The validity of the Gammarus : Asellus ratio as an index of organic pollution: abiotic and biotic influences

In freshwaters, Gammarus spp. are more sensitive to organic pollution than Asellus spp. and the relative abundance of the two taxa has been proposed a s a pollution index. We tested the validity of this by examining the relati onship between the Gammarus: Asellus (G:A) ratio and (1) a suite of physico -chemical variables, (2) established biotic (average score per taxon, ASPT) and richness (species richness (S) and Ephemeroptera, Plecoptera and Trich optera families richness (EPT family richness)) indices generated from the macro invertebrate community. In addition, we investigated a suspected biot ic interaction, predation, between Gammarus and Asellus. Both univariate an d multivariate analyses showed that the G:A ratio was sometimes responsive to changes in parameters linked to organic pollution, such as BOD5 and nitr ate levels. However, the G: A ratio also appeared responsive to variables n ot directly linked to organic pollution, such as conductivity and distance from source. There were significant positive correlations among the G:A rat io and the ASPT, S and EPT, indicating that changes in the relative abundan ces of Gammarus and Asellus were reflected in changes in the pollution sens itivity and richness of the wider macroinvertebrate community. A laboratory experiment revealed significant predation of Asellus aquaticus juveniles b y Gammarus duebeni celticus adults, but no reciprocal predation. We propose that the G:A ratio may be useful as a crude measure of organic pollution t hat could supplement more complex indices in a multimetric approach to poll ution monitoring or be used for monitoring individual sites, where a simple technique is required for monitoring purposes over a period of time. Also, we urge recognition of the possible role of biotic interactions among taxa used in the generation of pollution indices. (C) 2001 Elsevier Science Ltd . All rights reserved.