Lakes and impoundments are an important source of methane (CH4), a potent greenhouse gas, to the atmosphere. A recent analysis shows aquatic productivity (i.e., eutrophication) is an important driver of CH4 emissions from lentic waters. Considering that aquatic productivity will increase over the next century due to climate change and a growing human population, a concomitant increase in aquatic CH4 emissions may occur. We simulate the eutrophication of lentic waters under scenarios of future nutrient loading to inland waters and show that enhanced eutrophication of lakes and impoundments will substantially increase CH4 emissions from these systems (+30–90%) over the next century. This increased CH4 emission has an atmospheric impact of 1.7–2.6 Pg C-CO2-eq y−1, which is equivalent to 18–33% of annual CO2 emissions from burning fossil fuels. Thus, it is not only important to limit eutrophication to preserve fragile water supplies, but also to avoid acceleration of climate change. .Agricultural intensification and a growing human population are likely to increase the eutrophication of lakes and impoundments over the next century.  Here, the authors show that this enhanced eutrophication will substantially increase emissions of methane (+ 30–90%), a potent greenhouse gas, from these systems over the next century.

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Eutrophication will increase methane emissions from lakes and impoundments during the 21st century

Lakes and impoundments are important sources of greenhouse gases (GHG: i.e., CO2, CH4, N2O), yet global emission estimates are based on regionally-biased averages and elementary upscaling. We assembled the largest global dataset to date on emission rates of all three GHGs and found they covary with lake size and trophic state. Fitted models were upscaled to estimate global emission using global lake size inventories and a remotely-sensed global lake productivity distribution. Traditional upscaling approaches overestimated CO2 and N2O emission but underestimated CH4 by half. Our upscaled size-productivity weighted estimates (1.25-2.30 Pg of CO2-equivalents annually) are nearly 20% of global CO2 fossil fuel emission with ~75% of the climate impact due to CH4. Moderate global increases in eutrophication could translate to 5-40% increases in the GHG effects in the atmosphere, adding the equivalent effect of another 13% of fossil fuel combustion or an effect equal to GHG emissions from current land use change.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.1002/lol2.10073

Greenhouse gas emissions from lakes and impoundments: Upscaling in the face of global change

Nitrous oxide from streams and rivers: A review of primary biogeochemical pathways and environmental variables

Estimates of global riverine nitrous oxide (N2 O) emissions contain great uncertainty. We conducted a meta-analysis incorporating 169 observations from published literature to estimate global riverine N2 O emission rates and emission factors. Riverine N2 O flux was significantly correlated with NH4 , NO3 and DIN (NH4  + NO3 ) concentrations, loads and yields. The emission factors EF(a) (i.e., the ratio of N2 O emission rate and DIN load) and EF(b) (i.e., the ratio of N2 O and DIN concentrations) values were comparable and showed negative correlations with nitrogen concentration, load and yield and water discharge, but positive correlations with the dissolved organic carbon : DIN ratio. After individually evaluating 82 potential regression models based on EF(a) or EF(b) for global, temperate zone and subtropical zone datasets, a power function of DIN yield multiplied by watershed area was determined to provide the best fit between modeled and observed riverine N2 O emission rates (EF(a): R2  = 0.92 for both global and climatic zone models, n = 70; EF(b): R2  = 0.91 for global model and R2  = 0.90 for climatic zone models, n = 70). Using recent estimates of DIN loads for 6400 rivers, models estimated global riverine N2 O emission rates of 29.6-35.3 (mean = 32.2) Gg N2 O-N yr-1 and emission factors of 0.16-0.19% (mean = 0.17%). Global riverine N2 O emission rates are forecasted to increase by 35%, 25%, 18% and 3% in 2050 compared to the 2000s under the Millennium Ecosystem Assessment's Global Orchestration, Order from Strength, Technogarden, and Adapting Mosaic scenarios, respectively. Previous studies may overestimate global riverine N2 O emission rates (300-2100 Gg N2 O-N yr-1 ) because they ignore declining emission factor values with increasing nitrogen levels and channel size, as well as neglect differences in emission factors corresponding to different nitrogen forms. Riverine N2 O emission estimates will be further enhanced through refining emission factor estimates, extending measurements longitudinally along entire river networks and improving estimates of global riverine nitrogen loads.

/pdf/modeling-nitrous-oxide-emission-from-rivers-a-global-460qua152q.pdf

Modeling nitrous oxide emission from rivers: a global assessment

Emissions of nitrous oxide (N2O) from the world’s river networks constitute a poorly constrained term in the global N2O budget1,2. This N2O component was previously estimated as indirect emissions from agricultural soils3 with large uncertainties4–10. Here, we present an improved model representation of nitrogen and N2O processes of the land–ocean aquatic continuum11 constrained with an ensemble of 11 data products. The model–data framework provides a quantification for how changes in nitrogen inputs (fertilizer, deposition and manure), climate and atmospheric CO2 concentration, and terrestrial processes have affected the N2O emissions from the world’s streams and rivers during 1900–2016. The results show a fourfold increase of global riverine N2O emissions from 70.4 ± 15.4 Gg N2O-N yr−1 in 1900 to 291.3 ± 58.6 Gg N2O-N yr−1 in 2016, although the N2O emissions started to decline after the early 2000s. The small rivers in headwater zones (lower than fourth-order streams) contributed up to 85% of global riverine N2O emissions. Nitrogen loads on headwater streams and groundwater from human activities, primarily agricultural nitrogen applications, play an important role in the increase of global riverine N2O emissions. N2O emissions from rivers have increased globally by a factor of four between 1900 and 2016, with emissions starting to decline since the early 2000s. Most riverine N2O emissions come from smaller streams, driven primarily by the use of nitrogen fertilizers in agriculture.

Increased global nitrous oxide emissions from streams and rivers in the Anthropocene

Aquatic ecosystems are important sources of the greenhouse gas nitrous oxide. Measurements of nitrous oxide concentrations from 321 rivers, lakes and ponds in Canada reveal that some boreal aquatic systems can act as net nitrous oxide sinks.

Nitrous oxide sinks and emissions in boreal aquatic networks in Québec

Reservoir CO2 and CH4 emissions and their climate impact over the period 1900–2060

Lakes are undergoing striking physicochemical changes globally, including co‐occurring increases in dissolved organic carbon and nutrient concentrations, water color, and surface temperature. Although several experimental studies of lake browning and warming have been conducted over the last decade, knowledge remains limited as to the structural and functional responses of multitrophic plankton communities, especially under environmentally relevant physicochemical conditions. Using reverse osmosis to manipulate naturally occurring dissolved organic matter (DOM), we performed an enclosure experiment to evaluate the response of a planktonic food web (zooplankton–phytoplankton–bacterioplankton) to individual and combined increases in DOM and temperature, while accounting for changes in inorganic nutrients associated with DOM enrichment. We found that concomitant increases in DOM and temperature or inorganic nutrients elicited substantially greater biotic effects, but infrequently led to interactive effects. Overall, major plankton groups responded differently to manipulated factors, with most effects observed in standing stocks, community composition, and trophic structure, while metabolic (primary production and respiration) rates appeared to be generally less responsive. DOM enrichment had a clear stimulatory effect on phytoplankton, but weakly affected zooplankton. More specifically, DOM enrichment, alone or combined with inorganic nutrient amendments, decreased zooplankton : chlorophyll a ratios, implying a reduced trophic transfer efficiency and altered trophic structure. Warming generally increased bacterial abundance and cyanobacterial dominance, especially under DOM‐enriched conditions. Collectively, these results demonstrate that increasing local DOM, even by only ~ 2 mgC L−1, can enhance plankton responses to rising temperature or inorganic nutrients in the near‐surface layer of a clearwater lake, with potential implications for ecosystems facing co‐occurring environmental changes.

Dissolved organic matter mediates the effects of warming and inorganic nutrients on a lake planktonic food web

Small Contribution of Schoenoplectus acutus (Emergent Macrophyte) to Nitrogen Removal from Wastewater Effluent Input to a Restored Prairie Wetland Complex

C. Soued

Papers

Nitrous oxide sinks and emissions in boreal aquatic networks in Québec

Reservoir CO2 and CH4 emissions and their climate impact over the period 1900–2060

Dissolved organic matter mediates the effects of warming and inorganic nutrients on a lake planktonic food web

Small Contribution of Schoenoplectus acutus (Emergent Macrophyte) to Nitrogen Removal from Wastewater Effluent Input to a Restored Prairie Wetland Complex

Heterotrophic aquatic metabolism and sustained carbon dioxide emissions in a mineral-soil wetland restored with treated effluent.