Interaction of Climate Change and Eutrophication - Climate Change Impacts on Freshwater Ecosystems - page 133

faster than litter in any of the mesocosms. Nonetheless, it is remarkable that the
pattern of stem litter decomposition was distinct from that of leaf litter decomposing
in the same place. Thus, it appears that effects of warming and nitrogen enrichment
vary with litter quality, making forecasts of global change effects on litter
decomposition challenging. Consequently, predictions based only on temperature-
response relationships such as Q 10 models may be of limited value.
An assessment of whole-mesocosm metabolism by monitoring oxygen
concentrations over two 3-day periods on four occasions revealed that GPP, ER
and NEP were characterized by invariably low activity in summer, autumn and
winter. As a result, no clear differences were apparent among treatments during
these periods. In spring, NEP was also unaffected by warming. However, although
not statistically significant, a tendency emerged towards higher GPP and ER at
this time. Metabolism in N-enriched mesocosms also tended to be lower than in
unfertilized mesocosms, as had been seen for leaf decomposition and leaf-
associated respiration rates. Provided this pattern turns out to be robust when
substantiated by more data, it would imply that the stimulating effects of predicted
global warming on GPP and ER could be offset, at least in part, by sustained
nitrogen deposition. This contrasts with a finding in the UK experiments that
respiration by heterotrophs was stimulated by both warming and nitrogen
addition, which suggests the opposite. Distinct differences between systems
should be borne in mind, however. The UK system was very rich in nutrients,
dominated by submerged and free-floating plants; the Swiss system was in an
emergent plant stand in a much less eutrophic lake. Extension of such results on
a global basis will thus need many more experiments. However, it is clear that
important implications for carbon sequestration and the balance between aerobic
and anaerobic decomposition processes in freshwater systems can be expected.
Microevolution experiments in the UK and Danish mesocosms
Overall results of the three mesocosm experiments in the United Kingdom, Denmark
and Switzerland point to a general exacerbation of eutrophication symptoms with
warming. However, some of the observed responses were counterintuitive. Another
process that needs to be considered is evolutionary adaptation to warming. We used
two of the mesocosm systems of the Euro-limpacs project to investigate the
possibility that such adaptations can occur on short timescales.
When local communities inhabiting a lake or pond experience a temperature
increase, strong selection is imposed, to which the community may respond by
changes in species composition. For example, in the UK mesocosm experiments,
ostracods significantly increased with warming. In addition to species sorting,
however, each individual population may respond to selection pressures by
changing its trait values to become better adapted to the new conditions. These
microevolutionary changes may potentially allow local populations to cope with
climate change and may impact community composition by reducing the number
of species replacements (see Urban et al . 2008).
We studied evolutionary adaptation to temperature of various cladocerans in
the mesocosm experiments run in the United Kingdom and Denmark. Life-history
experiments on clones isolated from the cladoceran Simocephalus populations
inhabiting mesocosms in Denmark provided solid proof of microevolutionary