PhDThe carbon cycle modulates climate change, via the regulation of atmospheric CO2, and it
represents one of the most important ecosystem services of value to humans. However,
considerable uncertainties remain concerning potential feedbacks between the biota and the
climate. I used an ecosystem-level manipulative experiment in freshwater mesocosms to test
novel theoretical predictions derived from the metabolic theory of ecology (MTE), in an attempt
to understand the consequences of warming for aquatic communities and ecosystems. The yearlong
experiment simulated a warming scenario (A1B) expected by the end of the century. The
experiment revealed that (1) Ecosystem respiration increased at a faster rate than primary
production, reducing carbon sequestration by 13%. These results confirmed my theoretical
predictions based on the different activation energies of these two processes. Furthermore, I
provided a theoretical prediction that accurately quantified the precise magnitude of the
reduction in carbon sequestration observed experimentally, based simply on the activation
energies of these metabolic processes and the relative increase in temperature. (2) Methane
efflux increased at a faster rate than ecosystem respiration and photosynthesis in response to
temperature. This phenomenon was well described by the activation energies of these metabolic
processes. Therefore, warming increased the fraction of primary production emitted as methane
by 21%, and methane efflux represented a 9% greater fraction of ecosystem respiration.
Moreover, because methane is 21 times more potent as a greenhouse gas, relative to CO2, this
work suggests that warming may increase the greenhouse gas efflux potential of freshwater
ecosystems, revealing a previously unknown positive feedback between warming and the carbon
cycle. (3) Warming benefited smaller organisms and increased the steepness of the slope of the
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community size spectrum. As a result the mean body size of phytoplankton in the warmed
systems decreased by an order of magnitude. These results were down to a systematic shift in
phytoplankton community composition in response to warming. Furthermore, warming reduced
community biomass and total phytoplankton biomass, although zooplankton biomass was
unaffected. This resulted in an increase in the zooplankton to phytoplankton biomass ratio in the
warmed mesocosms, which could be explained by faster turnover within the phytoplankton
assemblages. Warming therefore shifted the distribution of phytoplankton body size towards
smaller individuals with rapid turnover and low standing biomass, resulting in a reorganisation of
the biomass structure of the food webs. The results of this thesis suggest that as freshwater
ecosystems warm they become increasingly carbon limited, resulting in a reduced capacity for
carbon sequestration, elevated greenhouse gas efflux potential, and altered body size and
biomass distribution