Assessing Acid Stress in Swedish Boreal and Alpine Streams Using Benthic Macroinvertebrates

Abstract

Sixty streams in northern Sweden were sampled for benthic macroinvertebrates in spring and autumn of 2000 as part of the European Union project AQEM (the Development and Testing of an Integrated Assessment System for the Ecological Quality of Streams and Rivers throughout Europe using Benthic Macroinvertebrates). Samples were taken using a harmonised multi-habitat sampling procedure and a large number of parameters describing the streams and their catchments were recorded for all sampling sites. From the stream water chemistry characteristics unbiased measures of acid stress were derived, based on the Swedish Ecological Quality Criteria (EQC) for acidity status. These criteria do not, however, assess the degree of human influence to a stream. Therefore, in the following analysis of classification, the Swedish EQC status criteria are applied as if they were a measure of human influence. Three new multimetric acid indices using benthic macroinvertebrates were developed for the AQEM project. These, together with a large number of other benthic macroinvertebrate indices including a number of indices aimed at detecting acid stress, were evaluated for their ability to detect acid stress, using both spring/early summer and autumn sampling. Surprisingly, the acid index that is in general use in northern Sweden worked well in spring/early summer, but because of a (probable) recolonisation of more acid sensitive taxa, the index values changed from classifying streams as affected in spring/early summer to classify streams as non-affected by acid stress in autumn. Another Swedish acid index (included in the Swedish Environmental Quality Criteria developed by the Swedish EPA) and developed for southern Sweden was strongly correlated between spring/early summer and autumn and could thus be sampled in the autumn and indicate acid stress from spring flood declines in pH. This is of great importance since the local County boards of northern Sweden generally argue that sampling has to be done in the spring to see the effects of acid stress, when sampling is difficult for logistic reasons. Whether or not the three indices developed for the AQEM project (and extensively based on the south Swedish acid index) were truly better than the south-Swedish acid index to assess acid stress in northern Sweden could not be clearly determined in the present study.