Interaction of Climate Change and Acid Deposition - Climate Change Impacts on Freshwater Ecosystems - page 167

been raised in Chapter 3, but is amplified by studies from the viewpoint of
acidification processes.
The decline in sulphate concentrations due to declining S deposition is believed
to have resulted in a general increase in DOC concentrations in acid-sensitive
lakes and streams in Europe and eastern North America since the mid-1980s,
possibly due to increased solubility of some components of the organic matter
under less acid conditions (Monteith et al . 2007). Monteith et al . (2007) analysed
time series from 522 remote lakes and streams in Europe and North America and
showed that increasing concentrations of DOC between 1990 and 2004 could be
explained by atmospheric deposition chemistry and catchment acidity (Monteith
et al . 2007). The relative change in DOC per year was strongly inversely related
to the change in sulphate and chloride concentrations in atmospheric deposition.
Thus, acid deposition to these ecosystems seems to have been partially buffered
by changes in organic acidity, and recovery from acidification has implied an
increased mobility of DOC and organic acids.
Vuorenmaa et al . (2006) studied trends in total organic carbon (TOC)
concentrations over the period 1987-2003 in 13 small forest lakes in Finland.
Recovery from acidification (reduced S deposition) and long-term changes in
runoff as potential drivers for the trends were examined. Results showed that
TOC concentrations have increased throughout Finland. Ten of the 13 lakes
showed a significant increasing TOC trend ( p < 0.05) and included both clear
water and humic lakes. The largest annual increase in TOC occurred in lakes
with the largest average concentrations. The magnitudes of the TOC trends
were not significantly related to the proportion of peat soils in the catchment
but catchment size was an important predictor. Decreasing S deposition and
improved acid-base status in soil due to the recovery from acidification implied
an increased mobilization of organic acids and TOC. There was little evidence
that the long-term increasing trend in TOC concentrations was related to long-
term changes in runoff.
Analysis of the long-term increase of DOC in two geochemically contrasting
forested catchments in the Czech Republic did not show a clear climatic effect
on DOC increases (Hruška et al . 2009). Between 1993 and 2007, DOC in stream
water increased significantly in both catchments: the mean annual increase was
0.42 mg l −1 yr −1 ( p < 0.001) in acidic Lysina and 0.43 mg l −1 yr −1 ( p < 0.001) in
alkaline Pluhuv Bor, resulting in cumulative increases of 64% and 65%,
respectively. The long-term increase in DOC was correlated with a decrease in
ionic strength (IS) at both catchments ( p < 0.001), which resulted from declining
atmospheric deposition. Only granitic Lysina was significantly acidified. Well-
buffered Pluhuv Bor, with serpentinite bedrock, showed very minor changes in
stream pH, as incoming SO 4 2− from the atmosphere was buffered by exchangeable
cations in the magnesium-rich soils. Thus, the observed DOC increase at Pluhuv
Bor was not due to changes in acidity. Climate change cannot be driving the
increases in DOC, because none of temperature, annual precipitation or discharge
(annual or weekly) showed statistically significant trends during the study
period.
Results of the MAGIC simulation show that the observed DOC increases will
significantly influence recovery of the Lysina catchment from acidification