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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]

Lakes are considered the second largest natural source of atmospheric methane (CH4). However, current estimates are still uncertain and do not account for diel variability of CH4 emissions. In this study, we performed high-resolution measurements of CH4 flux from several lakes, using an automated and sensor-based flux measurement approach (in total 4,580 measurements), and demonstrated a clear and consistent diel lake CH4 flux pattern during stratification and mixing periods. The maximum of CH4 flux were always noted between 10:00 and 16:00, whereas lower CH4 fluxes typically occurred during the nighttime (00:00-04:00). Regardless of the lake, CH4 emissions were on an average 2.4 higher during the day compared to the nighttime. Fluxes were higher during daytime on nearly 80% of the days. Accordingly, estimates and extrapolations based on daytime measurements only most likely result in overestimated fluxes, and consideration of diel variability is critical to properly assess the total lake CH4 flux, representing a key component of the global CH4 budget. Hence, based on a combination of our data and additional literature information considering diel variability across latitudes, we discuss ways to derive a diel variability correction factor for previous measurements made during daytime only. [ABSTRACT FROM AUTHOR]

The Belo Monte hydropower complex located in the Xingu River is the largest run-of-the-river (ROR) hydroelectric system in the world and has one of the highest energy production capacities among dams. Its construction received significant media attention due to its potential social and environmental impacts. It is composed of two ROR reservoirs: the Xingu Reservoir (XR) in the Xingu's main branch and the Intermediate Reservoir (IR), an artificial reservoir fed by waters diverted from the Xingu River with longer water residence time compared to XR. We aimed to evaluate spatiotemporal variations in CO2 partial pressure (pCO2) and CO2 fluxes (FCO2) during the first 2 years after the Xingu River impoundment under the hypothesis that each reservoir has contrasting FCO2 and pCO2 as vegetation clearing reduces flooded area emissions. Time of the year had a significant influence on pCO2 with the highest average values observed during the high-water season. Spatial heterogeneity throughout the entire study area was observed for pCO2 during both low- and high-water seasons. FCO2 , on the other hand, only showed significant spatial heterogeneity during the high-water period. FCO2 (0.90±0.47 and 1.08±0.62 µ mol m 2 d -1 for XR and IR, respectively) and pCO2 (1647±698 and 1676±323 µ atm for XR and IR, respectively) measured during the high-water season were on the same order of magnitude as previous observations in other Amazonian clearwater rivers unaffected by impoundment during the same season. In contrast, during the low-water season FCO2 (0.69±0.28 and 7.32±4.07 µ mol m 2 d -1 for XR and IR, respectively) and pCO2 (839±646 and 1797±354 µ atm for XR and IR, respectively) in IR were an order of magnitude higher than literature FCO2 observations in clearwater rivers with naturally flowing waters. When CO2 emissions are compared between reservoirs, IR emissions were 90 % higher than values from the XR during low-water season, reinforcing the clear influence of reservoir characteristics on CO2 emissions. Based on our observations in the Belo Monte hydropower complex, CO2 emissions from ROR reservoirs to the atmosphere are in the range of natural Amazonian rivers. However, the associated reservoir (IR) may exceed natural river emission rates due to the preimpounding vegetation influence. Since many reservoirs are still planned to be constructed in the Amazon and throughout the world, it is critical to evaluate the implications of reservoir traits on FCO2 over their entire life cycle in order to improve estimates of CO2 emissions per kilowatt for hydropower projects planned for tropical rivers. [ABSTRACT FROM AUTHOR]

River damming alters local hydrology, which influences aspects of the carbon cycle such as carbon dioxide (CO2) production and emissions. Run-of-the-river reservoirs (RORs) are designed to have a smaller flooded area than traditional storage reservoirs, maintaining a river flow similar to natural seasonal water level fluctuation. These features may potentially reduce the impacts of the dam on the natural carbon cycle. However, little information on the influence of RORs on the carbon cycle is available, especially for the Amazon or other large tropical rivers. The Belo Monte hydropower complex is a large ROR located in the Xingu River, a clearwater tributary of the Amazon River. It is composed of two reservoirs; the Xingu Reservoir (XR) with ROR characteristics and the Intermediate Reservoir (IR) with storage reservoir traits. Here we evaluate spatiotemporal variation of surface water CO2 partial pressure (pCO2), CO2 fluxes (FCO2), and gas exchange coefficients (k600) during the first two years after the impoundment of the Xingu River. Seasonal changes in the water level had a significant influence on pCO2 with the highest average values observed during high water. The FCO2 was more variable, although correlated with pCO2, throughout the two first years of river impoundment. Spatial heterogeneity was observed for pCO2 during both seasons while FCO2 showed significant spatial heterogeneity only during the high water period. High water FCO2 and pCO2 values were on the same order of magnitude as emissions measured in Amazonian clearwater rivers unaffected by impoundment, but low water values were an order of magnitude higher than previous observations in clearwater rivers with natural flowing waters. Finally, we observed variability in CO2 fluxes related to the type of environment (i.e., river channel, downstream the dams, outside reservoirs and flooded areas), among reservoirs and the land use of flooded areas after impoundment of the Belo Monte hydropower complex. For example, CO2 emissions were 15 % and 90 % higher for the IR compared to XR during high and low water season, respectively, indicating that storage reservoirs may be larger sources of CO2 to the atmosphere compared to RORs. Since many reservoirs are still planned to be constructed in the Amazon and throughout the world, it is critical to evaluate the implications of reservoir traits on CO2 fluxes over their entire life cycle in order to generate energy that has lower emissions per KW [ABSTRACT FROM AUTHOR]

Recent estimates indicate that the lower Amazon River outgasses significant amounts of carbon dioxide (CO2) that was not previously accounted for the global inland water carbon budget. Detailed evaluation of seasonal variability and controlling mechanisms behind the CO2 fluxes in this large and complex area remains incomplete. Previous observations throughout the Amazon basin showed that higher CO2 fluxes (FCO2) and partial pressure of CO2 (pCO2) occur during high water and higher wind intensity seasons. The influence of wind and water speed, depth of water column, as well as respiration of allochthonous and autochthonous organic matter, are frequently assigned as the main control variables. Here, we assess the influence of a set of biogeochemical and hydrodynamic parameters on the seasonal variation of FCO2 and pCO2 near the Amazon River mouth. FCO2, pCO2 and biogeochemical and hydrologic analyses were carried out from 2010 to 2016 during four different hydrological periods per year (N = 25) in the North Channel of the Amazon River mouth. FCO2 and pCO2 were used as independent variables and analyzed against 33 biogeochemical, hydrodynamic and meteorological parameters along the hydrological seasons. The highest FCO2 and pCO2 was obtained at high discharge season (11.28 ± 7.82 μmol m-2 s-1 and (4575 ± 429 μatm, respectively) when most of these parameters tend to be higher. Among the 33 parameters analyzed, the significant correlations with FCO2 and pCO2 (p < 0.05) observed were for water and air temperatures, dissolved oxygen, dissolved organic carbon, nitrate, dissolved inorganic nitrogen and pH. These variables could be considered suitable predictors for estimating pCO2 and FCO2 in the Amazon River mouth area. For a better estimation and understanding of carbon budgets in tropical rivers it is still required to verify and to quantify more deeply the relationship among CO2 evasion and others hydrodynamic, meteorological and biogeochemical variables. [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]

Spatial and temporal patterns in microbial biodiversity across the Amazon river-ocean continuum were investigated along ~675 km of the lower Amazon River mainstem, in the Tapajós River tributary, and in the plume and coastal ocean during low and high river discharge using amplicon sequencing of 16S rRNA genes in whole water and size-fractionated samples (0.2-2.0 mm and >2.0 mm). River communities varied among tributaries, but mainstem communities were spatially homogeneous and tracked seasonal changes in river discharge and co-varying factors. Co-occurrence network analysis identified strongly interconnected river assemblages during high (May) and low (December) discharge periods, and weakly interconnected transitional assemblages in September, suggesting that this system supports two seasonal microbial communities linked to river discharge. In contrast, plume communities showed little seasonal differences and instead varied spatially tracking salinity. However, salinity explained only a small fraction of community variability, and plume communities in blooms of diatom-diazotroph assemblages were strikingly different than those in other high salinity plume samples. This suggests that while salinity physically structures plumes through buoyancy and mixing, the composition of plume-specific communities is controlled by other factors including nutrients, phytoplankton community composition, and dissolved organic matter chemistry. Co-occurrence networks identified interconnected assemblages associated with the highly productive low salinity near-shore region, diatom-diazotroph blooms, and the plume edge region, and weakly interconnected assemblages in high salinity regions. This suggests that the plume supports a transitional community influenced by immigration of ocean bacteria from the plume edge, and by species sorting as these communities adapt to local environmental conditions. Few studies have explored patterns of microbial diversity in tropical rivers and coastal oceans. Comparison of Amazon continuum microbial communities to those from temperate and arctic systems suggest that river discharge and salinity are master variables structuring a range of environmental conditions that control bacterial communities across the river-ocean continuum. [ABSTRACT FROM AUTHOR]

The flux of methane ( CH4) from inland waters to the atmosphere has a profound impact on global atmospheric greenhouse gas ( GHG) levels, and yet, strikingly little is known about the dynamics controlling sources and sinks of CH4 in the aquatic setting. Here, we examine the cycling and flux of CH4 in six large rivers in the Amazon basin, including the Amazon River. Based on stable isotopic mass balances of CH4, inputs and outputs to the water column were estimated. We determined that ecosystem methane oxidation ( MOX) reduced the diffusive flux of CH4 by approximately 28-96% and varied depending on hydrologic regime and general geochemical characteristics of tributaries of the Amazon River. For example, the relative amount of MOX was maximal during high water in black and white water rivers and minimal in clear water rivers during low water. The abundance of genetic markers for methane-oxidizing bacteria ( pmoA) was positively correlated with enhanced signals of oxidation, providing independent support for the detected MOX patterns. The results indicate that MOX in large Amazonian rivers can consume from 0.45 to 2.07 Tg CH4 yr−1, representing up to 7% of the estimated global soil sink. Nevertheless, climate change and changes in hydrology, for example, due to construction of dams, can alter this balance, influencing CH4 emissions to atmosphere. [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]