Freshwater environments contribute 75% of the natural global methane (CH(4)) emissions. While there are indications that tropical lakes and reservoirs emit 58-400% more CH(4) per unit area than similar environments in boreal and temperate biomes, direct measurements of tropical lake emissions are scarce. We measured CH(4) emissions from 16 natural shallow lakes in the Pantanal region of South America, one of the world's largest tropical wetland areas, during the low water period using floating flux chambers. Measured fluxes ranged from 3.9 to 74.2 mmol m(-2) d(-1) with the average from all studied lakes being 8.8 mmol m(-2) d(-1) (131.8 mg CH(4) m(-2) d(-1)), of which ebullition accounted for 91% of the flux (28-98% on individual lakes). Diel cycling of emission rates was observed and therefore 24-h long measurements are recommended rather than short-term measurements not accounting for the full diel cycle. Methane emission variability within a lake may be equal to or more important than between lake variability in floodplain areas as this study identified diverse habitats within lakes having widely different flux rates. Future measurements with static floating chambers should be based on many individual chambers distributed in the various subenvironments of a lake that may differ in emissions in order to account for the within lake variability.

Inland water sediments receive large quantities of terrestrial organic matter [1][2][3][4][5] and are globally important sites for organic carbon preservation [5][6] . Sediment organic matter mineralization is positively related with temperature across a wide range of high-latitude ecosystems [6][7][8][9][10] , but the situation in the tropics remains unclear. Here we assessed temperature effects on the biological production of CO 2 and CH 4 in anaerobic sediments of tropical lakes in the Amazon and boreal lakes in Sweden. Based on conservative regional warming projections until 2100 11 , we estimate that sediment CO 2 and CH 4 production will increase 9-61 % above present rates. Combining the CO 2 and CH 4 as CO 2 equivalents (CO 2eq ) 11 , the predicted increase is 2.4 -4.5 times higher in tropical than boreal sediments. Although the estimated lake area in low latitudes is 3.2 times smaller than that of the boreal zone, we estimate that the increase in gas production from tropical lake sediments would be on an average 2.4 times higher for CO 2 and 2.8 times higher for CH 4 . The exponential temperature response of organic matter mineralization, coupled with higher increases in the proportion of CH 4 relative to CO 2 upon warming, suggests that the production of GHGs in tropical sediments will increase substantially. This represents a potential large-scale positive feedback to climate change. 2 Main textTropical and boreal biomes harbour approximately 50 % of the lakes on Earth 12 . These inland waters emit substantial amounts of carbon dioxide (CO 2 ; in the order of 0.5 Pg yr -1 ) 1,4,13,14 and methane (CH 4 ; 70 Tg yr -1 ) 15 . Organic matter escapes mineralization via burial in lake sediments, representing a global carbon (C) sink [13][14][15] . Cold conditions are favouring organic carbon (OC) preservation in lakes at northern latitudes [8][9][10]16 , whereas warm inland waters show intense organic degradation supporting high C emissions to the atmosphere 4,5,17,18 .Temperature and OC mineralization were recently shown to be strongly positively related in boreal lake sediments overlain by oxic water 9 . However, the majority of freshwater sediments below the uppermost layer (typically a few mm) are anoxic 19 , where the anaerobic biological degradation of OC releases not only CO 2 but also significant amounts of CH 4 15 . Although higher temperatures are also expected to increase metabolic responses 20 , the effects of changing temperatures on OC mineralization can depend on several factors including organic matter characteristics (e.g. the Carbon-QualityTemperature hypothesis) 21 . Thus, the temperature sensitivity of OC stocks at high latitudes previously reported [6][7][8]16 , may not be valid in the tropics where temperature sensitivity data is much more scarce 22 . We compared the anaerobic OC mineralization to CO 2 and CH 4 in tropical and boreal lake sediments along a temperature gradient. We simultaneously sampled a wide range of lake sediments from both tropical and boreal zones (see Suppl...

Tropical lake sediments are a significant source for the greenhouse gas methane. We studied function (pathway, rate) and structure (abundance, taxonomic composition) of the microbial communities (Bacteria, Archaea) leading to methane formation together with the main physicochemical characteristics in the sediments of four clear water, six white water and three black water lakes of the Amazon River system. Concentrations of sulfate and ferric iron, pH and δ C of organic carbon were usually higher, while concentrations of carbon, nitrogen and rates of CH production were generally lower in white water versus clear water or black water sediments. Copy numbers of bacterial and especially archaeal ribosomal RNA genes also tended to be relatively lower in white water sediments. Hydrogenotrophic methanogenesis contributed 58 ± 16% to total CH production in all systems. Network analysis identified six communities, of which four were comprised mostly of bacteria found in all sediment types, while two were mostly in clear water sediment. Terminal restriction fragment length polymorphism (T-RFLP) and pyrosequencing showed that the compositions of the communities differed between the different sediment systems, statistically related to the particular physicochemical conditions and to CH production rates. Among the archaea, clear water, white water, and black water sediments contained relatively more Methanomicrobiales, Methanosarcinaceae and Methanocellales, respectively, while Methanosaetaceae were common in all systems. Proteobacteria, Deltaproteobacteria (Myxococcales, Syntrophobacterales, sulfate reducers) in particular, Acidobacteria and Firmicutes were the most abundant bacterial phyla in all sediment systems. Among the other important bacterial phyla, clear water sediments contained relatively more Alphaproteobacteria and Planctomycetes, whereas white water sediments contained relatively more Betaproteobacteria, Firmicutes, Actinobacteria, and Chloroflexi than the respective other sediment systems. The data showed communities of bacteria common to all sediment types, but also revealed microbial groups that were significantly different between the sediment types, which also differed in physicochemical conditions. Our study showed that function of the microbial communities may be understood on the basis of their structures, which in turn are determined by environmental heterogeneity.