The temperature dependence of the carbon cycle in aquatic ecosystems

The carbon cycle modulates climate change via the regulation of atmospheric CO₂, and 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. We developed theoretical models derived from the metabolic theory of ecology (MTE), and tested them in an ecosystem-level manipulative experiment in freshwater mesocosms. The year-long experiment simulated a warming scenario (A1B; [IPCC, 2007]) expected by the end of the century. The key components of the carbon cycle - that is gross primary production (GPP), ecosystem respiration (ER) and CH₄ efflux (ME) - measured in our experiment were all strongly related to temperature. Their temperature dependence was typically constrained by the average activation energy of their particular metabolic pathway, and as predicted by our models, this increased progressively for GPP, ER and ME. Warming of 4 C decreased the sequestration of CO₂ by 13%, increased the fraction of primary production effluxing as methane by 20% and the fraction of ER as methane by 9%, in line with the offset in their respective activation energies. Because methane has 21 times the greenhouse gas radiative potential of CO₂, these results suggest aquatic ecosystems could drive a previously unknown positive feedback between warming and the carbon cycle. We then used a series of global data compilations of measurements of rates of primary production and respiration to better understand the temperature dependence of the carbon cycle in other aquatic ecosystems and to compare them with data from terrestrial systems. Our experimental results were mirrored by our global data compilations, with the effective activation energy for marine and freshwater primary production identical to GPP measured in our experiment. Similarly, the temperature dependences of respiration in estuaries, lakes and the ocean were indistinguishable from that of ER in our experiment. Finally, our study suggests that the temperature dependence of primary production and respiration in aquatic ecosystems might differ from those in terrestrial ecosystems, and this could be crucial in predicting the future response of the carbon cycle in these different systems to global warming.