Most lakes and reservoirs have surface CO 2 concentrations that are supersaturated relative to the atmosphere 1 . The resulting CO 2 emissions from lakes represent a substantial contribution to the continental carbon balance 2-4 . Thus, the drivers of CO 2 supersaturation in lakes need to be understood to constrain the sensitivity of the land carbon cycle to external perturbations 4-6 . Carbon dioxide supersaturation has generally been attributed to the accumulation of inorganic carbon in lakes where respiration exceeds photosynthesis 7,8 , but this interpretation has faced challenges 9-11 . Here we report analyses of water chemistry data from a survey of Spanish reservoirs that represent a range of lithologies, using simple metabolic models. We find that, above an alkalinity threshold of 1 mequiv. l −1 , CO 2 supersaturation in lakes is directly related to carbonate weathering in the watershed. We then evaluate the global distribution of alkalinity in lakes and find that 57% of the surface area occupied by lakes and reservoirs-particularly in tropical and temperate latitudes-has alkalinity exceeding 1 mequiv. l −1 . We conclude that lake inputs of dissolved inorganic carbon from carbonate weathering should be considered for the CO 2 supersaturation of lakes at both regional and global scales.There are two main mechanisms leading to CO 2 supersaturation in lakes and reservoirs: in situ net ecosystem production (NEP) imbalanced towards net heterotrophy 7 (that is, respiration exceeding photosynthesis); and inputs of groundwater or surface water with high dissolved inorganic carbon (DIC) content coming from both weathering of minerals and soil respiration in the watershed 10-14 . Among these, NEP is usually considered to be the main factor driving CO 2 supersaturation in lakes 7,8 . However, most studies relating CO 2 supersaturation to NEP have focused on relatively dilute, low-alkalinity lakes, despite the fact that CO 2 concentration in water is strongly modulated by the carbonate equilibrium (that is, the chemical reactions relating the different forms of DIC). Therefore, we lack conclusive evidence of the impact of NEP on CO 2 supersaturation in a range of systems showing contrasting DIC content.To test how DIC content can drive CO 2 supersaturation and modulate the relationship between NEP and CO 2 concentration, we use data from a nationwide study 15 including 202 measurements of dissolved oxygen (DO), DIC, alkalinity and CO 2 concentration in the surface layer of a set of Spanish reservoirs covering a wide range of DIC content and trophic states (Supplementary Table 1 and Data). We use the observed DO disequilibrium relative to the atmosphere as a surrogate for surface lake NEP, and investigate its impact on observed DIC and CO 2 concentrations considering three metabolic models. The models combine assumptions concerning the DIC loading from the watershed and the effect of lake NEP (Fig. 1a). All three models assume that the DIC generated during weathering of minerals in the watershed (DIC W ) dominates DIC...

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The potential for rivers to alter the flux of dissolved organic matter (DOM) from land to ocean is widely accepted. Yet anticipating when and where rivers behave as active reactors vs. passive pipes of DOM stands as a major knowledge gap in river biogeochemistry, resulting in uncertainties for global carbon models. Here, we investigate the controls on in-stream DOM dynamics by evaluating changes in DOM concentration and composition along several reaches of a medium-sized river network over one full hydrological year. Roughly half of the observations over time and space showed active reactor conditions and, among these, similar proportion of gains and losses was measured. High water residence times promoted the active over passive behavior of the reaches, while DOM properties and nitrate availability determined whether they supplied or removed DOM from the river. Among different DOM fractions, protein-like DOM both of terrestrial and aquatic origin seemed to drive bulk DOM patterns. Our study emphasizes the role of water residence time as a physical constraint for in-stream processes, and provides new insights into the key factors governing the net balance between in-stream gains and losses of DOM in rivers.

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Contamination routes of Listeria monocytogenes were examined for 3 years in an Iberian pork-processing plant that produced high-quality ready-to-eat meat products. Molecular subtypes of L. monocytogenes were determined by polymerase chain reaction-based serotyping and pulsed-field gel electrophoresis (PFGE) restriction analysis. A total of 541 L. monocytogenes isolates were recovered from the environment and equipment (n = 165), carcasses (n = 28), raw products (n = 321), and dry-cured products (n = 27). Only 29 different PFGE types were identified, 3 of which were repeatedly found to be persistent types and accounted for 73% of the isolates. One PFGE type dominated (45% of the isolates) and was mostly recovered from intermediate manufactured products and the environment of the manufacturing area. L. monocytogenes persistence appeared strongly linked to the manufacture of products and not to its sustained entrance with the raw material. Some clones were found to survive in the manufacturing area for 3 years. Controlling the contamination of raw ingredients, improving the compartmentalization, and changing the cleaning protocols resulted in reduced prevalence rates of L. monocytogenes on products; two persistent PFGE types were eliminated from the processing plant, although eradication of other adapted strains has not been achieved.