Four commonly employed biotic indices based on macrofauna composition: BMWP (British Monitoring Working Party score system), ASPT (Average Score Per Taxon), DSFI (Danish Stream Fauna Index) and FBI (Family level Biotic Index) were used to assess environmental quality of six small slow-flowing lowland streams (Masurian Lakeland, North-eastern Poland). Their efficiencies in determining the level of degradation, assessed by the simoultaneous measuring of the main abiotic parameters of water, were tested. The search for relationships between both abundance and percentages of invertebrate taxa and values of the main abiotic parameters with multiple regression analysis resulted in determining an indicative value for certain taxa. Results of discriminant function analysis show, that environmental quality can be predicted accurately on the basis of macrofauna composition in 94–100% of the sites. However, none of tested indices classified the sites to particular classes of degradation with sufficient accuracy. The relationships between the values of all tested indices and the values of Cumulative Index of Environmental Quality (CIEQ) based on abiotic parameters, expressed as coefficients of linear regression, were very low and ranged between 0.006 and 0.068. Therefore, a new regional, macrofauna-based biotic index, APODEMAC, is proposed as a better tool for environmental quality assessment. Values of APODEMAC are strongly dependent on values of CIEQ (R²= 0.641), and their seasonal variance is lower than that of other indices. Coefficient of linear regression between CIEQ values and APODEMAC was 9 to almost 100 times higher than for other tested biotic indices. The selection of indicator taxa used in APODEMAC procedure was based on comparisons between the taxa densities at sampling sites differing in terms of environmental quality and on the results of multiple regression and discriminant analysis. The importance of certain taxa of molluscs, leeches and dipterans in this procedure is much higher, while the importance of mayflies, stoneflies and caddisflies is much lower than in BMWP, ASPT, FBI and DSFI. The occurrence of the various larval insects in the studied environments seems to be limited rather by substrate type than by the chemical quality of stream water. Application of simple regional biotic indices like APODEMAC is suggested as an, in some cases, easier, cheaper and less time-consuming way of biological assessment compared to complicated and time-consuming implementation of the existing indices.
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