Understanding the impact of farming on aquatic ecosystems
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Organic materialOrganic material can enter water bodies by either point or diffuse sources with the main pathways being: runoff and leaching from slurries and manures applied to land; leakages from slurry or silage effluent stores; dirty water discharges from farm areas; use of dirty water for irrigation; and by direct inputs (faeces) from animals. About 90 million tonnes of livestock manures are collected from housed animals in England each year and spread to land.131 This represents around half of the material deposited to land each year; the remainder is deposited directly by grazing livestock. There is no known information on the total loads and source apportionment of organic material in surface waters of England and Wales. Inputs of organic material to water courses from agricultural sources are generally untreated and can be acute in the case of leakage from, or catastrophic failure of, slurry pits and farmyard manure storage facilities. By contrast, inputs from sewage treatment works are treated and tend to have low-level chronic effects. Milk, silage effluent, pig slurry and cattle slurry have typical biochemical oxygen demand (BOD) values of 100,000, 65,000, 25,000 and 17,000 mg l-1 of oxygen, respectively, all far higher than that of raw domestic sewage (300 mg l-1).132 Organic material is often associated with other pollutants that can compound its impact on aquatic ecology. Material derived from animal manures and slurries may contain faecal pathogens, ammonia and residual nitrate, phosphate and veterinary medicines. Some types of manure may also contain heavy metals such as from pigs and poultry which contain elevated levels of zinc and copper that may accumulate in soils to later be leached into water courses having direct toxicity effects on the receiving ecology. The EA WFD Article 5 risk assessments for rivers and lakes did not consider point or diffuse sources of organic material arising from agricultural activities. However, the risk of organic enrichment of transitional and coastal waters was assessed. The assessment considered the contribution of organic matter from riverine loads that would have included (but not quantified) inputs from farming. The results indicated that 13.4% (by area) of transitional water bodies and 0.2% (by area) of coastal water bodies were at risk: the ‘at risk’ water bodies were geographically spread around England and Wales.133 The greatest impacts on ecology occur where direct inputs of organic material enter watercourses due to storage failures, leakages etc. There were 55 such serious (category 1 and 2) water pollution incidents involving organic materials in England and Wales in 2006.134 Of these, slurry accounted for 75% of the incidents and other agricultural sources accounted for the remaining 25%. BOD is used as an indicator of organic material and is measured in rivers across the UK as part of the Harmonised Monitoring Scheme and in England and Wales for the chemical General Quality Assessment (GQA) Scheme. The results from the HMS in 2005 indicated that the average BOD of rivers in England and Wales was 1.6 mg l-1, with the highest values in the North West (2.4 mg l-1) and North East (2.3 mg l-1) regions and the lowest values in the South West (1.2 mg l-1) and Welsh (0.9 mg l-1) regions.135 BOD, dissolved oxygen and ammonia are used in the EA’s chemical GQA scheme for rivers which is used as a general indicator of organic pollution of rivers. Data from 2006 indicate that 69.5% of rivers (by length) were of good quality in England and Wales with the highest quality occurring in Wales (95% of rivers) and worst in Anglian region (47.4% of rivers).136 The results also indicate a general improvement of river quality since 1990. In terms of the proposed standards for BOD for implementation of the WFD, 18.7% and 3.7% of river water bodies (by length) in England and Wales, respectively, would be less than good ecological status, based on recent monitoring data.137 Biodegradation of organic material produces a high BOD and results in deoxygenation of the water column and sediment. Low dissolved oxygen levels can selectively remove the more pollution sensitive invertebrate species such as stonefly nymphs while encouraging the productivity of pollution tolerant organisms such as oligochaete worms (Tubificidae), midge larvae (Chironomidae) and bloodworms. In heavily polluted waters, the establishment of monocultures can occur whereby tubificid worms can become the dominant or sole species. The native white-clawed crayfish is particularly vulnerable to high BOD, becoming stressed when oxygen levels fall to below 5 mg l-1, and is vulnerable to spillages of materials with high BOD such as cattle slurry.138 Decreased oxygen levels can inhibit the passage of migratory fish such as Atlantic salmon and sea trout, as both these species have a high oxygen requirement.139 Early life stages, in particular eggs, are highly sensitive to low oxygen levels. In addition to its deoxygenating effects, organic material can cause physical degradation of the aquatic environment. The deposition and biodegradation of organic material on lake bottoms may produce a thick, rich anoxic mud, reducing the suitability as a habitat for infaunal and benthic organisms. Moreover the rich anoxic sediment may continue to provide a supply of nutrients to the water column even if the source of pollution is reduced. Deposition of organic material on beds and banks can cause de-stabilisation and reduce habitat suitability for rooted plants such as brook water crowfoot.140 Organic wastes can also contain some solid material, which can increase turbidity and reduce light penetration. Photosynthetic organisms such as algae and macrophytes may be eliminated at high concentrations. Deposition of solid material in organic waste can occur on streambeds, altering the substratum for the benthic communities and producing impacts similar to that of soil sediment. The effects of organic material in transitional and coastal waters are similar to those observed in freshwaters particularly in sediment deposition areas, although the capacity of transitional and coastal waters to dilute and disperse organic material is generally much greater. Because of the associated faecal pathogens, the effects of organic material in coastal waters are of more concern for human health risks in bathing waters and due to the contamination of shellfish. However it is possible that the deposition of organic material onto the beds of estuaries may alter the grain size distribution as a result of flocculation, reducing habitat suitability for certain organisms. Deposition of organic material onto hard rocky substrates will have an effect similar to that of soil sediment. Download 432.61 Kb. Share with your friends:
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