Measurements of sublethal effects on individual organisms indicate community-level impacts of pollution

1. Due to the cost and complexity of sampling biological communities, surrogate biological measures or concentrations of toxic contaminants are often used to indicate environmental impacts. Such indicators can be powerful tools, but their effectiveness requires evaluation. Mussels are widely used as bioindicators of environmental contamination. For example, physiological measurements on live mussels form the basis of scope for growth (SFG), an integrated indicator of environmental stress. However, the effectiveness of SFG as an indicator of community level effects has rarely been tested in the field. 2. In this study of six sites on the west coast of the UK, the diversity of macrofaunal communities associated with mussels was reduced at sites with low SFG (< 10 J g(-1) h(-1)) compared with those with high SFG (> 15 J g(-1) h(-1)). At smaller scales, variation in community structure was related to biomass of mussels, mass of coarse sediments and the fractal dimension of the surface of the mussel bed. 3. We measured hydrocarbon contamination as a first step in identifying contaminants that might underpin the relationship between SFG and diversity. Unresolved complex mixtures (UCM) are often the most abundant hydrocarbon contaminants in the environment, but have, until recently, been largely ignored. We found generally good accordance between UCM concentration in mussel tissues, SFG and diversity, but other pollutants are also likely to be involved. 4.Synthesis and applications. Our findings illustrate the complexity of relating changes in diversity to synoptic or sublethal measures of environmental stress in the field. However, the results represent a significant step towards a biotic indicator of environmental quality that integrates impacts across a range of levels of biological organization (from intra-individual to community). Such indicators may be of particular value in the implementation of the European Union Water Framework Directive in Europe and similar environmental legislation elsewhere.