Transitioning from anthropogenic to natural acidification in a humic
catchment in Norway: projections of deposition and climate change
effects
Abstract
Five decades of monitoring data (1974-2022) at the acidified,
acid-sensitive forested catchment of Langtjern in southern Norway
document strong chemical recovery and browning of surface water, related
to changes in sulfur (S) deposition. We used the process-oriented model
MAGIC to simulate water chemistry from 1860 to 2100 using historical and
projected deposition and climate. New in MAGIC is i) a solubility
control of dissolved organic carbon (DOC) from S deposition, which
allows inclusion of the changing role of organic acids in chemical
recovery, and ii) climate-dependency of weathering rates. MAGIC
successfully described measured chemical recovery and browning, and the
change towards organic acid dominated acidification status. Hindcasts of
pH suggested lower preindustrial pH than previously modelled with MAGIC,
simulated without sulfate-dependency of DOC solubility. Climate
scenarios indicated substantially wetter climate, leading to increased
base cation losses and slight reacidification of the surface waters. A
sensitivity analysis of weathering rates revealed that a doubling of
weathering rates is needed to reach pre-industrial ANC in 2100, given
that S deposition is expected to be reduced to a minimum. We conclude
that impacts of climate change are most likely to lead to slight
reacidification of surface waters, and that enhanced weathering rates
could partly compensate this trend.