| Summary: | Quality classification of water bodies commonly hinges upon the results of biotic indices. Biotic indices should reliably detect environmental change caused by anthropogenic stress; distinguish between different levels of disturbance; and be applicable in different areas. This study assesses current methods used in the assessment of benthic ecosystem health in transitional and coastal waters. Specifically, this study considers the performance of macrozoobenthos based biotic and diversity indices. Data utilised in the assessment covered a range of sites and environmental gradients including long term monitoring sites in Scotland; sites impacted by fish farms, organic waste discharge, and chemical effluent; estuarine sites; and sites from Galway Bay, Ireland, one of which was impacted by river discharge. Currently used indices of environmental status are based mainly on structural ecosystem properties and may not encompass all aspects of ecosystem health, such as functioning. Structural and functional based assessment methods were evaluated by comparing the performance of a range of standard benthic abundance indices and approaches focussing on intrinsic biological characteristics. Indices did not perform consistently in response to different types of impact – organic, chemical and physical, indicating some indices are unsuitable for the detection of multiple stressors. Index quality classifications agreed best in the most impacted sites but performed unpredictably in moderate conditions. Variability of indices increased as disturbance increased, decreasing the statistical certainty and confidence in the index values. Structural indices were found to be more variable than functional indices but the sensitivity of functional indices to anthropogenic disturbance needs further testing to determine whether they are able to detect low level disturbance. Functional indices may not be advantageous in regular monitoring over traditional methods but may provide a more informative assessment of ecosystem health. Use of biological traits may also give an indication of the type or cause of disturbance. Classification of moderate-good conditions using benthic indices is particularly ambiguous and distinguishing natural from anthropogenic disturbance remains one of the biggest challenges. The results indicate that complementarity of approaches is important in the assessment of quality of coastal and transitional benthic aquatic systems.
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