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In the littoral zone, at the land–water interface of lakes, the areal productivity of aquatic vegetation rivals that of rainforests, resulting in a potentially very high carbon (C) turnover. Whereas tidal wetlands at the land–ocean interface are included in global C budgets, lake littoral zones are currently not accounted for, despite the total shoreline of lakes being estimated at four times longer than that of the global ocean. Here we quantify the littoral net atmospheric C sink using mass balance and a model of C export from the littoral to the pelagic zone. We argue that ignoring littoral C turnover in lakes potentially results in biased estimates of continental C cycling. In our global estimate, we show that the estimated global C balance of lakes may reverse from a net C source to a net C sink (that is, net C burial > net C outgassing). In addition, a large part of the C outgassed in the pelagic might originate from the littoral, implying that previous estimates of terrestrial C inputs to inland waters were too high. We argue that quantifying and modelling lake littoral C fluxes are essential to more accurately estimate the feedback between the continents and climate.

There is substantial variation in estimates of the respiratory quotient (RQ), i.e., molar ratio of produced CO₂ and consumed O₂ during microbial mineralization of organic matter (OM). While several studies have examined RQ's controlling factors in terrestrial or aquatic ecosystems, there are no broader cross-ecosystem comparisons, and there is a lack of general understanding of the extrinsic (environmental) and intrinsic (organic matter composition) controls on RQ. In this study, we examine RQ across a broad range of environments, including soils, aquatic sediments, lake and coastal water. We measured CO₂ production and O₂ consumption using membrane inlet mass spectrometry (MIMS). We also assessed the microbial metabolic profiles using BIOLOG EcoPlates and determined the energy content of the natural OM with bomb calorimetry and its elemental composition. We show that RQ differs significantly between the ecosystem types and strongly deviates from the frequently assumed value of 1. In addition, microbial mineralization across the different studied ecosystems is correlated with the bulk energy content of the OM (kJ g^-1 organic carbon). Finally, RQ was correlated to the metabolic profiles of microorganisms, as estimated based on BIOLOG EcoPlates. We argue that an increased use of cross-ecosystem experimental studies will enhance the understanding of the factors controlling carbon cycling.

Lake shores are fringed with aquatic plants, but their very high productivity has been overlooked in global lake carbon budgets. We estimate the carbon fluxes of lake littoral zones and show that the carbon balance of lakes can reverse from a carbon source to a carbon sink when these zones are included. 

Reservoirs are hotspots for methane (CH₄) emissions. However, to date, the effects of terrestrial organic matter (OM) input and degradation on CH₄ emissions from large reservoirs remain largely unknown. From May 2020 to April 2021, we conducted monthly sampling campaigns at 100 sites in Lake Qiandao (580 km²), a mega-reservoir in China, and made monthly vertical profile observations from March to September 2023. We estimated an annual mean FCH₄ flux of 0.26 g C m^–2 yr^–1 (1.51 × 10⁸ g C yr^–1). Elevated FCH₄ and enriched δ¹³C-CH₄ coincided with low dissolved oxygen (DO) concentrations, high levels of organic suspended solids, terrestrial organic matter, nutrients, depleted δ¹⁸O-H₂O, and low carbon isotope fractionation (α_C) in the inflowing lake regions. Dissolved CH₄ (cCH₄) correlated positively to the relative abundance of aliphatic compounds. Anoxic bioincubation experiments revealed rapid degradation of riverine organic matter, accompanied by a 56-fold increase in cCH₄, δ¹³C-CH₄ enrichment (to −32.25‰), and a significant decrease in α_C to 1.02. These findings indicate that acetoclastic CH₄ production makes a substantial contribution to cCH₄ and thus FCH₄. Based on multiple lines of evidence, we conclude that input of terrestrial organic matter and its subsequent degradation lead to DO depletion, and their OM degradation byproducts serve as carbon substrates that promote CH₄ emissions.

Photochemical degradation of dissolved organic matter (DOM) has been the subject of numerous studies; however, its regulation along the inland water continuum is still unclear. We aimed to unravel the DOM photoreactivity and concurrent DOM compositional changes across 30 boreal aquatic ecosystems including peat waters, streams, rivers, and lakes distributed along a water residence time (WRT) gradient. Samples were subjected to a standardized exposure of simulated sunlight. We measured the apparent quantum yield (AQY), which corresponds to DOM photomineralization per photon absorbed, and the compositional change in DOM at bulk and individual compound levels in the original samples and after irradiation. AQY increased with the abundance of terrestrially derived DOM and decreased at higher WRT. Additionally, the photochemical changes in both DOM optical properties and molecular composition resembled changes along the natural boreal WRT gradient at low WRT (<3 years). Accordingly, mass spectrometry revealed that the abundance of photolabile and photoproduced molecules decreased with WRT along the boreal aquatic continuum. Our study highlights the tight link between DOM composition and DOM photodegradation. We suggest that photodegradation is an important driver of DOM composition change in waters with low WRT, where DOM is highly photoreactive.

Freshwater lakes and reservoirs cover a small fraction of the Earth, however their emission of the greenhouse gas methane (CH₄) from the sediment to the atmosphere is disproportionately high. Currently, there is still a limited understanding of the links between sediment characteristics and CH₄ formation. Earlier studies have indicated that sediment age and nitrogen content are related to sediment CH₄ formation rates, but it is uncertain such relationships are valid across gradients of sediment characteristics. We therefore measured potential CH₄ formation rates in multiple layers of sediment sampled from nine lakes situated in the temperate, boreal and alpine biogeographic regions of Sweden, thus differing in productivity, catchment and climate properties. Potential CH₄ formation varied over 3 orders of magnitude, and was broadly related to the quantity and reactivity of organic matter, and generally decreased with sediment depth. Sediment age and total nitrogen content were found to be the key controlling factors of potential CH₄ formation rates, together explaining 62% of its variability. Moreover, the model developed from the Swedish lake sediment data was able to successfully predict the potential CH₄ formation rates in reservoirs situated in different biogeographic regions of Brazil (R² = 0.62). Therefore, potential CH₄ formation rates in sediments of highly contrasting lakes and reservoirs, from Amazonia to alpine tundra, could be accurately predicted using one common model (RMSE = 1.6 in ln-units). Our model provides a valuable tool to improve estimates of CH₄ emission from lakes and reservoirs, and illustrates the fundamental regulation of microbial CH₄ formation by organic matter characteristics.

Lakes and reservoirs are important emitters of methane, a strong greenhouse gas, to the atmosphere. Methane is produced in absence of oxygen by specific microbes that degrade the organic matter in the sediment. Currently, it is still uncertain which specific sediment properties control the production of methane, and if such properties are the same across lakes and reservoirs located in different ecosystem. To test this, we collected sediment cores from several lakes across different ecosystems in Sweden, and we measured potential methane formation rates. Methane formation rates varied greatly among lakes and was related to the quality and quantity of organic matter in the sediment. From this experiment, we calculated an empirical model that can predict methane formation rates as a function of sediment age and nitrogen content. Moreover, we found that our model could well predict potential methane formation rates in tropical reservoirs. In conclusion, sediment age and nitrogen content are universal controlling factors of methane formation rates across lakes and reservoirs in different ecosystems, from tropics to arctic tundra. Our findings provide a valuable tool to improve estimates of methane emission from lakes and reservoirs and illustrates how sediment characteristics play a crucial role in regulating methane formation rates.

Aquatic dissolved organic matter (DOM) is a crucial component of the global carbon cycle, and the extent to which DOM escapes mineralization is important for the transport of organic carbon from the continents to the ocean. DOM persistence strongly depends on its molecular properties, but little is known about which specific properties cause the continuum in reactivity among different dissolved molecules. We investigated how DOM fractions, separated according to their hydrophobicity, differ in biodegradability across three different inland water systems. We found a strong negative relationship between hydrophobicity and biodegradability, consistent for the three systems. The most hydrophilic fraction was poorly recovered by solid-phase extraction (SPE) (3-28% DOC recovery) and was thus selectively missed by mass spectrometry analysis during SPE. The change in DOM composition after incubation was very low according to SPE-ESI (electrospray ionization)-mass spectrometry (14% change, while replicates had 11% change), revealing that this method is sub-optimal to assess DOM biodegradability, regardless of fraction hydrophobicity. Our results demonstrate that SPE-ESI mass spectrometry does not detect the most hydrophilic and most biodegradable species. Hence, they question our current understanding of the relationships between DOM biodegradability and its molecular composition, which is built on the use of this method.

The highest CH4 production rates can be found in anoxic inland water surface sediments however no model quantifies CH4 production following fresh particular organic matter (POM) deposition on anoxic sediments. This limits our capability of modeling CH4 emissions from inland waters to the atmosphere. To generate such a model, we quantified how the POM supply rate and POM reactivity control CH4 production in anoxic surface sediment, by amending sediment at different frequencies with different quantities of aquatic and terrestrial POM. From the modeled CH4 production, we derived parameters related to the kinetics and the extent of CH4 production. We show that the extent of CH4 production can be well predicted by the quality (i.e., C/N ratio) and the quantity of POM supplied to an anoxic sediment. In particular, within the range of sedimentation rates that can be found in aquatic systems, we show that CH4 production increases linearly with the quantity of phytoplankton-derived and terrestrially derived POM. A high frequency of POM addition, which is a common situation in natural systems, resulted in higher peaks in CH4 production rates. This suggests that relationships derived from earlier incubation experiments that added POM only once, may result in underestimation of sediment CH4 production. Our results quantitatively couple CH4 production in anoxic surface sediment to POM sedimentation flux, and are therefore useful for the further development of mechanistic models of inland water CH4 emission.

Inland waters receive and process large amounts of colored organic matter from the terrestrial surroundings. These inputs dramatically affect the chemical, physical, and biological properties of water bodies, as well as their roles as global carbon sinks and sources. However, manipulative studies, especially at ecosystem scale, require large amounts of dissolved organic matter with optical and chemical properties resembling indigenous organic matter. Here, we compared the impacts of two leonardite products (HuminFeed and SuperHume) and a freshly derived reverse osmosis concentrate of organic matter in a set of comprehensive mesocosm- and laboratory-scale experiments and analyses. The chemical properties of the reverse osmosis concentrate and the leonardite products were very different, with leonardite products being low and the reverse osmosis concentrate being high in carboxylic functional groups. Light had a strong impact on the properties of leonardite products, including loss of color and increased particle formation. HuminFeed presented a substantial impact on microbial communities under light conditions, where bacterial production was stimulated and community composition modified, while in dark potential inhibition of bacterial processes was detected. While none of the browning agents inhibited the growth of the tested phytoplankton Gonyostomum semen, HuminFeed had detrimental effects on zooplankton abundance and Daphnia reproduction. We conclude that the effects of browning agents extracted from leonardite, particularly HuminFeed, are in sharp contrast to those originating from terrestrially derived dissolved organic matter. Hence, they should be used with great caution in experimental studies on the consequences of terrestrial carbon for aquatic systems.

Eutrophication of fresh waters results in increased CO2 uptake by primary production, but at the same time increased emissions of CH4 to the atmosphere. Given the contrasting effects of CO2 uptake and CH4 release, the net effect of eutrophication on the CO2-equivalent balance of fresh waters is not clear. We measured carbon fluxes (CO2 and CH4 diffusion, CH4 ebullition) and CH4 oxidation in 20 freshwater mesocosms with 10 different nutrient concentrations (total phosphorus range: mesotrophic 39 µg/L until hypereutrophic 939 µg/L) and planktivorous fish in half of them. We found that the CO2-equivalent balance had a U-shaped relationship with productivity, up to a threshold in hypereutrophic systems. CO2-equivalent sinks were confined to a narrow range of net ecosystem production (NEP) between 5 and 19 mmol O2 m?3 day?1. Our findings indicate that eutrophication can shift fresh waters from sources to sinks of CO2-equivalents due to enhanced CO2 uptake, but continued eutrophication enhances CH4 emission and transforms freshwater ecosystems to net sources of CO2-equivalents to the atmosphere. Nutrient enrichment but also planktivorous fish presence increased productivity, thereby regulating the resulting CO2-equivalent balance. Increasing planktivorous fish abundance, often concomitant with eutrophication, will consequently likely affect the CO2-equivalent balance of fresh waters.