Understanding the impact of farming on aquatic ecosystems
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- Eutrophication
- Acidification
- Toxic effects
Nitrate/nitrogenThe principal farming activities leading to the pollution of water bodies by nitrate is the application of inorganic fertilisers and animal manures to arable and pastoral land. Nitrate is highly soluble and is readily leached from soils in water bodies via cracks and preferential (subsurface) pathways.22 It can also directly enter water bodies via run-off from land and through atmospheric deposition of NOx and ammonia: the latter is rapidly oxidised when deposited on land. Nitrates lost from farmland may often be accompanied by other agricultural pollutants which may compound the impact on ecology, for instance nitrates lost from fertilisers and slurries applied to land are often accompanied by soil sediment and organic matter. Unlike phosphorous, however, nitrate is unlikely to be incorporated with soil sediments due to its high solubility. At the national level (England and Wales) agriculture is the predominant source (60.6% in 2000/2001) of the total nitrogen load to all surface waters, with 32.1% coming from sewage treatment works.23 Total nitrogen loads to sea showed no clear trends between 1991 and 2003.24 There are clear regional differences with agricultural areas such as the South West and Anglian regions having the highest nitrogen loads from agriculture while more populous areas such as the Thames region have a higher proportion of nitrogen loads arising from sewage treatment works. The EA’s WFD Article 5 risk assessments estimated that 37.9% of river water bodies (45.0% by length), 19.9% of transitional water bodies (8.5% by area) and 13.1% of coastal water bodies (4.9% by area) were at risk from diffuse nitrogen inputs.25 26 The risk assessment for rivers included nitrogen inputs from agriculture and atmospheric deposition but did not consider any ecological effects of nitrogen enrichment. For transitional and coastal waters, diffuse (riverine) inputs and direct discharges were considered with no specific apportionment for agricultural inputs. No risk assessment was undertaken for diffuse nitrogen in lakes. As of 2007 there are 137 sensitive areas in England and Wales designated under the Urban Waste Water Treatment Directive as being eutrophic or likely to become eutrophic in the near future without action being taken. These sensitive areas are a mixture of rivers, lakes (reservoirs), harbours and estuaries, and hence there will be a mixture of which nutrient will be most pertinent (nitrogen and/or phosphorus) in terms of ecological impact and actions required. One of the criteria for the identification of polluted waters under the Nitrates Directive is whether waters are eutrophic or will become eutrophic in the near future without action being taken. As a result, a total of 55% of England was designated as a Nitrate Vulnerable Zone (NVZ) in October 2002 though not all were necessarily designated because of the eutrophication criteria. Defra is currently reviewing and consulting on increasing NVZs to 70% of England or applying the Action Programmes required under the Directive to the whole of England. This latest review recognises that nitrogen may also play a role in the eutrophication of freshwaters. There are also 12 problem or potential problem areas identified under OSPAR's comprehensive procedure for the assessment of eutrophication. These are estuaries, bays or harbours and correspond to areas designated under there UWWT and/or Nitrates Directive. Nitrate concentrations are high in water draining from much of the agricultural land in England.27 For example in 2006, 28% of rivers had high concentrations of nitrate (greater than 30 mg l-1)28 with the highest concentrations being found in the Midlands, Anglian and Thames regions, which have some of the largest areas under intense agricultural production. Similarly, a review of nitrate concentrations in rivers in England between 1999 and 2004 reported that the mean nitrate concentration varied regionally from 15 mg l-1 in North West region to 39 mg l-1 in Anglian region.29 Typically, nitrate concentrations are higher in lowland areas dominated by arable agriculture than in areas dominated by pastoral farming30 and lower in wetter areas where high quantities of rainfall dilute the nitrate within soils before being leached or washed into water courses.31 Concentrations in drainage waters from grassland systems are highly variable depending upon the intensity of stocking and management.32 Nitrate concentrations are increased where manures are used, even under ‘best practice’. The implications of the proposed WFD environmental standards for dissolved inorganic nitrogen have been recently assessed.33 Of the 60 transitional and coastal water bodies with sufficient monitoring data, 65% (by number) would be classified as less than good ecological status. There are 136 transitional and 99 coastal water bodies identified for the WFD, and hence this initial classification may not be fully representative of all water bodies. Nitrate occurs naturally in all surface waters but elevated concentrations of nitrate can cause deterioration of ecological quality in three main ways: eutrophication, acidification and toxic effects. Eutrophication: Nutrients (e.g. nitrogen and phosphorus) in the appropriate amounts (i.e. background levels) are essential to maintain an adequate primary productivity, which in turn is essential to support higher trophic levels and to maintain a healthy ecosystem structure and function. In general, excessive nutrients of anthropogenic origin cause an increase in plant growth (eutrophication), which in still waters causes increased phytoplankton biomass, often dominated by harmful or toxic species, leading to increased turbidity and decreased light transparency. In rivers this may be seen as increased attached algal growth or even excessive growth of higher plants. In estuaries and coastal waters, excessive growth of opportunistic macroalgal species can result in the formation of dense algal mats, which reduce light penetration to submerged communities and cover intertidal sediments. Sea grasses may also experience severely retarded growth as a result of the growth of opportunistic macroalgal species.34 These are examples of the direct effects of nutrient enrichment. As a consequence of increased plant growth, there is an imbalance between the processes of plant/algal production and consumption, followed by sedimentation of organic matter, stimulation of microbial decomposition and oxygen consumption with depletion of bottom-water oxygen in stratified water bodies (indirect effects). Thus, eutrophication causes not only nuisance increases in plant growth but also adverse changes in species diversity as well as reduced suitability for human use and consumption. In freshwaters, phosphorus is most likely to limit plant growth but some water bodies are limited by nitrogen rather than phosphorus and are therefore vulnerable to nitrate-driven eutrophication.35 One study, for example, found that phytoplankton growth was limited by nitrogen in one third of the 30 upland lakes studied.36 Nitrate /nitrogen rather than phosphorus is often the key limiting nutrient in saline waters, although estuaries may be limited by phosphorous at their freshwater extreme.37 There are also many water body type-specific physical characteristics that influence the manifestation of eutrophication. Such factors as water velocity, mixing characteristics, retention time, water transparency and water temperature will fundamentally affect the response of any particular water body to elevated nutrient concentrations. In the context of the WFD these factors are taken into account in the quantification of type-specific reference conditions which equate to ‘natural’ undisturbed ecological conditions and anchor the classification of ecological status against which the achievement of good ecological status is judged. Although macroalgal and toxic algal blooms are commonly associated with nutrient enrichment and eutrophication, they can also occur naturally when, for example, stimulated by favourable meteorological conditions and physical processes such as advective heat transport.38 The incidence of naturally occurring algal blooms should be taken into account in the reference conditions of a water body: an increase in the occurrence compared to reference conditions may indicate eutrophication related bloom events. Under particular environmental conditions, the decay of algal blooms can lead to a build up of bacteria which can release toxins that affect fish and lead to mass mortalities. A significant example is the Hungerford fish mortality of 1998 when more than a million fish were killed.39 Acidification: When the nitrate anion is leached from soils it may be accompanied by a hydrogen ion or proton (H+) which may in some circumstances (such as in water bodies with low acid neutralising capacity) contribute to the acidification of surface waters.40 Acidification can exacerbate the loss of macrophytes due to eutrophication and promote their replacement by filamentous green algae and rushes which thrive in acidic conditions. Acidic conditions can also cause the proliferation of certain macroinvertebrates that are more tolerant of low pH conditions (such as the mayfly Leptophlebia vespertina), and a loss of those that are intolerant (such as the mayfly, Baetis rhodani).41 Few fish are able to survive in highly acidic conditions; a pH of around 4.5 is fatal to most species.42 Little is known about how much acidification can be attributed to nitrates derived from agriculture. The contribution that ammonia emissions from agricultural activities make to acidification is discussed under ammonia. Toxic effects: Elevated nitrate concentrations can have direct toxic effects on certain freshwater animals. Conversion of nitrate ions to nitrite ions in the body can cause respiratory problems in fish and crayfish. Nitrates have also been implicated in the decline of amphibian populations as they can cause impaired swimming ability and decreased body size in many species. It is thought that freshwater organisms may be more sensitive to nitrate enrichment than those in coastal or estuarine systems because nitrate is more toxic at low salinities.43 Download 432.61 Kb. Share with your friends:
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