Long-term records and modelling of acidification, recovery, and liming at Lake Hovvatn, Norway

Lake Store Hovvatn and the adjacent Lake Lille Hovvatn (Norway) are acidified owing to long-term deposition of S and N. By 1974, pH was 4.46 and acid-neutralizing capacity was –42 µequiv.·L–1. Following a lake SO4 reduction from 92 to 33 µequiv.·L–1, pH had increased to 4.8 and acid-neutralizing capacity had increased to –8 µequiv.·L–1 by 2003. The acidification history is well reconstructed using the dynamic model MAGIC. The model predicts that the lakes will not, however, recover to conditions adequate to support a self-reproducing brown trout (Salmo trutta) population. Lake Store Hovvatn was first limed in 1981 and subsequently annually or biannually until 1999, at which time the entire catchment was limed. Liming increased pH to above target levels of 6.0 and reduced inorganic Al to below 5 µequiv.·L–1 in the main water body. Only after the terrestrial liming in 1999, however, was pH potentially adequate for egg survival in the lake during winter, as pH at shallow depths below the ice stayed above 5.5. The results indicate that even the dramatic reduction in acid deposition in Europe will be insufficient to provide water quality adequate for fish populations; such lakes will require some sort of liming for many decades to come.