Recovery of watercourses from acidification slowed down by processes in damp banks of forest streams, Swedish researchers find

Audrey Dixon

ALNARP, Sweden , February 3, 2012 (press release) – Researchers at the Swedish University of Agricultural Sciences (SLU) report that the recovery of watercourses from acidification is slowed down by processes in the damp banks of forest streams (riparian zone). For instance, in the forest stream banks you find aluminium, which will transform into a form that is toxic to fish, when transported into the streams.

The deposition of acidifying substances has decreased substantially since the early 1990's, leading to a decreased acidification pressure on lakes and streams. The recovery rate has been much slower than expected, though.

Aluminium, which is toxic to fish in its inorganic form, is one of the examples illustrating that the recovery rate differ in different parts of the landscape. New findings are described in a special issue of the journal Ambio* summarising results from the Swedish integrated monitoring programme (IM)**.

“We see that the reduced acidification pressure does not give an equal impact in all parts of the forest landscape. On higher grounds, aluminium now precipitates in the soil and is trapped to a much higher degree, as expected. But in damp and low lying areas we still find high levels of aluminium bound to humic substances. Since the streams are still quite acidic, the aluminium is partly transformed to its toxic form, when these humic substances enter the streams,” says Stefan Löfgren, Senior Research Officer at SLU.

The reason that forest streams is still quite acidic is in large part due to the weather conditions during the past decades, generating an increased runoff of water from the humus-rich, shallow soil layers. This in turn, has given an increased outflow of organic acids from the ground and makes the streams acidic, despite that the acidifying deposition has decreased.

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