On the community and ecosystem level consequences of warming

Abstract

The 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 3 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.