Are biological indices BMPWʹ and ASPTʹ and their significance regarding water quality seasonally dependent? Factors explaining their variations

Biological indices based on macroinvertebrates offer advantages for testing water quality. Nevertheless, a common criticism is that these indices show seasonal dependence. To elucidate this matter, 60 sits were sampled over a two-year cycle, and the BMWPʹ and ASPTʹ biotic indices (adaptation of the Biological Monitoring Working Party Score System and Average Score Per Taxon for the Iberian Peninsula) were calculated. Annual variations in the indices were studied (considering, separately, only non-polluted sites, and grouping polluted and clean-water sites together) and compared for changes in temperature, the factor most characteristic of seasonality. Moreover, the correlations with physical factors (altitude, slope of stretch, flow, temperature, distance from the source) and chemical parameters (pH, conductivity, nitrates, nitrites, ammonium, phosphates, chlorides, sulphates, calcium, magnesium, sodium, potassium, COD, iron, copper, zinc, lead, nickel, oils and fats, detergents and pesticides) were analysed. Both indices were found to be negatively related with parameters indicative of pollution. From a multiple regression, a good score prediction using physical and chemical data (for BMWPʹ as for ASPTʹ) was obtained. For non-polluted sites, the variability of the BMWPʹ (and its significance with respect to water quality) in relation to seasonality was quite reduced and not significant, but the ASPTʹ showed a significant dependence on temperature. The relationship of these indices to temperature in all sites (polluted and clean) was negative in both cases, indicating that the relationship is caused more by pollution than by seasonality.