such as lakes and reservoirs. We see it as dense algal and plant growth which is being driven by the enrichment of the water with phosphorus and nitrogen. These nutrients encourage the growth of aquatic plants which use photosynthesis and hence we see the formation of things like algal blooms and water hyacinths. Nutrients from animal manures, and fertilisers used in agriculture and elsewhere, and sewage, can travel via surface runoff into water bodies. However, it is important to state that eutrophication can also occur naturally over long periods, as water bodies fill with sediment.

Sources

1. Point source: This is where nutrient loadings can come from individual points such as a discharge from a sewage treatment plant, industrial plants, fish farms or a burst slurry tank.

2. Non-point source: This implies loss of nutrients from widespread human activities that have no specific identifiable point of discharge or entry into watercourses. This includes things like nitrogen and phosphorous lost from fertilised agricultural land but also atmospheric deposition of nitrogen.

The three main sources of nutrient deposition are:

  • Erosion and leaching from farmland.
  • Sewage from cities and industrial wastewater.
  • Atmospheric deposition of nitrogen.
  • While nitrogen will move in water, phosphorus is moved by water. Phosphorus is generally attached to clay and is moved as the sediment in brown water washed off fields following heavy rain.

    Effects of Eutrophication

    When nutrient levels increase in water, phytoplankton and other photosynthetic plants grow rapidly and produce algal blooms. They then limit the amount of dissolved oxygen available in the water. And when these algae die and decay, the oxygen level in the water is further depleted resulting in the possible suffocation of marine life.

    These and other aspects cause a deterioration in water quality and its possible loss as a source of clean drinking water. And it obviously affects the attractiveness of that water body as a recreational amenity.

    Prevention

    Limiting total fertiliser use (organic and artificial) on farmland is seen by many as the best way to reduce the risk of nutrient loss over time. Ironically, reducing nitrogen loading on land is also seen as a way to minimise other forms of nitrogen loss, such as ammonia, nitrous oxide and leaching.

    Many see the strengthening of regulations around non-point source pollution as being important to help control eutrophication, as this is thought to be the biggest source of nutrient loss.

    Placing barley straw into water bodies inhibits algal growth. This has been known for many years and was tested by Teagasc back in the 2000s. Barley straw has been used in different ways, which vary from bales being dropped into rivers and streams feeding into static water bodies, to rows of bales placed into lake systems.

    It is still not clear why, or how, barley straw suppresses algal growth but it is accepted that it does this when used at the correct rate. Teagasc puts the optimal use rate of barley straw at 10g/m2 for areas with average problem intensity and 25g/m2 for water areas with severe algal problems.

    Curative actions

    Where eutrophication is present, “curative” actions can be considered but not every solution may be practical or cost-effective. Procedures can be implemented, such as:

    Dredging: The removal by drainage of the top 10-20cm of sediment that tends to be high in phosphorus will help reduce that supply source in the water. Phosphorus mainly resides in sediment and removing some of this may help reduce levels in slow-moving rivers or ponds, but this would be more difficult in a bigger lake.

    Oxygenation: Adding oxygen to a water body would help to reduce some of the negative effects of eutrophication but it would not reduce the nitrogen or phosphorus levels. However, it would help to restore the ecological condition of the water and prevent the loss of the marine life present.

    Oxygen is added by increasing contact between air and water.

    This might be done by pumping air into the water, having a working fountain, installing a waterfall feature at the entry to the water body or, in bigger lakes, active use of propeller boats and jet skis will help add oxygen.