Interaction of Climate Change and Eutrophication - Climate Change Impacts on Freshwater Ecosystems - page 121

exacerbate some symptoms of eutrophication in freshwaters, but not others. For
wetlands and streams, this conclusion is mainly based on experiments in paired
sites in Iceland, supplemented with space-for-time cross-comparisons of systems
from different climate regions. For lakes, we have used several approaches,
including space-for-time comparisons, experiments, palaeolimnology and
modelling. The key questions asked were

Do nutrients structure ecosystems in different ways under current and
anticipated future climatic conditions?

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Will changing climate interact with increased nutrient supply to alter
ecosystem processes?

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Will changing climate aggravate eutrophication symptoms?

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Can effects of climate change be distinguished from those of eutrophication?

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Are there lessons to be learnt from the past to understand future problems
better?

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Can we mitigate negative effects of climate change on ecosystems in terms of
enhanced eutrophication?

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These questions are addressed in the following four sections on the basis of the
different approaches used, and the final section attempts to provide some general
conclusions.

Changes in trophic structure

Space-for-time analyses of large data sets from lakes
Eutrophication was formerly seen largely in terms of nutrient flows entering rivers
and lakes from the catchment, and this is still fundamentally the basis of assessing
eutrophication effects. However, once nutrients have entered a basin, the way
that they are apportioned may have different consequences. For example, the
absolute and relative amounts of vascular plants and algae produced are dependent
on the structure of the food web (also called trophic structure). Internal processes,
such as the recycling of nutrients from sediments, may be altered, sometimes
exacerbating the effects of increased external loads (Jeppesen et al . 2009).
Based on comparisons across latitudinal gradients, and presuming free
migration of species northwards, major changes in trophic structure are to be
expected in a future warmer European climate (Jeppesen et al . 2007; 2010).
The fish community will change, with higher dominance of zooplanktivorous
and omnivorous fish, including the thermo-tolerant carp ( Cyprinus carpio )
(Lehtonen 1996), implying increased predation on zooplankton and,
consequently, less grazing on phytoplankton (less top-down control), and a
higher algal biomass (and chlorophyll a ) per unit of phosphorus. A study of 84
shallow European lakes from northern Sweden to Spain showed that the ratio
of fish biomass (expressed as catch per net-night in multi-mesh sized gillnets) to
zooplankton biomass increased southwards whilst the zooplankton:phytoplankton
biomass ratio decreased in the same direction, both substantially (Gyllström
et al . 2005).