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full suite of physical, geochemical and microfossil analyses were conducted from
dated replicate sediment cores at each site.
Despite chronological uncertainties associated with problems of matching
radiometric dates from the sediment record to the calendar dates of the
meteorological record, the study showed a good correlation between increasing
temperature and the organic matter content in the sediment and between
increasing temperature and changes in the composition of the diatom record.
Both suggest that changes in algal composition and overall lake productivity have
been responding to temperature increases of the last few decades and support the
interpretations above describing the probable relationship between organic
matter and temperature inferred for earlier periods.
One of the most striking observations at a number of mountain sites in Europe
has been an increase in the abundance of planktonic diatoms, especially of the
genus
Cyclotella
(Rühland
et al
. 2008). Perhaps the best example is from Lake
Redo in the Spanish Pyrenees (Catalan
et al
. 2002). Sediment core analysis from
this lake showed that two species of planktonic diatom,
Fragilaria nanana
and
Cyclotella pseudostelligera
, appeared in the lake in the late 19th-early 20th
century and became more abundant over the last few decades. An examination of
the current phytoplankton seasonality in the lake showed that these species are
dominant in late summer and early autumn, appearing towards the end of the
summer stratification period and reaching maximum abundance in September
and October, respectively. A close correspondence between the increase in these
taxa in the core record and the October air temperature (Fig. 2.6) suggests that
increasing autumn water temperature has been the main reason, directly or
indirectly, for the change.
Similar examples of planktonic diatom increases have been recorded from
sediments of lakes in other remote high-altitude and high-latitude regions (e.g.
Sovari & Korhola 1998; Koinig
et al
. 2002; Sovari
et al
. 2002; Rühland & Smol
2005; Smol
et al
. 2005; Solovieva
et al
. 2005, 2008).
Although the grounds for interpreting such changes in remote lakes as a
consequence of increasing temperature are credible, the potential separate or
additional effect of long-range transported nutrient pollution cannot be excluded
in some cases. For example, Wolfe
et al
. (2001) showed that for two remote
mountain lakes in the Rocky Mountains, the striking increase in the abundance
of
Asterionella formosa
in the uppermost sediments of both lakes corresponded
to an increase in the concentration of lead (Pb) in the core and to a decrease in
the values of the stable isotope
15
N, indicating that the plankton increase was
more likely to have been caused by air pollution than by global warming. Studies
of present-day nutrient dynamics of remote lakes across Europe and North
America have confirmed the extent to which atmospheric inorganic N deposition
is an important nutrient source for such lakes (Bergström & Jansson 2006),
illustrating the need for caution in attributing productivity increases entirely to
the direct impact of climate change at some sites.
The problem in disentangling the separate but interacting influences of nutrient
deposition and climate warming is that both can cause symptoms of eutrophication
that are recorded in an identical way in lake sediments, viz. by an increase in
organic matter and by a change in the relative abundance of planktonic diatoms.
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