Bioindication in Norwegian rivers using non-diatomaceous benthic algae: The acidification index periphyton (AIP)

Impacts of acidification on aquatic communities have been obvious in Scandinavia since the end of the 19th century and recent model simulations show that in Norway, acidification of surface waters will continue to be an issue in the coming decades. Here, we present a new index based on non-diatomaceous benthic algae (acidification index periphyton, AIP) that can be used to describe the mean annual acidity of Norwegian rivers. The AIP was applied to 608 samples from unlimed rivers all over Norway and values ranged from 5.35 to 7.28, thus covering a range from acid to neutral conditions. Application of the AIP to both limed sites and sites that formerly were acidified demonstrate that the algal community reacts with a several years delay to both river liming and natural recovery. The AIP is most sensitive between mean annual pH values of approximately 5.5 and 7.0 and can be especially useful in detecting the first signs of an acidification trend or the last steps of a recovery process.