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Globally, Freshwater Ecosystems Emit More CO2 Than the Burning of Fossil Fuels
Abstract and Figures
Freshwater emits substantial volumes of CO2 to the atmosphere. This has largely gone unnoticed in global carbon budgets. My aim was to quantify the CO2 emanating from freshwater from 66° N to 47° S latitudes via in situ bacterial respiration (BR). I determined BR (n = 326) as a function of water temperature. Freshwater is emitting CO2 at a rate of 58.5 Pg C y−1 (six times that of fossil fuel burning). Most is emitted from the Northern Hemisphere. This is because the high northern summer temperatures coincide with most of the world’s freshwater. Diffuse DOC sources, for example dust, may be driving high freshwater BR. However, many sources remain elusive and not individually quantified in the literature. We must include freshwater CO2 emissions in climate models. Identifying, quantifying and managing freshwater’s diffuse sources of Dissolved Organic Carbon (DOC) will hopefully provide us with another opportunity to change our current climate trajectory.
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The carbon cycle is of fundamental importance to estimate the influence of anthropogenic emissions on the atmospheric CO2 concentration, and thus, to classify the impact of these emissions on global warming. Different models have been developed, which under simplified assumptions can well reproduce the observed CO2 concentration over recent years, but they also lead to contradictory interpretations of the human impact. Here we consider, how far such suppositions are substantiated or must be made responsible for significant misinterpretations. We present detailed own calculations based on the Conservation Law, which reproduce all details of the measured atmospheric CO2 concentration over the Mauna Loa Era. In particular, they allow to deduce an upper limit of 35% for the anthropogenic contribution to the observed increase of CO2 over the Mauna Loa Era, and a more likely value of 14%. Under non-equilibrium conditions between the Earth's surface and troposphere this even gives a lower bound of only 3.5%. The importance of only one unitary time scale for the removal of anthropogenic and natural CO2 emissions from the atmosphere, characterized by an effective absorption time, is discussed.
Inland aquatic ecosystems are valuable sentinels of anthropic-associated changes (e.g., agriculture and tourism). Eutrophication has become of primary importance in altering aquatic ecosystem functioning. Quantifying the CO2 emissions by inland aquatic ecosystems of different trophic statuses may provide helpful information about the role of eutrophication on greenhouse gas emissions. This study investigated diel and seasonal carbon dioxide (CO2) concentrations and emissions in three tropical karst lakes with different trophic statuses. We measured CO2 emissions using static floating chambers twice daily during the rainy/warm and dry/cold seasons while the lakes were thermally stratified and mixed, respectively. The CO2 concentration was estimated by gas chromatography and photoacoustic spectroscopy. The results showed a significant seasonal variation in the dissolved CO2 concentration (CCO2) and the CO2 flux (FCO2), with the largest values in the rainy/warm season but not along the diel cycle. The CCO2 values ranged from 13.3 to 168.6 µmol L−1 averaging 41.9 ± 35.3 µmol L−1 over the rainy/warm season and from 12.9 to 38.0 µmol L−1 with an average of 21.0 ± 7.2 µmol L−1 over the dry/cold season. The FCO2 values ranged from 0.2 to 12.1 g CO2 m−2 d−1 averaging 4.9 ± 4.0 g CO2 m−2 d−1 over the rainy/warm season and from 0.1 to 1.7 g CO2 m−2 d−1 with an average of 0.8 ± 0.5 g CO2 m−2 d−1 over the dry/cold season. During the rainy/warm season the emission was higher in the eutrophic lake San Lorenzo (9.1 ± 1.2 g CO2 m−2 d−1), and during the dry/cold the highest emission was recorded in the mesotrophic lake San José (1.42 ± 0.2 g CO2 m−2 d−1). Our results indicated that eutrophication in tropical karst lakes increased CO2 evasion rates to the atmosphere mainly due to the persistence of anoxia in most of the lake’s water column, which maintained high rates of anaerobic respiration coupled with the anaerobic oxidation of methane. Contrarily, groundwater inflows that provide rich-dissolved inorganic carbon waters sustain emissions in meso and oligotrophic karstic tropical lakes.
The generation and consumption of single carbon molecules (CO2 and CH4) by aquatic microbial communities is an essential aspect of the global carbon budget. Organic carbon flow (warm sunlit regimes) is depicted as beginning at the surface with autochthonous fixation followed by biomass settling to sediments, CO2 respi-ration to the atmosphere, and outflow. We sought to broaden understanding of C1 cycling and consortia by examining the microbial community of a below-ice lake water column in which both input and output are likely disrupted due to ice cover. By analysing the microbial community composition and co-occurrence network of an ice-covered lake timeseries, we were able to identify potential consortia involved in C1 cycling. The network confirmed known associations supporting the efficacy of such analyses but also pointed to previously unknown potential associations. Fur-ther and contrary to typical organic carbon flow under warm sunlit regimes, we found support for upward flow of recently fixed carbon in cold low-light conditions under-ice in winter.
Inland waters transport, transform and retain significant amounts of dissolved organic carbon (DOC) that may be biologically reactive (bioreactive) and thus potentially degraded into atmospheric CO2. Despite its global importance, relatively little is known about environmental controls on bioreactivity of DOC as it moves through river systems with varying water residence time (WRT). Here we determined the influence of WRT and landscape properties on DOC bioreactivity in 15 Swedish catchments spanning a large geographical and environmental gradient. We found that the short-term bioreactive pools (0–6 d of decay experiments) were linked to high aquatic primary productivity that, in turn, was stimulated by phosphorus loading from forested, agricultural and urban areas. Unexpectedly, the percentage of long-term bioreactive DOC (determined in 1-year experiments) increased with WRT, possibly due to photo-transformation of recalcitrant DOC from terrestrial sources into long-term bioreactive DOC with relatively lower aromaticity. Thus, despite overall decreases in DOC during water transit through the inland water continuum, DOC becomes relatively more bioreactive on a long time-scale. This increase in DOC bioreactivity with increasing WRT along the freshwater continuum has previously been overlooked. Further studies are needed to explain the processes and mechanisms behind this pattern on a molecular level.
p>Accurate assessment of anthropogenic carbon dioxide (<span classCombining double low line"inline-formula">CO2 ) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere - the "global carbon budget" - is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil <span classCombining double low line"inline-formula">CO2 emissions (<span classCombining double low line"inline-formula"> E FF ) are based on energy statistics and cement production data, while emissions from land use and land-use change (<span classCombining double low line"inline-formula"> E LUC ), mainly deforestation, are based on land use and land-use change data and bookkeeping models. Atmospheric <span classCombining double low line"inline-formula">CO2 concentration is measured directly and its growth rate (<span classCombining double low line"inline-formula"> G ATM ) is computed from the annual changes in concentration. The ocean <span classCombining double low line"inline-formula">CO2 sink (<span classCombining double low line"inline-formula"> S OCEAN ) and terrestrial <span classCombining double low line"inline-formula">CO2 sink (<span classCombining double low line"inline-formula"> S LAND ) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (<span classCombining double low line"inline-formula"> B IM ), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as <span classCombining double low line"inline-formula">±1 σ . For the last decade available (2008-2017), <span classCombining double low line"inline-formula"> E FF was <span classCombining double low line"inline-formula">9.4±0.5 GtC yr<span classCombining double low line"inline-formula">ĝ'1 , <span classCombining double low line"inline-formula"> E LUC <span classCombining double low line"inline-formula">1.5±0.7 GtC yr<span classCombining double low line"inline-formula">ĝ'1 , <span classCombining double low line"inline-formula"> G ATM <span classCombining double low line"inline-formula">4.7±0.02 GtC yr<span classCombining double low line"inline-formula">ĝ'1 , <span classCombining double low line"inline-formula"> S OCEAN <span classCombining double low line"inline-formula">2.4±0.5 GtC yr<span classCombining double low line"inline-formula">ĝ'1 , and <span classCombining double low line"inline-formula"> S LAND <span classCombining double low line"inline-formula">3.2±0.8 GtC yr<span classCombining double low line"inline-formula">ĝ'1 , with a budget imbalance <span classCombining double low line"inline-formula"> B IM of 0.5 GtC yr<span classCombining double low line"inline-formula">ĝ'1 indicating overestimated emissions and/or underestimated sinks. For the year 2017 alone, the growth in <span classCombining double low line"inline-formula"> E FF was about 1.6 % and emissions increased to <span classCombining double low line"inline-formula">9.9±0.5 GtC yr<span classCombining double low line"inline-formula">ĝ'1 . Also for 2017, <span classCombining double low line"inline-formula"> E LUC was <span classCombining double low line"inline-formula">1.4±0.7 GtC yr<span classCombining double low line"inline-formula">ĝ'1 , <span classCombining double low line"inline-formula"> G ATM was <span classCombining double low line"inline-formula">4.6±0.2 GtC yr<span classCombining double low line"inline-formula">ĝ'1 , <span classCombining double low line"inline-formula"> S OCEAN was <span classCombining double low line"inline-formula">2.5±0.5 GtC yr<span classCombining double low line"inline-formula">ĝ'1 , and <span classCombining double low line"inline-formula"> S LAND was <span classCombining double low line"inline-formula">3.8±0.8 GtC yr<span classCombining double low line"inline-formula">ĝ'1 , with a <span classCombining double low line"inline-formula"> B IM of 0.3 GtC. The global atmospheric <span classCombining double low line"inline-formula">CO2 concentration reached <span classCombining double low line"inline-formula">405.0±0.1 ppm averaged over 2017. For 2018, preliminary data for the first 6-9 months indicate a renewed growth in <span classCombining double low line"inline-formula"> E FF of <span classCombining double low line"inline-formula">+ 2.7 % (range of 1.8 % to 3.7 %) based on national emission projections for China, the US, the EU, and India and projections of gross domestic product corrected for recent changes in the carbon intensity of the economy for the rest of the world. The analysis presented here shows that the mean and trend in the five components of the global carbon budget are consistently estimated over the period of 1959-2017, but discrepancies of up to 1 GtC yr<span classCombining double low line"inline-formula">ĝ'1 persist for the representation of semi-decadal variability in <span classCombining double low line"inline-formula">CO2 fluxes. A detailed comparison among individual estimates and the introduction of a broad range of observations show (1) no consensus in the mean and trend in land-use change emissions, (2) a persistent low agreement among the different methods on the magnitude of the land <span classCombining double low line"inline-formula">CO2 flux in the northern extra-tropics, and (3) an apparent underestimation of the <span classCombining double low line"inline-formula">CO2 variability by ocean models, originating outside the tropics. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding the global carbon cycle compared with previous publications of this data set (Le Quéré et al., 2018, 2016, 2015a, b, 2014, 2013).</p
Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere - the "global carbon budget" - is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (E-FF) are based on energy statistics and cement production data, while emissions from land use and land-use change (E-LUC), mainly deforestation, are based on land use and land -use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly and its growth rate (G(ATM)) is computed from the annual changes in concentration. The ocean CO2 sink (S-OCEAN) and terrestrial CO2 sink (S-LAND) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (B-IM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as +/- 1 sigma. For the last decade available (2008-2017), E-FF was 9.4 +/- 0.5 GtC yr(-1), E-LUC 1.5 +/- 0.7 GtC yr(-1), G(ATM) 4.7 +/- 0.02 GtC yr(-1), S-OCEAN 2.4 +/- 0.5 GtC yr(-1), and S-LAND 3.2 +/- 0.8 GtC yr(-1), with a budget imbalance B-IM of 0.5 GtC yr(-1) indicating overestimated emissions and/or underestimated sinks. For the year 2017 alone, the growth in E-FF was about 1.6 % and emissions increased to 9.9 +/- 0.5 GtC yr(-1). Also for 2017, E-LUC was 1.4 +/- 0.7 GtC yr(-1), G(ATM) was 4.6 +/- 0.2 GtC yr(-1), S-OCEAN was 2.5 +/- 0.5 GtC yr(-1), and S-LAND was 3.8 +/- 0.8 GtC yr(-1), with a B-IM of 0.3 GtC. The global atmospheric CO2 concentration reached 405.0 +/- 0.1 ppm averaged over 2017. For 2018, preliminary data for the first 6-9 months indicate a renewed growth in E-FF of +2.7 % (range of 1.8 % to 3.7 %) based on national emission projections for China, the US, the EU, and India and projections of gross domestic product corrected for recent changes in the carbon intensity of the economy for the rest of the world. The analysis presented here shows that the mean and trend in the five components of the global carbon budget are consistently estimated over the period of 1959-2017, but discrepancies of up to 1 GtC yr(-1) persist for the representation of semi-decadal variability in CO2 fluxes. A detailed comparison among individual estimates and the introduction of a broad range of observations show (1) no consensus in the mean and trend in land -use change emissions, (2) a persistent low agreement among the different methods on the magnitude of the land CO2 flux in the northern extra-tropics, and (3) an apparent underestimation of the CO2 variability by ocean models, originating outside the tropics. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding the global carbon cycle compared with previous publications of this data set (Le Quere et al., 2018, 2016, 2015a, b, 2014, 2013). All results presented here can be downloaded from
We estimate global terrestrial gross primary production (GPP) based on models that use satellite data within a simplified light-use efficiency framework that does not rely upon other meteorological inputs. Satellite-based geometry-adjusted reflectances are from the MODerate-resolution Imaging Spectroradiometer (MODIS) and provide information about vegetation structure and chlorophyll content at both high temporal (daily to monthly) and spatial (∼1 km) resolution. We use satellite-derived solar-induced fluorescence (SIF) to identify regions of high productivity crops and also evaluate the use of downscaled SIF to estimate GPP. We calibrate a set of our satellite-based models with GPP estimates from a subset of distributed eddy covariance flux towers (FLUXNET 2015). The results of the trained models are evaluated using an independent subset of FLUXNET 2015 GPP data. We show that variations in light-use efficiency (LUE) with incident PAR are important and can be easily incorporated into the models. Unlike many LUE-based models, our satellite-based GPP estimates do not use an explicit parameterization of LUE that reduces its value from the potential maximum under limiting conditions such as temperature and water stress. Even without the parameterized downward regulation, our simplified models are shown to perform as well as or better than state-of-the-art satellite data-driven products that incorporate such parameterizations. A significant fraction of both spatial and temporal variability in GPP across plant functional types can be accounted for using our satellite-based models. Our results provide an annual GPP value of ∼140 Pg C year - 1 for 2007 that is within the range of a compilation of observation-based, model, and hybrid results, but is higher than some previous satellite observation-based estimates.
The perhumid region of the coastal temperate rainforest (CTR) of Pacific North America is one of the wettest places on Earth and contains numerous small catchments that discharge freshwater and high concentrations of dissolved organic carbon (DOC) directly to the coastal ocean. However, empirical data on the flux and composition of DOC exported from these watersheds are scarce. We established monitoring stations at the outlets of seven catchments on Calvert and Hecate islands, British Columbia, which represent the rain-dominated hypermaritime region of the perhumid CTR. Over several years, we measured stream discharge, stream water DOC concentration, and stream water dissolved organic-matter (DOM) composition. Discharge and DOC concentrations were used to calculate DOC fluxes and yields, and DOM composition was characterized using absorbance and fluorescence spectroscopy with parallel factor analysis (PARAFAC). The areal estimate of annual DOC yield in water year 2015 was 33.3 Mg C km⁻² yr⁻¹, with individual watersheds ranging from an average of 24.1 to 37.7 Mg C km⁻² yr⁻¹. This represents some of the highest DOC yields to be measured at the coastal margin. We observed seasonality in the quantity and composition of exports, with the majority of DOC export occurring during the extended wet period (September–April). Stream flow from catchments reacted quickly to rain inputs, resulting in rapid export of relatively fresh, highly terrestrial-like DOM. DOC concentration and measures of DOM composition were related to stream discharge and stream temperature and correlated with watershed attributes, including the extent of lakes and wetlands, and the thickness of organic and mineral soil horizons. Our discovery of high DOC yields from these small catchments in the CTR is especially compelling as they deliver relatively fresh, highly terrestrial organic matter directly to the coastal ocean. Hypermaritime landscapes are common on the British Columbia coast, suggesting that this coastal margin may play an important role in the regional processing of carbon and in linking terrestrial carbon to marine ecosystems.
A large fraction of the organic carbon derived from land that is transported through inland waters is decomposed along river systems and emitted to the atmosphere as carbon dioxide (CO 2). The Amazon River outgasses nearly as much CO 2 as the rainforest sequesters on an annual basis, representing ∼25% of global CO 2 emissions from inland waters. However, current estimates of CO 2 outgassing from the Amazon basin are based on a conservative upscaling of measurements made in the central Amazon, meaning both basin and global scale budgets are likely underestimated. The lower Amazon River, from Óbidos to the river mouth, represents ∼13% of the total drainage basin area, and is not included in current basin-scale estimates. Here, we assessed the concentration and evasion rate of CO 2 along the lower Amazon River corridor and its major tributaries, the Tapajós and Xingu Rivers. Evasive CO 2 fluxes were directly measured using floating chambers and gas transfer coefficients (k 600) were calculated for different hydrological seasons. Temporal variations in pCO 2 and CO 2 emissions were similar to previous observations throughout the Amazon (e.g., peak concentrations at high water) and CO 2 outgassing was lower in the clearwater tributaries compared to the mainstem. However, k 600-values were higher than previously reported upstream likely due to the generally windier conditions, turbulence caused by tidal forces, and an amplification of these factors in the wider channels with a longer fetch. We estimate that the lower Amazon River mainstem emits 0.2 Pg C year −1 within our study boundaries, or as much as 0.48 Pg C year −1 if the entire spatial extent to the geographical mouth is considered. Including these values with updated basin scale estimates and estimates of CO 2 outgassing from small streams we estimate that the Amazon running waters outgasses as much as 1.39 Pg C year −1 , increasing the global emissions from inland waters by 43% for a total of 2.9 Pg C year −1. These results highlight a large missing gap in basin-scale carbon budgets along the complete continuum of the Amazon River, and likely most other large river systems, that could drastically alter global scale carbon budgets.
Many freshwater systems receive substantial inputs of terrestrial organic matter. Terrestrially derived dissolved organic carbon (t‐DOC) inputs can modify light availability, the spatial distribution of primary production, heat, and oxygen in aquatic systems, as well as inorganic nutrient bioavailability. It is also well‐established that some terrestrial inputs (such as invertebrates and fruits) provide high‐quality food resources for consumers in some systems.
In small to moderate‐sized streams, leaf litter inputs average approximately three times greater than the autochthonous production. Conversely, in oligo/mesotrophic lakes algal production is typically five times greater than the available flux of allochthonous basal resources.
Terrestrial particulate organic carbon (t‐POC) inputs to lakes and rivers are comprised of 80%–90% biochemically recalcitrant lignocellulose, which is highly resistant to enzymatic breakdown by animal consumers. Further, t‐POC and heterotrophic bacteria lack essential biochemical compounds that are critical for rapid growth and reproduction in aquatic invertebrates and fishes. Several studies have directly shown that these resources have very low food quality for herbivorous zooplankton and benthic invertebrates.
Much of the nitrogen assimilated by stream consumers is probably of algal origin, even in systems where there appears to be a significant terrestrial carbon contribution. Amino acid stable isotope analyses for large river food webs indicate that most upper trophic level essential amino acids are derived from algae. Similarly, profiles of essential fatty acids in consumers show a strong dependence on the algal food resources.
Primary production to respiration ratios are not a meaningful index to assess consumer allochthony because respiration represents an oxidised carbon flux that cannot be utilised by animal consumers. Rather, the relative importance of allochthonous subsidies for upper trophic level production should be addressed by considering the rates at which terrestrial and autochthonous resources are consumed and the growth efficiency supported by this food.
Ultimately, the biochemical composition of a particular basal resource, and not just its quantity or origin, determines how readily this material is incorporated into upper trophic level consumers. Because of its highly favourable biochemical composition and greater availability, we conclude that microalgal production supports most animal production in freshwater ecosystems.
Carbon dioxide (CO2) emission from fluvial systems represents a substantial flux in the global carbon cycle. However, variation in fluvial CO2 fluxes at the air–water interface as well as its drivers are poorly understood, especially in non-forested headwaters. Here, we measured CO2 concentration and fluxes in 14 lowland open-canopy streams (Pampean region, Argentina) that cover a wide range of land use and water quality. We also analyzed drivers of CO2 concentration and fluxes, including factors related to metabolism, gas solubility, alkalinity, and gas transfer. Metabolic rates varied considerably among the
study sites, but most streams (i.e., 8 out of the 11 where we were able to estimate ecosystem metabolism) were net heterotrophic. Metabolic differences among sites were mostly driven by the aromatic carbon content and the percent of the stream reach covered by primary producers. All streams, even those that were not net heterotrophic were CO2 supersaturated. Alkalinity combined with in-stream primary production explained 52.3% of the variance in the partial pressure of CO2 (pCO2), but our observations suggest that pCO2 might be controlled by external groundwater inputs of dissolved inorganic carbon rather than by internal metabolism. All streams were net emitters of CO2 to the atmosphere. Significantly more variance in CO2 flux was explained by gas transfer velocity (63.7%) than by pCO2 (21.9%). We also observed high spatial heterogeneity in CO2 fluxes within each stream, which was associated with flow variation and the presence of different macrophyte and algae patches. Overall, our results indicate that CO2 emission in these extremely low turbulence streams is controlled by a
combination of external and internal biogeochemical processes and limited atmospheric exchange.
Invisible to the naked eye lies a tremendous diversity of organic molecules and organisms that make major contributions to important biogeochemical cycles. However, how the diversity and composition of these two communities are interlinked remains poorly characterized in fresh waters, despite the potential for chemical and microbial diversity to promote one another. Here we exploited gradients in chemodiversity within a common microbial pool to test how chemical and biological diversity covary and characterized the implications for ecosystem functioning. We found that both chemodiversity and genes associated with organic matter decomposition increased as more plant litterfall accumulated in experimental lake sediments, consistent with scenarios of future environmental change. Chemical and microbial diversity were also positively correlated, with dissolved organic matter having stronger effects on microbes than vice versa. Under our experimental scenarios that increased sediment organic matter from 5 to 25% or darkened overlying waters by 2.5 times, the resulting increases in chemodiversity could increase greenhouse gas concentrations in lake sediments by an average of 1.5 to 2.7 times, when all of the other effects of litterfall and water color were considered. Our results open a major new avenue for research in aquatic ecosystems by exposing connections between chemical and microbial diversity and their implications for the global carbon cycle in greater detail than ever before.
Freshwater ecosystems constitute a small fraction of our planet but play a disproportionately large and critical role in the global carbon cycle.