Norwegian lakes show widespread recovery from acidification; results from national surveys of lakewater chemistry 1986-1997

Surveys of 485 lakes in Norway conducted in 1986 and again in 1995 reveal w idespread chemical recovery from acidification. Sulphate concentrations in lakes have decreased by 40% in acidified areas in southern Norway. This dec rease has been compensated about 25% by decreases in concentrations of base cations and about 75% by increased Acid Neutralising Capacity (ANC). The i ncreased ANC in turn reflects lower concentrations of acidic cations Alumin ium (Aln+) and Hydrogen (H+). A sub-set of 78 of the 485 lakes sampled year ly between 1986 and 1997 shows that, at first most of the decrease in non-m arine sulphate (SO4*) was compensated by a decrease in base cations, such t hat ANC remained unchanged. Then as SO4* continued to decrease, the concent rations of non-marine calcium and magnesium ((Ca+Mg)*) levelled out. Conseq uently, ANC increased, and H+ and Aln+ started to decrease. In eastern Norw ay, this shift occurred in 1989-90, and came slightly later in southern and western Norway. Similar shifts in trends in about 1991-92 can also be seen in the non-acidified areas in central and northern Norway. This shift in t rends is not as pronounced in western Norway, perhaps because of the confou nding influence of sea-salt episodes on water chemistry. This is the first documented national-scale recovery from acidification due to reduced acid d eposition. Future climate warming and potentially increased N-leaching can counteract the positive trends in recovery from acidification.