A critical limit for acid neutralizing capacity in Norwegian surface waters, based on new analyses of fish and invertebrate responses

The status of fish and invertebrate populations was analysed in the context of surface water acidification and loss of acid neutralizing capacity in Norwegian lakes and streams. The invertebrate data came from 165 sites, and the fish data included populations in 1095 lakes, plus the Atlantic salmon populations in 30 rivers. The status of both fish and invertebrates was strongly related to both acid neutralization capacity ANC (Σ base cations - Σ strong acid anions) and the concentration of labile aluminium. Ca2+ and TOC mederated the toxicity of both low pH and high aluminium. The critical level of ANC varied among fish species, with Atlantic salmon being the most sensitive, followed by brown trout. Perch were the most tolerant of low pH/high Aln+. Atlantic salmon status appears to be a good indicator of acidification of rivers, and trout is a useful indicator for lakes. Based on an evaluation of fish and invertebrate populations, a critical lower limit of ANC = 20 μequiv./l is suggested as the tolerance level in Norwegian surface waters.