Type of article: Special features to Ecological Research, “The Future
Environment and Role of Multiple Air Pollutants”.
Transitioning from anthropogenic to natural acidification in a humic
catchment in Norway: projections of deposition and climate change
effects
Heleen A. de Wit1,2*, François
Clayer1, Øyvind Kaste1 and Magnus
Norling1
1 Norwegian Institute for Water Research, Oslo, Norway
2 Centre for Biogeochemistry in the Anthropocene, Department of
Biosciences, University of Oslo, Norway
*corresponding author
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.
Keywords: organic acids, weathering, climate change, chemical recovery,
forest
Introduction
Reductions in anthropogenic emissions of sulfur (S) and nitrogen (N) to
the atmosphere has resulted in wide-scale chemical recovery of surface
waters in acid-sensitive regions in Europe and North America (Garmo et
al. 2014; Skjelkvale et al. 2005). Reductions in sulfate
(SO4), rather than in N deposition have been the
dominant driver of water chemical recovery since N deposition change has
been less distinct than for S, and N-retention in catchments diminish
its impact on elemental runoff (Watmough et al. 2005).
Chemical recovery is impacted by other factors than reduced
SO4 deposition and associated changes in inorganic
cations and anions. Reduced air pollution has also led to widespread
browning of surface waters (de Wit et al. 2021; Monteith et al. 2007),
where other factors such as precipitation also impact dissolved organic
matter (DOM) concentrations (de Wit et al. 2016). Increases in DOM are
relevant for assessing chemical recovery of surface waters since
increases in organic acidity partly compensate for declines in mineral
acidity (Evans et al. 2008a), also illustrated by the organic-acid
adjusted acid neutralizing capacity (ANCoaa) which better describes
acidification in humic lakes than ANC only based on major ions (Lydersen
et al. 2004). Factors that affect surface water acidification on a more
local or regional basis are seasalt spray, through mobilization of
protons and aluminium (Hindar et al. 2004), catchment disturbances such
as tree die-back from insect attacks, leading to mobilization of S and N
(Oulehle et al. 2021), and hydrology through post-drought mobilization
of SO4 (Clark et al. 2006) and variation in flow-paths
(Evans et al. 2008b). Possibly, chemical recovery may be enhanced by
climate-induced increased weathering rates (Augustin et al. 2015)
resulting in higher rates of base cation replenishment in soils, which
have lost base cations through decades of mobilization and leaching due
to acid deposition (Watmough et al. 2005).
Because of the high success of historical emission reductions of S, the
potential for further reductions in Europe will be limited (Grennfelt et
al. 2020; Schopp et al. 2003). That implies that other factors such as
climate and land use will become relatively more important for future
chemical recovery of surface waters (Kopacek et al. 2016; Vuorenmaa et
al. 2017) and for the time required for reaching pre-industrial water
quality, if at all possible (Helliwell & Simpson 2010).
For assessment of future surface water acidification, the
process-oriented model MAGIC (Cosby et al. 1985; Norling et al. this
issue) is commonly used (Larssen 2005; Posch et al. 2019). MAGIC has in
its core descriptions of acid-base soil chemistry and elemental mass
balances and has been further developed to include forest N cycling and
C storage which potentially enable simulation of catchment disturbances
and climate change effects on element cycling and surface water
chemistry (Valinia et al. 2021). A new version of MAGIC called
MAGIC-Forest is implemented in the model development framework Mobius
(Norling et al. 2021) enabling flexible sensitivity analysis and
addition of model features (Norling et al., this issue) ,
Simulation of effects of ‘confounding factors’ on expected recovery of
surface water is necessary for credible predictions in the current era
where factors other than deposition will influence surface water
acidification status (de Wit et al. 2023). Until recently, browning of
surface waters was not implemented in MAGIC while weathering rates were
calibrated to a constant value. In the current era of low S deposition,
changes in organic acids and weathering are likely to become more
important drivers of surface water acidity status.
Here, we aim to test the MAGIC model on its ability to describe five
decades of acidification and recovery in the acidified, forested
Langtjern catchment using new features in MAGIC, e.g. browning
(sulfate-control) and climate-dependency of weathering, and predict
acidification under further reductions of atmospheric deposition and
climate change. Our research questions are the following: i. can we
reproduce empirical charge balances at Langtjern, including estimates of
organic charge; ii. Given observed increases in DOM at Langtjern, can
MAGIC reproduce acidification and recovery, and produce credible
hindcasts and forecasts of pre-industrial and future ANC under changes
in acid deposition? iii. How high must weathering rates be to reach
preindustrial ANC in 2100, given projections of climate and deposition?
The model calibration will be anchored in five decades of streamwater
monitoring at Langtjern.