5.3.1 Eutrophication caused by fertiliser excesses in run-off
Excess nutrients in agricultural run-off can cause eutrophication
Source: BBC GCSE Bitesize
Agricultural run-off containing nitrates and phosphates from excessive fertiliser use can lead to waterways (both freshwater and marine) becoming enriched with nutrients, beyond levels that can be absorbed or dissipated by the natural system. This enrichment can promote algal blooms. These may directly damage ecosystems through the release of toxins, or prevent sunlight reaching aquatic plants growing in deeper water. These plants are then unable to photosynthesise, and so die and decay. The decay process uses up oxygen, leading to hypoxic (low oxygen) conditions, causing further death and decay of aquatic organisms. This process is called eutrophication.
Global prevalence and impact of nutrient excesses and resulting eutrophication
Sutton et al. (2013)
The Our Nutrient World report summarises research into global nitrogen (N) and phosphorous (P) flows between 2000 and 2010. The cited studies find that:
- There is a 120 million tonnes per year surplus of N in agricultural soils.
- 95 million tonnes of N and 2-7 million tonnes of P enter freshwater systems (aquifers and rivers) from agricultural systems each year.
- 40-66 million tonnes of N and 9 million tonnes of P enter the ocean from rivers each year.
The UK as an example
It is estimated that approximately 60% of nitrates and 25% of phosphorus in UK water bodies has agricultural origins (Watts, et al., n.d.).
“Currently, only 24% of surface water bodies in England and 36% of surface water bodies in Wales meet ‘good ecological status’ as defined by the Water Framework Directive. 22% of water bodies achieve good status in Northern Ireland and in Scotland 65% of water bodies are deemed good or better, but for the 35% which are failing, agriculture is deemed to be a major pressure.” (Watts, et al., n.d.)
The contribution of aquaculture
Data concerning the contribution of aquaculture to the problems of excessive nutrients are more difficult to find, as aquaculture is usually considered to be on the receiving end of water pollution problems or as a part of possible solutions (see for example the study by Rose, et al. (2014) discussing the potential role of shellfish aquaculture in managing nitrogen in coastal waters). However, some studies have sought to quantify aquacultural contributions to marine or freshwater nutrient loading. One study (Karakassis, et al., 2005) reported that fish farming contributes less than 5% of anthropogenic nutrient addition to the Mediterranean, which experiences an annual increase in N and P of just 0.01%, making the contribution of fish farming in this case arguably negligible. For a more detailed discussion of the contribution of aquaculture to excessive nutrients in the environment, see the Olsen, Holmer and Olsen (2008) report by the Norwegian Seafood Research Fund (FHF).
5.3.2 Pesticide contamination
- Pesticides (insecticides and herbicides) sprayed onto fields can accumulate in sediments that become washed into water bodies.
- Interstitial waters (water trapped in sediments or in pores in sedimentary rocks) can become particularly concentrated with pesticides.
- One study has shown that such interstitial water contamination can inhibit photosynthesis in microalgae, suggesting the ecological impact that pesticide contamination can have.
- Pesticides also pose a toxicity threat to both humans and wildlife. For example, pesticides are a known source of arsenic in soils and ground waters, compounding problems of naturally occurring arsenic in rocks, the accumulation of which in food systems is a serious health threat, affecting ~130 million people worldwide.
Magnusson, et al. (2013)
The UK as an example
Although only very small percentages of surface or ground water bodies in the UK fail to meet ‘good status’ requirements as a result of pesticides, 15% of Drinking Water Protected areas in England and Wales are at risk of failing standards, owing to very stringent drinking water safety standards (Watts, et al., n.d.).
China as an example
In recent years, excessive fertiliser and pesticide usage in China – and their impacts – have come under intense scrutiny. For a general discussion of soil and water pollution in China, considering both fertilisers and pesticides together, see the following news articles:
- Meng, Y. (2012) The damaging truth about Chinese fertiliser and pesticide use. Chinadialogue [online].
- Watts, J. (2010) Chinese farms cause more pollution than factories, says official survey. The Guardian [online].
- Patton, D. (2015) China farm pollution worsens, despite moves to curb excessive fertilisers, pesticides. Reuters [online]
For more specific research pertaining to China’s fertiliser and pesticide situation, see:
- Sun, B., Zhang, L., Yang, L., Zhang, F., Norse, D. and Zhu, Z. (2012) Agricultural non-point source pollution in China: causes and mitigation measures. Ambio, 41(4), 370-379.
- Li, H., Zeng, E.Y., and You, J. (2014) Mitigating Pesticide Pollution in China Requires Law Enforcement, Farmer Training, and Technological Innovation. Environmental toxicology and chemistry / SETAC 33 (5), 963–71.
Statistics concerning China’s pollution can also be found in Chinese here.
5.3.3 Sediment and silting
Sediment and silting
Agriculture is a major source of excess sediment in waterways (e.g. it is estimated that 75% of sediment that is polluting water bodies in the UK is as a result of farming). This can be caused by upland drainage management practices that lead to greater volumes and rates of runoff into waterways; this may cause faster river flows and subsequent increased downstream erosion of river banks, producing excess sediment. Along with the eutrophication and toxicity associated with chemicals contained within sediments, sediment can act as a physical pollutant by:
- increasing the turbidity of the water (i.e. making it cloudier) reducing the depth to which light can penetrate, with negative consequences for photosynthetic plants and algae in the water and ecosystems they support.
- leading to silting, i.e. deposition of sediment on the riverbed, which changes the dynamics of the water flow. This increases the risk of flooding as well as potentially creating blockages for human users of the waterways.