Interaction of Climate Change and Eutrophication - Climate Change Impacts on Freshwater Ecosystems - page 140

small effect. While the effect of N-enrichment confirms results of previous studies
(Velthof et al . 1996), the lack of a temperature effect was unexpected and might
be explained by antagonistic effects of warming on the ratio of denitrification
end products. The temperature difference in the study sites was around 12 °C and
thus twice the estimated maximum temperature rise anticipated by the end of the
21st century (max. 6.4 °C; IPCC 2007). This result implies that a positive feedback
loop between global warming and enhanced N 2 O emissions from wetlands is by
no means a certainty.
Palaeolimnology and modelling
To improve our understanding of how climate change might affect water quality
in the future, we need a better knowledge of how it has modified lake ecosystems
in the past. This information is contained within historical and palaeolimnological
records, but it is difficult to extract because many of the lakes that have been
affected by climate change have experienced changes in nutrient input over a
similar period. In the Euro-limpacs project, several approaches have been applied
to selected lake sites in an attempt to disentangle the effects of nutrient enrichment
and climate change and assess interactions between the two. Statistical modelling
was used on long-term monitoring data to identify trends and relationships
among variables recorded seasonally (e.g. Ferguson et al . 2008), process-based
modelling was applied to sites where historical catchment and lake data are
available to provide information on interactions among variables (e.g. Elliott
et al . 2005; Elliott & May 2008), and palaeolimnology was used to reconstruct
environmental change from the remains of biota preserved in lake sediments and,
thereby, to provide data over a longer period (e.g. Marchetto et al . 2004;
Battarbee et al . 2005; Manca et al . 2007).
Insights from the application of these methods are illustrated here for Loch
Leven, the largest shallow lake in Great Britain, which has been monitored
fortnightly since 1968 and where significant changes in both the climate and the
nutrient availability have been recorded (Carvalho & Kirika 2003). In response
to severe eutrophication problems, phosphorus input from external sources,
mostly a woollen mill, was reduced during the 1990s. The total phosphorus (TP)
loading, which had steadily risen to 20 t yr −1 by 1985, was reduced to 8 t TP yr −1
by 1995 (Bailey-Watts & Kirika 1999). The period of eutrophication and recovery
also spans a period when there has been a measurable impact of climate change
on the lake. Winter ice cover has become less frequent and less extensive, spring
air temperatures have increased markedly and winter rainfall has significantly
increased (Ferguson et al . 2008).
In response to the reductions in point-source nutrient loading, there have been
significant decreasing trends of soluble reactive phosphorus (SRP) concentrations
in summer, autumn and winter, mainly because of decreases in the last decade
(1995-2005). There has, however, been an increasing trend in winter nitrate
concentrations, probably due partly to the significantly increased winter rainfall.
Chlorophyll a concentration has decreased in spring as years have
become warmer, but increased during the rest of the year (Ferguson et al . 2009).