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Review of the evidence for organic pollution thresholds to protect rivers with special designations for wildlife

By John Iwan Jones, John Davy-Bowker, John Murphy, Virginie Keller, Richard Williams and Cynthia Davies


Natural England is responsible for overseeing the management and assessment of sites with national and international designations for wildlife, including Sites of Special Scientific Interest (SSSIs) and Special Areas of Conservation (SACs). A key aspect of this work is the definition of Conservation Objectives for each site, that define favourable conditions and inform management decisions relating to the site. The purpose of this report is to review key quantitative information on the response of the characteristic biological community of river types in England and the UK to organic pollution stress gradients, in order to reconsider the adequacy of the targets set for protecting SSSI condition. The stress under consideration here is the toxic effect caused either directly or indirectly by organic pollution, not the effect of increased nutrient loading.\ud Organic pollution can result in oxygen depletion and increased ammonia concentrations with severe consequences for river organisms. Organic effluents frequently contain large quantities of suspended solids which reduce the light available and alter river bed characteristics rendering it unsuitable for many organisms.\ud Objective quantitative information on the sensitivity of river organisms to organic pollution has been derived using different approaches, carefully controlled laboratory assessments (e.g. LC50), field scale experimental manipulations, case histories and correlation of field survey data. The different approaches vary in the degree of control and realism. Evidence available from a variety of sources is reviewed and the reponse of the biological community to determinands associated with organic pollution presented. Fish are particularly prone to the toxic effects of unionised ammonia and of oxygen stress. Both unionised ammonia and oxygen stress can cause death of invertebrates, but they are typically less sensitive to ammonia than fish. Field scale simulated pollution events and exposure to polluted natural waters indicate that, whilst laboratory trials provide an indication of sensitivity, they do not accurately reflect the response of organisms in the field.\ud New analyses of field data were undertaken at both family and species level to determine the response of macroinvertebrates to organic pollution. The approach was to develop a ranking of taxa based on their tolerance to organic pollution, as measured by a combination of BOD, oxygen and ammonium concentration based on partial ordination. Infrequent taxa were included in the analyses passively, to give an indication of their sensitivity without biasing the results. The three macroinvertebrate families most sensitive to organic pollution were identified as Odontoceridae, Goeridae and Rhyacophilidae (incl. Glossosomatidae). The species level analysis was compromised by a lack of data from more polluted sites. It is recommended that more species level data are collected from organically polluted sites before a repeat of this analysis is attempted. \ud Management thresholds of organic pollution for SSSIs should be able to protect the most sensitive taxa characteristic of the habitat, and correspondingly thresholds should differ among sites according to the fauna. The tolerance of the most sensitive taxa typically present can then be used to establish the thresholds for each river type. RIVPACS 9-endgroup river types predict the fauna and mean abundance in such rivers; these were matched to NE (and JNCC) river types. Thresholds are set corresponding to concentrations where 80% of the occurrences of the most sensitive macoinvertebrate familes are in less polluted sites. Two thresholds are proposed:\ud I: 10th %ile DO = 85%, Mean BOD = 1.8 mg l-1, 90th %ile Total Ammonia (NH3 -N) = 0.23 mg l-1\ud II: 10th %ile DO = 79%, Mean BOD = 2.0 mg l-1, 90th %ile Total Ammonia (NH3 -N) = 0.29 mg l-1\ud Data are compiled from SSSI sites where available and compliance with these thresholds investigated. Data on various determinands associated with organic pollution are presented for SSSI sites where available.\u

Topics: Ecology and Environment
Publisher: NERC/Centre for Ecology & Hydrology
Year: 2008
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