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The CO2 flux (FCO2) from lakes to the atmosphere is a large component of the global carbon cycle and depends on the air–water CO2 concentration gradient (ΔCO2) and the gas transfer velocity (k). Both ΔCO2 and k can vary on multiple timescales and understanding their contributions to FCO2 is important for explaining variability in fluxes and developing optimal sampling designs. We measured FCO2 and ΔCO2 and derived k for one full ice‐free period in 18 lakes using floating chambers and estimated the contributions of ΔCO2 and k to FCO2 variability. Generally, k contributed more than ΔCO2 to short‐term (1–9 d) FCO2 variability. With increased temporal period, the contribution of k to FCO2 variability decreased, and in some lakes resulted in ΔCO2 contributing more than k to FCO2 variability over the full ice‐free period. Increased contribution of ΔCO2 to FCO2 variability over time occurred across all lakes but was most apparent in large‐volume southern‐boreal lakes and in deeper (> 2 m) parts of lakes, whereas k was linked to FCO2 variability in shallow waters. Accordingly, knowing the variability of both k and ΔCO2 over time and space is needed for accurate modeling of FCO2 from these variables. We conclude that priority in FCO2 assessments should be given to direct measurements of FCO2 at multiple sites when possible, or otherwise from spatially distributed measurements of ΔCO2 combined with k‐models that incorporate spatial variability of lake thermal structure and meteorology. [ABSTRACT FROM AUTHOR]

Lakes contribute 9%–19% of global methane (CH4) emissions to the atmosphere. Dissolved molecular oxygen (DO) in lakes can inhibit the production of CH4 and promote CH4 oxidation. DO is therefore often considered an important regulator of CH4 emissions from lakes. Presence or absence of DO in the water above the sediments can affect CH4 production and emissions by (a) influencing if methane production can be fueled by the most reactive organic matter in the top sediment layer or rely on deeper and less degradable organic matter, and (b) enabling CH4 accumulation in deep waters and potentially large emissions upon water column turnover. However, the relative importance of these two DO effects on CH4 fluxes is still unclear. We assessed CH4 fluxes from two connected lake basins in northern boreal Sweden where one was experimentally oxygenated. Results showed no clear difference in summer CH4 emissions attributable to water column DO concentrations. Large amounts of CH4 accumulated in the anoxic hypolimnion of the reference basin but little of this may have been emitted because of incomplete mixing, and effective methane oxidation of stored CH4 reaching oxic water layers. Accordingly, ≤24% of the stored CH4 was likely emitted in the experimental lake. Overall, our results suggest that hypolimnetic DO and water column CH4 storage might have a smaller impact on CH4 emissions in boreal forest lakes than previous estimates, yet potential fluxes associated with water column turnover events remain a significant uncertainty in lake CH4 emission estimates. Plain Language Summary: Lakes represent 9%–19% of the total sources of the greenhouse gas methane (CH4), and the concentration of dissolved oxygen is considered important for regulating lake CH4 cycling. In this study we measured CH4 emissions from two connected lake basins, one where we increased concentrations of dissolved oxygen in the water column (Experimental basin), and one that was left in a natural state, resulting in deep water oxygen depletion (Reference basin). In the Experimental basin, the dissolved oxygen addition resulted in negligible CH4 accumulation in the water column. In the Reference basin, CH4 accumulated in anoxic water layers. However, no clear difference in CH4 emissions among the two basins could be detected during the summer. The basins only partially mixed in fall and spring and depending on the degree and intensity of water column mixing, we estimated that 0%–24% of CH4 stored in the water column was released upon fall and spring turnover, while the rest was converted to carbon dioxide by methane‐oxidizing bacteria. Accordingly, water column anoxia may not be important for CH4 emissions from all boreal lakes but can represent a main share of the yearly CH4 emissions in some lakes. Key Points: Hypolimnetic oxygenation strongly reduced CH4 storageLake CH4 fluxes during summer stratification period were not influenced by water column dissolved oxygen concentrations0%–24% of the stored CH4 escaped oxidation and was emitted at turnover periods, constituting 0%–44% of the yearly CH4 emissions [ABSTRACT FROM AUTHOR]

Winter methane (CH4) accumulation in seasonally ice‐covered lakes can contribute to large episodic emissions to the atmosphere during spring ice melt. Biological methane oxidation can significantly mitigate such CH4 emissions, but despite favorable CH4 and O2 concentrations, CH4 oxidation appears constrained in some lakes for unknown reasons. Here we experimentally test the hypothesis that phosphorus (P) availability is limiting CH4 oxidation, resulting in differences in ice‐out emissions among lakes. We observed a positive relationship between potential CH4 oxidation and P concentration across 12 studied lakes and found an increase in CH4 oxidation in response to P amendment, without any parallel change in the methanotrophic community composition. Hence, while an increase in sedimentary CH4 production and ebullitive emissions may happen with eutrophication, our study indicates that the increase in P associated with eutrophication may also enhance CH4 oxidation. The increase in CH4 oxidation may hence play an important role in nutrient‐rich ice‐covered lakes where bubbles trapped under the ice may to a greater extent be oxidized, reducing the ice‐out emissions of CH4. This may be an important factor regulating CH4 emissions from high latitude lakes. Plain Language Summary: Methane produced by microorganisms in lake sediments accumulates under the ice in seasonally ice‐covered lakes and is released into the atmosphere when the ice melts. A specialized group of bacteria can consume methane in the water and significantly reduce methane emissions. In some lakes, however, this consumption is limited for unclear reasons. A recent study suggests that phosphorus, an essential nutrient for plants and algae in aquatic environments, could cause this limitation. Here, we tested the influence of phosphorus on the bacterial consumption of methane in 12 lakes with different phosphorus concentrations and made experiments with the addition of phosphorus. Our results indicate that methane consumption was higher in lakes with more phosphorus in the water and that phosphorus addition enhanced the consumption of methane. Despite higher production of methane expected from nutrient‐rich lakes, our study shows that increased consumption of methane may reduce part of the enhanced production, limiting the emissions. This neutralization may be higher in seasonally frozen lakes where methane is trapped under the ice, allowing more time for methane consumption. Key Points: Positive relationship between methane oxidation and phosphorus observed among lakes was supported by a phosphorus amendment experimentThe methanotrophic community composition was not affected by phosphorus amendmentsEnhanced methane oxidation in nutrient‐rich lakes may neutralize more methane trapped under the ice and reduce ice‐off emissions [ABSTRACT FROM AUTHOR]

Most measurements of respiration rates in large tropical rivers do not account for the influence of river flow conditions on microbial activity. We developed a ship‐board spinning incubation system for measuring O2 drawdown under different rotation velocities and deployed the system along the lower Amazon River during four hydrologic periods. Average respiration rates in incubation chambers rotated at 0.22 m s−1 and 0.66 m s−1 were 1.4 and 2.4 times higher than stationary chambers, respectively. On average, depth‐integrated respiration rates in chambers spun at 0.22 m s−1 and 0.66 m s−1 accounted for 64% ± 22% and 104% ± 36% of CO2 outgassing rates, respectively, in mainstem sites. Continuous measurements of in situ pCO2 were also made along with cross‐channel profiles of river velocity. A positive correlation between river velocity and pCO2 was observed along the lower river (r2 = 0.67–0.96) and throughout a tidal cycle. [ABSTRACT FROM AUTHOR]

Methane ( CH4) fluxes from world rivers are still poorly constrained, with measurements restricted mainly to temperate climates. Additional river flux measurements, including spatio-temporal studies, are important to refine extrapolations. Here we assess the spatio-temporal variability of CH4 fluxes from the Amazon and its main tributaries, the Negro, Solimões, Madeira, Tapajós, Xingu, and Pará Rivers, based on direct measurements using floating chambers. Sixteen of 34 sites were measured during low and high water seasons. Significant differences were observed within sites in the same river and among different rivers, types of rivers, and seasons. Ebullition contributed to more than 50% of total emissions for some rivers. Considering only river channels, our data indicate that large rivers in the Amazon Basin release between 0.40 and 0.58 Tg CH4 yr−1. Thus, our estimates of CH4 flux from all tropical rivers and rivers globally were, respectively, 19-51% to 31-84% higher than previous estimates, with large rivers of the Amazon accounting for 22-28% of global river CH4 emissions. [ABSTRACT FROM AUTHOR]

Abstract: Amphibians are among the most threatened vertebrates in the world and this is also true for those inhabiting the Atlantic Forest hotspot, living in ecosystems that are highly degraded and threatened by anthropogenic activities. We present a data set containing information about amphibian communities sampled throughout the Atlantic Forest Biome in South America. The data were extracted from 389 bibliographic references (articles, books, theses, and dissertations) representing inventories of amphibian communities from 1940 to 2017. The data set includes 17,619 records of 528 species with taxonomic certainty, from 1,163 study sites. Of all the records, 14,450 (82%) were classified using the criterion of endemism; of those, 7,787 (44%) were considered endemic and 6,663 (38%) were not. Historically, multiple sampling methods were used to survey amphibians, the most representative methods being active surveys (82.1%), surveys at breeding sites (20%), pitfall traps (15.3%), and occasional encounters (14.5%). Species richness averaged 15.2 ± 11.3 (mean ± SD), ranging from 1 to 80 species per site. We found a low dominance in the communities, with 10 species occurring in about 26% of communities: Physalaemus cuvieri (4.1%), Dendropsophus minutus (3.8%), Boana faber (3.1%), Scinax fuscovarius (2.8%), Leptodactylus latrans (2.7%), Leptodactylus fuscus (2.6%), Boana albopunctata (2.3%), Dendropsophus nanus (1.6%), Rhinella ornata (1.6%), and Leptodactylus mystacinus (1.6%). This data set represents a major effort to compile inventories of amphibian communities for the Neotropical region, filling a large gap in the data on the Atlantic Forest hotspot. We hope this data set can be used as a credible tool in the proposal of new studies on amphibian sampling and even in the development of conservation planning for these taxa. This information also has great relevance for macroecological studies, being foundational for both conservation and restoration strategies in this biodiversity hotspot. No copyright or proprietary restrictions are associated with the use of this data set. Please cite this data paper when the data are used in publications or teaching events. [ABSTRACT FROM AUTHOR]