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These observations suggest that as sulphur deposition continues to decrease,
temperature change is likely to emerge as the key driver of lake acidity at many
sites; pH values at non-polluted sites may reach levels only previously experienced
in the warmer early Holocene (Larsen
et al
. 2006). The results also suggest that
some of the increases in alkalinity that have taken place over recent decades (e.g.
Monteith & Evans 2005) might be attributed to increasing temperature as well
as reducing acid deposition. However, for regions, e.g. in North-West Europe,
where climate change may cause an increase in precipitation, such an effect might
be offset as higher discharge and the associated deposition of sea salts in stormy
conditions may depress alkalinity. So far, there is no palaeolimnological evidence
for this effect, but the analysis of long-term monitoring data from the United
Kingdom supports the hypothesis (D. T. Monteith, personal communication).
Identifying the role of climate change on lakes suffering from eutrophication
from lake sediment records is equally difficult. All lakes with populated or
agricultural catchments are influenced by nutrient pollution to some extent, and
as water temperature increase tends to produce symptoms identical to those of
eutrophication in terms of algal productivity, hypolimnetic oxygen stress and
nutrient recycling, the palaeolimnological record is not capable of differentiating
between the different stresses, at least using standard techniques. Current palaeo-
ecological research is attempting to disentangle these stresses using statistical
approaches beginning with sites where both long-term instrumental climatic data
and nutrient concentration data are available. At Loch Leven, a large, shallow
lake in lowland Scotland, the eutrophication signal in the sediment record
outweighs any evidence of climate as a control on the diatom community on a
decadal to centennial scale. However, at an inter-annual scale, there are several
changes in species composition in the recent fossil record that may be attributed
to climate variability. In particular, peaks in
Aulacoseira ambigua
and
A. granulata
and its variety
angustissima
in 2003-4, 1998-9 and 1986-7 appear to be
associated with wetter, and possibly windier and cooler, summers (H. Bennion
et al
.,
personal communication). At Lago Maggiore, a deep, pre-alpine lake in Northern
Italy, the combination of Cladocera data from the sediment record with
contemporary zooplankton and fish data has provided insights into past changes
in the interaction between trophic dynamics of the lake and climate change
(Manca
et al
. 2007), whereby evidence of increased instability in recent
zooplankton communities follows the occurrence of extreme meteorological
events and changes in fish predation, which, in turn, may be attributed to
increased temperature.
Conclusions
The palaeoenvironmental record indicates that climate has varied naturally and
continually on a range of different timescales influenced principally by variability in
the orbit of the earth around the sun, by changes in solar irradiance and in the
emission of dust from volcanic eruptions. Instrumental records also show that
climate system is also naturally dynamic and has its own internal modes of variability
that can lead to significant oscillations in weather patterns on inter-annual and
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