Interaction of Climate Change and Eutrophication - Climate Change Impacts on Freshwater Ecosystems - page 145

mesocosm experiments); and (iii) higher nutrient and carbon turnover and higher
productivity, though perhaps unchanged or reduced NEP.
There is evidence that nitrogen, now increasingly seen as just as important as
phosphorus in freshwater systems (Elser et al . 2007), may have counterintuitive
effects, for example, by inhibiting decomposition, but sometimes also interacting
with temperature to change the structure of aquatic plant communities in ways that
will give more problems, such as increases in nuisance free-floating plants. There is
also evidence that increased temperature will be associated with rapid expansion in
exotic species of plants, already causing management problems, and fish such as
carp, as more thermo-sensitive fish fail to survive. Where they have been introduced,
carp are regarded as a problem to all but a section of the angling community that
avidly seeks to catch large specimens. Important ecosystem processes in wetlands,
such as denitrification, show complex responses to interactions between nutrient
enrichment and warming. While increased loading of nitrogen increases denitrification
in wetlands, antagonistic effects of warming on the ratio of denitrification end
products may prevent emissions of N 2 O, a potent greenhouse gas, from increasing
at the same rate. This implies that warming under nutrient-enriched conditions
need not necessarily lead to positive feedbacks on greenhouse gas production and
emissions, though more experiments are needed to get a more clear picture.
Will changing climate aggravate eutrophication symptoms?
There is clear evidence from the space-for-time comparison of data, controlled
experiments in ponds and lakes and the paired-site studies in Iceland that climate
warming will exacerbate the symptoms of eutrophication. However, the effects will
be complex and will vary depending on initial conditions and location. Shifts in
trophic structure will lead to higher risk of eutrophication symptoms such as algal
blooms and perhaps higher risk of cyanobacterial dominance in larger lakes and of
filamentous algae in shallow lakes and streams. Some symptoms may be unaffected.
For example, no experimental evidence to date suggests that macrophyte-dominated
clear-water lakes will shift to a turbid state. Indeed, results of the mesocosm
experiments provide no support for the presumption that increases in nutrient
supply alone drive the displacement of aquatic plants. Other factors such as increased
salinity, raised water levels, changes in fish communities, mechanical damage or
damage by vertebrate grazers or toxins to invertebrate grazers or the plants
themselves will generally be involved. In Icelandic wetlands, symptoms such as
increased N 2 O emission may even be abated following global warming, but further
studies are needed to verify this finding for a wider range of wetland types.
Can effects of climate change be distinguished from those
of eutrophication?
It is patently difficult to distinguish climate change effects from eutrophication effects
in long-term records and palaeolimnological studies, since the symptoms accompanying
both changes are rather similar and both have occurred simultaneously, at least in the
last 150 years. However, the shift in trophic structure, phenology and life histories of
organisms (e.g. age and size of first reproduction, longevity, seasonal dynamics) can