Assessing the acidity of Swedish streams using benthic macroinvertebrates and weighted averaging (WA) regression and calibration

Weighted averaging (WA) regression and calibration methods are commonly used in paleoecology to infer environmental conditions in the past, but few stream studies have used these approaches on contemporary macroinvertebrate data sets. Here we examined if WA regression and calibration methods could better our understanding of the effects of acidification on Swedish stream ecosystems. Macroinvertebrate and water chemistry data from 232 stream sites in the northern and 154 stream sites in the southern parts of the country were used for model calibration. In addition, 42 randomly selected sites were used for model validation (25 in the north and 17 in the south). Inferred WA pH was compared to inferences of pH obtained using four commonly used acidity indices. Performance of the different methods was assessed using the r 2 -values and root mean square errors (RMSE) of regressions between inferred and observed pH. WA approaches were found to be slightly better than the four acidity indices studied here. However, both WA and the acidity indices performed poorly at the test sites in southern Sweden. This study shows that WA approaches could be useful for assessing acidity of Swedish streams, but the overall low r 2 -values indicate that factors other than pH also are affecting the macroinvertebrate assemblages confounding the pH-organism response relationship.