Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 55

During the period 1990-2004, the observed increases in DOC concentrations
in Europe and eastern North America have clearly been driven mainly by
reductions in atmospheric sulphur deposition, resulting from international
legislation to regulate pollutant emissions since the 1980s (Evans et al . 2006;
Vuorenmaa et al . 2006; Monteith et al . 2007). As sulphur deposition has fallen,
reductions in acidity and ionic strength have allowed more DOC to remain in
solution in soil water, and therefore to be leached to surface waters.
European sulphur emissions have fallen dramatically (by around three quarters)
since their 1980 peak (Vestreng et al . 2007), and while some further reductions
are likely, they clearly will not be on the scale of those observed in the past. On
the other hand, climatic changes are expected to continue, or even accelerate,
into the future. Some field studies have suggested that a large proportion of DOC
is derived from recently fixed carbon and any response by net primary production
to changes in temperature or soil moisture may therefore be expected to influence
the size of the organic matter pool available for decomposition and, hence, DOC
production. Further, laboratory studies have confirmed that DOC production
per se increases with rising temperature as a result of microbial activity, particularly
in peats subject to drying (e.g. Evans et al . 2005)
Experimental data suggest that rates of soil DOC production are increased
under higher temperatures and in response to a shift from anaerobic to aerobic
conditions in saturated soils (Clark et al . 2009). A study by Tipping et al . (1999)
also indicated that climatic warming will increase the production of potentially
soluble organic matter. On the other hand, more severe drying during droughts
may have the opposite effect on DOC leaching; field manipulation experiments on
podsolic heathland soils in Wales (see below) showed decreasing microbial activity
and DOC concentrations in response to experimental drought, with increased
DOC observed following soil re-wetting (Toberman et al . 2008). Overall, Evans
et al . (2006) concluded for UK surface waters that warming (of around 0.6°C)
since the 1980s could account for a small proportion of the observed increase in
DOC, but that reductions in acid deposition were the dominant driver of change
to date. Statistical analysis of long-term data records from Storgama, Birkenes and
Langtjern (Norway) indicate that climate variables explain a significant part of the
seasonal variation in DOC concentrations (de Wit et al . 2008), while the long-
term increase in TOC is related to reduced acid deposition. Most of the seasonal
variation is apparently related to temperature and precipitation.
In coastal areas, climate may also impact DOC through decadal-scale variations
in the deposition of sea-salt ions (Evans et al . 2001). High levels of sea-salt
deposition, which are linked to high wind speeds and hence to positive phases of
the North Atlantic Oscillation, may affect DOC solubility through a mechanism
analogous to the effect of sulphur deposition, by causing transient periods of
acidification and increased ionic strength, suppressing DOC release. This
mechanism also appears to have contributed to observed DOC increases in some
areas (Monteith et al . 2007).
In summary, these findings suggest that, as sulphate deposition returns towards
background (i.e. pre-industrial) levels, climatic factors such as discharge and
temperature may become the major drivers of variability in dissolved organic
matter.