Understanding the impact of farming on aquatic ecosystems
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AmmoniaMajor sources of ammonia include livestock farming such as pig and poultry farms. Other sources of ammonia emissions include direct volatilisation from mineral fertilisers, animal manures, agricultural crops and a wide range of non-agricultural sources including sewage, catalytic converters, wild animals, seabirds and industrial processes.59 Ammonia can enter water bodies via a number of pathways including atmospheric deposition, run-off from land, leaching and direct excretions from animals. Ammonia deposited from air to land is rapidly oxidised to nitrate. Ammonia is the least stable form of nitrogen in water and is easily transformed to nitrate in oxygenated waters. It is also readily converted to nitrate and nitrite in soils where it is applied in fertilisers. As a result a small proportion of the nitrogen that is lost to water bodies is in the form of ammonia. Deposition of ammonia can lead to terrestrial and aquatic eutrophication and acidification. The source of ammonia and the pathway by which it reaches the water body can have an important bearing on the degree of ecological effect. The greatest ecological impact will occur where livestock have direct access to water and are able to defecate directly into streams, rivers and lakes. As this direct input of ammonia will be highly concentrated it will be unlikely to be converted to nitrate quickly. Leakages of slurry pits and farmyard manure storage facilities also pose a significant risk if there has been insufficient storage time before the spillage to allow the conversion of ammonia to nitrate. By comparison, the concentrations of ammonia in discharges from sewage treatment works are generally low following treatment and are quickly converted to nitrate. Losses of ammonia from farmland will be lower where slow movement of ammonia through cracks and spaces in soils promotes conversion to nitrate and the adsorption and retention of ammonia in the soil. Consequently, ammonia contained in organic material and slurries applied as fertiliser is more likely to be converted to nitrate before being leached from the soil. Ammonia losses from farmland are also dependent on the season and meteorological conditions at the time of application of fertilisers and manures. For example, there is a greater risk of volatilisation of ammonia from fertilisers and loss to the atmosphere during the summer months. If heavy rains follow application of fertilisers to fields then a large percentage will be lost to watercourses as surface run-off and is likely to be accompanied by sediment and other organic material. It has been estimated that the UK released 263.4 kt of ammonia to air, water and soil in 2005 and that agriculture accounted for 80% of emissions60 with cattle alone accounting for 44% of the total.61 These emissions arise from livestock housing, the storage, treatment and application of all types of animal manures and the use of inorganic fertilisers. Ammonia emissions are highest in western England and East Anglia, where the dominant sources of ammonia are cattle, pigs and poultry. Emissions are lowest in upland areas of northern England and Wales. There has been a steady decline in atmospheric ammonia emissions from agriculture since the early 1990s largely due to declining livestock numbers and the use of nitrogen based fertiliser. Predictions indicate that this decline will continue due to the influence of the Common Agriculture Policy (CAP) reform and IPPC Directive regulations on both livestock numbers and management practices.62 However, it is predicted that atmospheric ammonia emissions will be the largest contributor to acidification, eutrophication and secondary particulate matter at the European level by 2020 partly reflecting the success of European polices to reduce SO2 and NOx emissions.63 Atmospheric ammonia emissions are affected by a large number of factors that make the interpretation of experimental data difficult and emission estimates uncertain64. The total annual discharge of ammonia to sea from riverine and direct discharges in England and Wales has declined steadily from 65 kt yr-1 in 1991 to around 40 kt yr-1 in 2003.65 Sewage discharges contribute the majority of ammonia to UK coastal waters. Riverine inputs, which reflect both diffuse and point land-based sources, have also shown a slight decline and continue to account for around 20% of total inputs. Atmospheric deposition of nitrogen (including ammonia) to surface water bodies (excluding marine waters) has been estimated to be only a small proportion (~0.4%) of the total load.66 However, in lakes with large surface areas compared with their total catchments and marine waters, atmospheric deposition of nitrogen can constitute a significant part of the total inputs.67 For example it has been estimated that atmospheric deposition accounts for 25% of total nitrogen inputs into the Baltic Sea. The EA’s WFD Article 5 risk assessment only included point source discharges (e.g. sewage treatment works and industrial discharges) of ammonia to rivers, transitional and coastal waters and did not assess the risk of diffuse sources of ammonia. In terms of the effects of atmospheric emissions, the EA risk assessments indicated that 21% (of 432) of lakes (16.4% by area) and 2.9% of river water bodies (2.9% by length) are at risk from acidification: the assessment included the deposition of ammonia with NOx and sulphate. In 2005, the average concentration of ammonia in UK waters was 0.12 mg l-1 N, with the highest concentrations being found in the North West (0.43 mg l-1 N) and North East (0.20 mg l-1 N) regions and the lowest concentrations in the South West (0.07 mg l-1 N) and Welsh (0.04 mg l-1 N) regions.68 Ammonia occurs naturally in surface waters but at elevated concentrations it can become toxic to aquatic organisms: the un-ionised form (NH3) is very toxic to aquatic life whilst the ionised form is virtually non-toxic (NH4+).69 Current environmental quality standards (EQSs, expressed as annual average concentrations) for the protection of freshwater and saltwater fish are 0.015 mg l-1 NH3-N and 0.021 mg l-1 NH3-N, respectively indicating that saltwater fish generally have a higher tolerance to ammonia than freshwater fish. Standards for total ammonia (expressed as 90%ile concentrations) have also been proposed for the classification of ecological status under the WFD: these are 0.3 mg N/l and 0.6 mg N/l for the boundary between good and moderate ecological status for upland and low alkalinity rivers, and lowland and high alkalinity rivers, respectively.70 These proposed WFD standards have been established in terms of effects on macroinvertebrate communities. No standards have been proposed for lakes, transitional and coastal waters. At low concentrations (<0.1 mg l-1 NH3) ammonia irritates the gills of some fish species, leading to skin and gill hyperplasia which leaves the gills vulnerable to parasitic infection and disease. A study in which rainbow trout were subjected to prolonged exposure of ammonia at low levels showed a reduced success in egg hatching, reduced growth rates and morphological changes as well as pathological changes in the gills, liver and kidneys.71 At higher concentrations, (>0.1 mg l-1 NH3) ammonia damages the skin, gills, eyes and internal organs of fish.72 Negative physiological effects can result in reduced feeding activity and reduced body size.73 Elevated levels of ammonia in the environment can impair ammonia excretion in fish or cause a net uptake resulting in elevated concentrations in the body. This causes convulsions and eventually death. Ammonia can be lethal to fish at concentrations less than 1 mg l-1. Freshwater coarse fish are less sensitive to ammonia than salmonids; coarse species typically have an LC50 (lethal concentrations for 50% mortality) of 0.3-2.5 mg l-1 whereas salmonids have an LC50 in the range 0.06-0.91 mg l-1. However, a small number of species, such as the weather loach and mudskipper, can tolerate high concentrations of ammonia.74 Atlantic salmon are amongst the species most susceptible to high ammonia concentrations whilst species such as inland silverside and red drum are able to tolerate higher concentrations. Ammonia is also toxic to invertebrates, with toxicity being greatest during the early life stages and the effects similar to those found in fish as mentioned above, and to Nitrosomonas and Nitrobacter bacteria, where it inhibits the nitrification process which converts ammonia to less toxic nitrate.75 Evidence is now emerging that nitrogen (NOx and ammonia) deposition in upland areas may be having an eutrophication effect on UK upland waters76, and nitrate-driven eutrophication may be occurring in some freshwater bodies.77 Further work is required on how nitrogen is affecting eutrophication in upland lakes in the UK.78 The UKTAG’s preliminary assessment of the implications of the proposed standards for ammonia for implementation of the WFD indicates that 17.3% of river water bodies in England (by length) and 2.7% (by length) in Wales would be less than good ecological status.79 This assessment was based on measured levels of ammonia in rivers as part of the EA’s GQA scheme and would reflect the impact of ammonia from all sources. The effects of ammonia in transitional and coastal waters are similar to those observed in freshwaters, but often exacerbated by high salinities and warm temperatures. The toxicity of ammonia can be further increased by interactions with other chemical pollutants such as copper, cyanide, phenol, zinc and chlorine. In well aerated waters, however, ammonia is converted to less toxic nitrate. Ammonia is also toxic to crustaceans, but to a lesser extent than fish. Prawns are particularly sensitive, whereas eastern oyster are particularly tolerant. At a lower trophic level, growth of algae is retarded when exposed to concentrations of ammonia greater than 0.24 mg l-1 over 10 days.80 Download 432.61 Kb. Share with your friends:
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