Environmental Variables and Macroinvertebrate Community Structure in Drainage Ditches on the Somerset Levels

1. Studies of macroinvertebrate assemblages have tended to find relationships between environmental variables and the species present in the assemblage. Here, I looked at the relationship between assemblages and environmental variables in six drainage ditches in a small area of the Somerset Levels, UK. 2. I sampled aquatic macroinvertebrates and a range of environmental physicochemical variables from the six ditches, and investigated the differing relationships between the environmental variables and a range of assemblage datasets (overall assemblage, Coleoptera, Hemiptera, Odonata, and Mollusca). 3. Environmental variables surveyed were relatively homogeneous, with the exception of calcium, conductivity and nitrate. Diversity, species richness, and evenness for overall and single-taxon assemblages varied between sites, with no discernable pattern between large and small ditches. 4. Different taxonomic groups reacted strongly to different environmental variables, and no clear deterministic pattern is expressed, either overall or within taxa. 5. Species distributions appear to be largely due to chance, rather than significant interaction with the physicochemical environment, between ditches which are close enough together to be within the dispersal range of many taxa, and between which the chemical environment does not radically alter