Understanding the impact of farming on aquatic ecosystems
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- Combined effects of different pressures
- Indicators of pollution
Other pollutantsPhosphate fertilisers can contain cadmium and as such represent a cadmium input to arable soils, where it is rapidly adsorbed by particulate matter, and potentially to adjacent water bodies.153 Cadmium can also be found in trace quantities in manure, where it is probably associated with zinc complements given to livestock, and again this represents a potential input to agricultural soils and thereby to water. There appears to be no information on how important this potential agricultural source of cadmium is in relation to the total load of cadmium in surface water bodies in England and Wales though it has been estimated that between 1.4 to 6.5 g ha-1 yr-1 of cadmium can be input to agricultural soils through the application of manure/compost.154 Other estimates put the cadmium load to UK soils as between 11.3 and 25 tonnes yr-1 155 and in 2005 the direct and riverine inputs of cadmium to UK seas were calculated to be between 4.9 and 8 tonnes.156 Oil and fuels are the most frequently reported type of pollutant of inland waters in England and Wales. In 2006, there were 605 serious (category 1 and 2) water pollution incidents, of which oil and fuels were implicated in 89 (14.7%).157 The proportion of these that originated from agricultural sources is recorded but not publicly available. Combined effects of different pressuresAlthough combined effects of different pressures are common in the aquatic environment, it is important to recognise that it is difficult to interpret these impacts from information on individual pressures. There are clearly many potential interactions in the environment, whereby the combined effects could result in different effects or effects larger or smaller than those that might be expected on the basis of the individual pressures. This is particularly evident for the combined effects of chemicals (such as pesticide and veterinary medicines) and soil sediments or organic matter where the bioavailability of the chemicals and their resulting toxicity to receiving water organisms is modified. The problem of assessing the nature of combined effects is particularly problematic when these act in opposing ways on particular biological elements. This is exemplified by the potential effects on the biomass of aquatic plants in a water body of elevated concentrations of nitrate and/or phosphate and a herbicide. Whilst the elevated nitrate and/or phosphate concentrations will tend to act to increase aquatic plant biomass through eutrophication, the presence of the herbicide at sub-lethal concentrations will tend to restrict the growth of aquatic plants. As a result no change in biomass may be evident even though the two pressures are outside of “normal” conditions. Current knowledge on the responses of aquatic biota to combined pressures from nutrients and priority substances has been summarised by O’Toole and Irvine (2006)158. These issues illustrate why it is necessary to use a series of indicators for a particular biological quality element (e.g. phytoplankton, phytobenthos, macrophytes, macroalgae, angiosperms, benthic invertebrates or fish) to assess potential impact. As described in section 3, a number of different metrics (e.g. the abundance of particular species, overall diversity) may be used to estimate the status of a quality element. Different metrics may be used to indicate the impact of different types of pressure on the element159. In other cases, monitoring results for different metrics may be combined to give a representative picture of the impact of a particular type of pressure (or range of pressures) on the quality element. In terms of the use individual quality elements, tools are being developed for the classification of ecological status that takes into account the response of the quality element to different pressures160. For example, RIVPACs, the classification tool for river benthic invertebrates, is being developed for the assessment of toxicity, acidification, hydrological, and perhaps morphological pressures, as well as organic pollution pressure for which it was initially developed. For example, using multiple metrics may be appropriate where none of the metrics on their own give a sufficiently reliable indication that the quality element has been adversely impacted as a result of human activities. The relationship between anthropogenic pressures and ecological status may vary according to the sensitivity of river ecosystems and combinations of pressures. Agriculture and urbanisation are considered the principal impairment sources in the literature but the relative influence of combined pressures coming from agricultural or urban land use is not well established.161 Nevertheless large scale analysis comparing pressure-impact relationships across different spatial scales, countries and ecoregions would still be necessary for the implementation of the WFD. Indicators of pollutionPotential indicators of pollution arising from farming range from sub-organism level determinands (e.g. tissue analysis of persistent bioaccumulative substances in an appropriate species) through the presence and abundance of key species, to indices which integrate information about the structure or function of whole biological communities. Several different types of ecological indicator exist and those chosen depend on their required role in the assessment process. Ecological indicators can be divided into three classes162: early warning indicators that detect impending changes; compliance indicators that detect changes in characteristics beyond acceptable limits; diagnostic indicators that show the causes of deviations from “normal conditions”. Indicators are commonly used to determine the impact of various pollutants and disturbances. The indicator used for a defined part of the monitored system needs to be able to inform stakeholders about the threat to a given element of the ecosystem.163 The results should be unambiguous in their response to pressures, although this may not always be possible given the complexity of the systems being monitored.164 The validation process for indicators is also important, but unfortunately is rarely fully implemented.165 The expression of the indicator should be directly related to changes in the pressure so that the implications of changes in the indicator are evident. Furthermore there should be an understanding of the variability in the relationship so that stakeholders can assign a level of uncertainty to any management decisions that are based on the ecological indicator data. It is unlikely that a single indicator will be indicative of a specific farming-related pressure. Instead a series of indicators are likely to be needed from which the extent and magnitude of impacts of different pressures can be elucidated.166 In addition, many water bodies will be subject to the same pressure from a number of sources (e.g. nutrient enrichment from agriculture and sewage treatment works) and, as a consequence, it will not always be straightforward to apportion the observed impacts to farming. Download 432.61 Kb. Share with your friends:
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