The carbon cycle modulates climate change, via the regulation of atmospheric CO 2 , and it represents one of the most important services provided by ecosystems. However, considerable uncertainties remain concerning potential feedback between the biota and the climate. In particular, it is unclear how global warming will affect the metabolic balance between the photosynthetic fixation and respiratory release of CO 2 at the ecosystem scale. Here, we present a combination of experimental field data from freshwater mesocosms, and theoretical predictions derived from the metabolic theory of ecology to investigate whether warming will alter the capacity of ecosystems to absorb CO 2 . Our manipulative experiment simulated the temperature increases predicted for the end of the century and revealed that ecosystem respiration increased at a faster rate than primary production, reducing carbon sequestration by 13 per cent. These results confirmed our theoretical predictions based on the differential activation energies of these two processes. Using only the activation energies for whole ecosystem photosynthesis and respiration we provide a theoretical prediction that accurately quantified the precise magnitude of the reduction in carbon sequestration observed experimentally. We suggest the combination of whole-ecosystem manipulative experiments and ecological theory is one of the most promising and fruitful research areas to predict the impacts of climate change on key ecosystem services.

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Organism size is one of the key determinants of community structure, and its relationship with abundance can describe how biomass is partitioned among the biota within an ecosystem. An outdoor freshwater mesocosm experiment was used to determine how warming of $4 1C would affect the size, biomass and taxonomic structure of planktonic communities. Warming increased the steepness of the community size spectrum by increasing the prevalence of small organisms, primarily within the phytoplankton assemblage and it also reduced the mean and maximum size of phytoplankton by approximately one order of magnitude. The observed shifts in phytoplankton size structure were reflected in changes in phytoplankton community composition, though zooplankton taxonomic composition was unaffected by 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. Overall, warming shifted the distribution of phytoplankton size towards smaller individuals with rapid turnover and low standing biomass, resulting in a reorganization of the biomass structure of the food webs. These results indicate future environmental warming may have profound effects on the structure and functioning of aquatic communities and ecosystems.

Until recently, denitrification was thought to be the only significant pathway for N 2 formation and, in turn, the removal of nitrogen in aquatic sediments. The discovery of anaerobic ammonium oxidation in the laboratory suggested that alternative metabolisms might be present in the environment. By using a combination of 15 ؊ . We observed a shift in the significance of anaerobic ammonium oxidation to N 2 formation relative to denitrification, from 8% near the head of the estuary to less than 1% at the coast. The relative importance of anaerobic ammonium oxidation was positively correlated (P < 0.05) with sediment organic content. This report of anaerobic ammonium oxidation in organically enriched estuarine sediments, though in contrast to a recent report on continental shelf sediments, confirms the presence of this novel metabolism in another aquatic sediment system.Since the 1970s, substantial research has focused on the ability of estuarine sediments to attenuate riverine nitrogen (N) loads before they affect coastal seas (4,18,19,31,33). Estuarine sediments are essentially anaerobic below a few surface millimeters, and the mineralization of organic matter proceeds via alternate electron acceptors such as NO 3 Ϫ and SO 4 2Ϫ (20,27). In turn, the reduction of NO 3 Ϫ removes NO 3 Ϫ from the overlying waters. Until recently, it was largely thought that NO 3 Ϫ could be either reduced to N 2 gas via denitrification (a facultative metabolism mediated by a variety of bacteria) and lost from the system or reduced to ammonium (NH 4 ϩ ) by fermentative metabolisms and hence conserved within the sediments (dissimilatory nitrate reduction to ammonium [DNRA]) (8,23). It had been demonstrated that changes in sediment organic loadings and estuarine NO 3 Ϫ concentrations may affect the partitioning between these two end products of NO 3 Ϫ reduction (11, 12). The discovery within the laboratory (17) of anaerobic ammonium oxidation revealed a novel metabolism that could short circuit the N cycle, bypassing what was previously thought to be a critical aerobic nitrification phase and potentially providing an alternative pathway for N 2 gas formation in the environment (Fig. 1).Originally, it was thought (17) that anaerobic ammonium oxidation coupled the oxidation of NH 4 ϩ to the reduction of NO 3 Ϫ :Further work, however, showed that the oxidation of ammonium was actually coupled to the reduction of nitrite rather than nitrate (34, 35):The application of this process to the treatment of nitrogenous waste has received a great deal of attention (9, 10), and more recently, the organism responsible has been classified as a new autotrophic planctomycete (28). Anaerobic ammonium oxidation was recently reported to account for as much as 24 and 60% of N 2 formation in continental shelf sediments in relatively deep water (380 and 695 m, respectively) but less than 2% of N 2 formation in eutrophic shallow coastal bay sediments (30). The drop in the significance of anaerobic ammonium oxidation for N 2 formation relative to denitrificatio...

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