Understanding the impact of farming on aquatic ecosystems
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PhosphorusThe principal farming activities that lead to pollution of water bodies by phosphorus are the application of inorganic fertilisers and animal manures to arable and pastoral land. Soil erosion by water and overland flow are the principal pathways by which phosphorus enters water bodies.44 Leaching and subsurface flow, particularly via macropores or cracks, may also be important in some situations such as on poorly permeable soils. The major point sources of phosphorus are from sewage treatment works and industrial discharges. In many water bodies there may also be internal loads reflecting historic external loads that are generated through a variety of chemical and biological processes acting upon phosphorus stored within sediment and organic matter. The relative contributions of phosphorus from external and internal sources are highly variable in both space and time.45 The proportional loads of total phosphorus (TP) and soluble reactive phosphorus (SRP) to the waters of Great Britain from agriculture have been estimated to be 23-28% and 19%, respectively.46 The most important source in both cases was households, which contributed 60.7% and 67.4% of TP and SRP, respectively. There were regional differences in the proportions of TP loads contributed by agriculture with the highest proportion of 62% occurring in the Western Wales RBD and 10% in the Thames RBD. The total annual discharge (riverine and direct discharges) of orthophosphate to sea in England and Wales between 1991 and 2003 was variable and showed no clear trends. 47 Riverine inputs, which reflect both diffuse and point land-based sources, accounted for 40-75% of total inputs and were highest in the southern North Sea and lowest in the Atlantic. Emissions of phosphorus from sewage treatment works in rivers subject to both point and diffuse sources of phosphorus are thought to pose a greater risk of eutrophication than emissions from agriculture.48 This is because loads of soluble, and hence immediately bioavailable, phosphorus from sewage treatment works enter the river continually throughout the year and are at minimum dilution during the spring and summer when plant growth is high and most sensitive to phosphorus concentrations in the water column. In contrast, the majority of the annual load of phosphorus from agricultural land is attached to soil particles and not immediately bioavailable. It is delivered to rivers and lakes during storm runoff events in autumn and winter, and a significant proportion of the load will be immediately flushed out of the system by the high river flows. Some of this particulate phosphorus may be deposited on the river bed and can enhance plant growth in subsequent growing seasons either directly via root uptake or indirectly via desorption and diffusion into the water column. In slow-flowing rivers accumulated sediment may therefore provide an instream reservoir of phosphorus that may mask the effect of any changes in point source discharges. In general, however, the lower bioavailability of phosphorus from diffuse sources means that point sources can provide the most significant risk of eutrophication in rivers, even in rural areas with high agricultural phosphorus losses.49 However, it has also been demonstrated that intensively managed grassland can also be a significant source of dissolved reactive phosphorus in some catchments from the surface applications of manures, accumulation of soil phosphorus and the creation of bypass flow pathways that encourage the loss of P from soil to water50. The EA’s WFD Article 5 risk assessments estimated that 47.4% (out of 5858) of river water bodies (50.3% by length), and 36.8% (out of 432) lake water bodies (40.2% by area) were at risk from diffuse phosphorus (agricultural) inputs.51 Other diffuse sources such as septic tanks were not considered. Under the UWWTD, 108 rivers, 4 canals, 8 lakes and10 reservoirs in England and Wales have been designated as Sensitive Areas (eutrophic). This means that they are eutrophic or may become eutrophic in the near future if preventative action is not taken. Lakes and rivers can be designated as sensitive areas only if they show elevated phosphorus concentrations and evidence of change in response parameters including diatoms and macrophytes (for rivers and lakes), chlorophyll-a and water clarity (for lakes).52 Eutrophication due to elevated phosphorus concentrations is a serious and widespread problem in rivers in England and Wales. In pristine rivers concentrations of SRP are very low – typically <30μg l-1. In 2006, 55.3% of rivers in England and 10.7% of rivers in Wales had concentrations of phosphate >100 μg l-1.53 Higher levels were generally found in the lowlands of central and east England and in southern England and in rivers flowing through areas dominated by arable farming. However as this assessment is only based on dissolved inorganic phosphorus (SRP) the figures do not provide a profile of total phosphorus losses from agriculture, where a large part of the component will be in particulate, as opposed to soluble, form.54 Although phosphorus concentrations >100 μg l-1 are considered to be indicative of existing or future eutrophication problems, the sensitivity of rivers to eutrophication depends upon water chemistry and hydrology. Calcareous (hard water) catchments are less sensitive to phosphorus enrichment than non-calcareous (soft water) catchments. Furthermore, rivers in lowland areas are at greater risk of eutrophication because deposition of sediment leads to the accumulation of a reservoir of instream particulate phosphorus and high retention time facilitates uptake of phosphorus by diatoms and plants. Lakes have been defined as eutrophic if they have mean phosphate concentrations in excess of 35 μg l-1, mean chlorophyll-a concentrations greater than 2.5μg l-1 and a mean Secchi disk water transparency of less than 6 m.55 Lakes tend to become naturally more eutrophic with (geologic) time and as lakes naturally show a wide variation in trophic status, high phosphorus concentrations alone are not indicative of anthropogenic eutrophication. Until the EA’s WFD monitoring programmes was made operational in December 2006, there was no comprehensive monitoring scheme for lake water quality in England and Wales and few data are currently available to make a national assessment of lake trophic status. However, a review of research studies on 129 lakes in England and Wales found that 69% of the lakes had total phosphorus concentrations greater than 100 μg l-1 P.56 In a study for English Nature of 102 SSSI sites which were suspected of being eutrophic, 84% showed symptoms.57 These studies suggest that a majority of lakes in England and Wales are nutrient enriched to some extent. More recently, the UK Technical Advisory Group (UKTAG) on the WFD proposed new standards for phosphorus concentrations in rivers and lakes. These standards are type specific and would equate to the achievement of good ecological status in terms of phosphorus. An assessment of the implications of these new standards estimated that 63.3% of river water bodies and 67% of lake water bodies in England and 12.8% of rivers and 80% of lakes in Wales would be reported as worse than Good Status with at least 50% confidence for rivers and 95% confidence for lakes.58 This preliminary classification is in respect of all sources of phosphorus and not just that due to agricultural sources. It should also be noted that UKTAG has expressed some concern on the validity of data used to classify lakes. Although nitrogen (nitrate) is thought to be the key limiting nutrient in saline waters, phosphorus can also contribute towards eutrophication. Estuaries, in particular, may be limited by phosphorus at their freshwater extreme, grading through to nitrogen limitation at their seaward end. However, high suspended sediments and turbidity in estuaries commonly means that light rather than nutrients often limits algal growth. The WFD Article 5 risk assessments did not address diffuse phosphorus in transitional or coastal waters. The main impact and effect of enrichment of freshwater water bodies by phosphorus is eutrophication. Eutrophication has already been described in the section on nitrate. Download 432.61 Kb. Share with your friends:
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