Aquatic Ecosystem Variability and Climate Change – A Palaeoecological Perspective - Climate Change Impacts on Freshwater Ecosystems - page 27

their most important attributes because these enable stratigraphic correlations
with data derived from other archives to be made with a minimum of ambi-
guity. In a novel study of the annually laminated sediments of Kassjön, northern
Sweden, Simpson and Anderson (2009) were able to match chronologically
the sediment record from the lake with the tree-ring-based temperature record
from the same region in northern Sweden. They compared a 400-year period
from −200 BC to AD 200, a period of known climate variability, before the
introduction of agriculture into the region. The aim was to assess how much
variability in the diatom record could be attributed to natural temperature vari-
ability. As both the climate proxy (tree rings) and the biological response
(diatoms) were based on regularly spaced samples in time, classic time-series
statistics could be used for the analysis. Although the reasons for the correlation
are as yet unclear, the results show a clear correspondence between the climatic
variability and the observed magnitudes of diatom assemblage shifts on a range
of timescales.
Recent change
Remote regions
Given the natural variability of the climate system and the complexity of ecosystem
response to climate change on different timescales, it is not easy to use
contemporary observations to separate trends in aquatic ecosystem behaviour
that can be attributed to anthropogenic climate change from those caused by
natural climate forcing. And it is doubly difficult to identify climate impacts on
aquatic systems for those systems that have been, and are, strongly influenced in
other ways by human activity over recent decades and centuries. Such influences,
principally from land-use change and from pollution, are far stronger than climate
change. The palaeolimnological record, however, can provide a sufficiently long
time frame for these different influences to be disentangled, but only the most
remote regions on earth, free from pollution, provide good locations where
unambiguous evidence for recent limnological change attributable to climate
change can be potentially identified. Even in such regions, the confounding
impact of long-distance transported pollutants, especially from nutrients, cannot
always be easily discounted (cf. Wolfe et al . 2001).
Nevertheless, many palaeolimnologists have focussed research over recent
years on lake sediment records in arctic-alpine regions above or beyond the
timber line, regions also where the effects of global warming are projected to be
greatest (e.g. Smol & Douglas 2007). In Europe, there has been a special interest
in alpine lakes. In one large-scale study that included lakes from all the major
mountain regions, the recent sediment record was compared with instrumental
temperature records to assess how much of the variability in the record could be
attributed to temperature change over the last 200 years (Battarbee et al . 2002).
Homogenized instrumental temperature records constructed from meteorological
station data were tuned to each mountain site using corrections for lapse-rate
differences and for shading effects (Agustí-Panareda & Thompson 2002), and a