Air–water diffusive gas flux is commonly determined using measurements of gas concentrations and an estimate of gas transfer velocity (k
600
) usually derived from wind speed. The great heterogeneity ...of aquatic systems raises questions about the appropriateness of using a single wind-based model to predict k
600
in all aquatic systems. Theoretical considerations suggest that wind speed to k
600
relationships should instead be system-specific. Using data collected from aquatic systems of different sizes, we show that k
600
is related to fetch and other measures of ecosystem size. Lake area together with wind speed provided the best predictive model of gas transfer velocity and explained 68% of the variability in individual k
600
measurements. For a moderate wind speed of 5 m·s
−1
, predicted k
600
varied from 6 cm·h
−1
in a small 1 ha lake to over 13 cm·h
−1
in a 100 km
2
system. Wave height is also shown to be a promising integrative predictor variable. The modulating influence of system size on wind speed – gas transfer velocity relationships can have a large impact on upscaling exercises of gas exchange at the whole landscape level.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The color of freshwaters, often measured as absorbance, influences a number of ecosystem services including biodiversity, fish production, and drinking water quality. Many countries have recently ...reported on increasing trends of water color in freshwaters, for which drivers are still not fully understood. We show here with more than 58000 water samples from the boreal and hemiboreal region of Sweden and Canada that absorbance of filtered water (a₄₂₀) co-varied with dissolved organic carbon (DOC) concentrations (R² = 0.85, P<0.0001), but that a₄₂₀ relative to DOC is increased by the presence of iron (Fe). We found that concentrations of Fe significantly declined with increasing water retention in the landscape, resulting in significantly lower Fe concentrations in lakes compared to running waters. The Fe loss along the aquatic continuum corresponded to a proportional loss in a₄₂₀, suggesting a tight biogeochemical coupling between colored dissolved organic matter and Fe. Since water is being flushed at increasing rates due to enhanced runoff in the studied regions, diminished loss of Fe along the aquatic continuum may be one reason for observed trends in a₄₂₀, and in particular in a₄₂₀/DOC increases. If trends of increased Fe concentrations in freshwaters continue, water color will further increase with various effects on ecosystem services and biogeochemical cycles.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Methane (CH₄) emissions from aquatic systems should be coupled to CH₄ production, and thus a temperature-dependent process, yet recent evidence suggests that modeling CH₄ emissions may be more ...complex due to the biotic and abiotic processes influencing emissions. We studied the magnitude and regulation of two CH₄ pathways—ebullition and diffusion—from 10 shallow ponds and 3 lakes in Québec. Ebullitive fluxes in ponds averaged 4.6 ± 4.1 mmol CH₄ m-2 d-1, contributing ~56% to total (diffusive + ebullitive) CH₄ emissions. In lakes, ebullition only occurred in waters < 3 m deep, averaging 1.1 ± 1.5 mmol CH₄ m-2p d-1, and when integrated over the whole lake, contributed only 18% to 22% to total CH₄ emissions. While pond CH₄ fluxes were related to sediment temperature, with ebullition having a stronger dependence than diffusion (Q10, 13 vs. 10; activation energies, 168 kJ mol-1 vs. 151 kJ mol2-1), the temperature dependency of CH₄ fluxes from lakes was absent. Combining data from ponds and lakes shows that the temperature dependency of CH₄ diffusion and ebullition is strongly modulated by system trophic status (as total phosphorus), suggesting that organic substrate limitation dampens the influence of temperature on CH₄ fluxes from oligotrophic systems. Furthermore, a strong phosphorus-temperature interaction determines the dominant emission pathway, with ebullition disproportionately enhanced. Our results suggest that aquatic CH₄ ebullition is regulated by the interaction between ecosystem productivity and climate, and will constitute an increasingly important component of carbon emissions from northern aquatic systems under climate and environmental change.
No Longer a Paradox DelSontro, Tonya; del Giorgio, Paul A.; Prairie, Yves T.
Ecosystems (New York),
09/2018, Letnik:
21, Številka:
6
Journal Article
Recenzirano
Lakes play an important role in the global carbon cycle, emitting significant amounts of the carbonic greenhouse gases, CO₂ and methane (CH₄). Nearly all lake studies have reported oxygenated surface ...waters oversaturated with (and thus continuously emitting) CH₄, yet no consistent explanation exists to account for why CH₄, which is produced in anoxic zones and consumed in the presence of oxygen, remains in oxic waters across the range of lake sizes. Here, we developed a physical model that defines the spatial CH₄ distribution in the surface waters of lakes as a function of CH₄ transport from the littoral zone including air–water gas exchange, and tested this in a set of 14 lakes that ranged widely in size (0.07–19,000 km²). Although the model adequately resolved the overall CH₄ decline within a lake relative to distance from shore across the range of lake sizes, discrepancies between observations and predictions suggest that other processes modulate surface CH₄ distributions. Coupled trends in the stable carbon isotopic signature of CH₄ further indicate that the spatial pattern in 30% of the lakes was dominated by a net loss via oxidation, whereas a net input of ¹³C-depleted CH₄ dominated the spatial pattern in 70% of the lakes, suggesting the predominance of pelagic CH₄ production in the oxic epilimnia of these lakes. The spatial patterns imposed by the interaction between physical and biological processes may result in a size-dependent underestimation of wholelake CH₄ emissions when based on center samples. Whereas the actual contributions of oxidation and eplimnetic CH₄ production are still not well understood, our results demonstrate that the ubiquitous CH₄ oversaturation observed in most lakes can be explained through the interaction between horizontal transport of littoral CH₄, air–water gas exchange and the balance between epilimnetic CH₄ oxidation and production.
Methanogenesis has traditionally been assumed to occur only in anoxic environments, yet there is mounting, albeit indirect, evidence of methane (CH4) production in oxic marine and freshwaters. Here ...we present the first direct, ecosystem-scale demonstration of methanogenesis in oxic lake waters. This methanogenesis appears to be driven by acetoclastic production, and is closely linked to algal dynamics. We show that oxic water methanogenesis is a significant component of the overall CH4 budget in a small, shallow lake, and provide evidence that this pathway may be the main CH4 source in large, deep lakes and open oceans. Our results challenge the current global understanding of aquatic CH4 dynamics, and suggest a hitherto unestablished link between pelagic CH4 emissions and surface-water primary production. This link may be particularly sensitive to widespread and increasing human influences on aquatic ecosystem primary productivity.
Lakes are a major component of boreal landscapes, and whereas lake CO₂emissions are recognized as a major component of regional C budgets, there is still much uncertainty associated to lake ...CH₄fluxes. Here, we present a large‐scale study of the magnitude and regulation of boreal lake summer diffusive CH₄fluxes, and their contribution to total lake carbon (C) emissions, based on in situ measurements of concentration and fluxes of CH₄and CO₂in 224 lakes across a wide range of lake type and environmental gradients in Québec. The diffusive CH₄flux was highly variable (mean 11.6 ± 26.4 SD mg m⁻² d⁻¹), and it was positively correlated with temperature and lake nutrient status, and negatively correlated with lake area and colored dissolved organic matter (CDOM). The relationship between CH₄and CO₂concentrations fluxes was weak, suggesting major differences in their respective sources and/or regulation. For example, increasing water temperature leads to higher CH₄flux but does not significantly affect CO₂flux, whereas increasing CDOM concentration leads to higher CO₂flux but lower CH₄flux. CH₄contributed to 8 ± 23% to the total lake C emissions (CH₄ + CO₂), but 18 ± 25% to the total flux in terms of atmospheric warming potential, expressed as CO₂‐equivalents. The incorporation of ebullition and plant‐mediated CH₄fluxes would further increase the importance of lake CH₄. The average Q₁₀of CH₄flux was 3.7, once other covarying factors were accounted for, but this apparent Q₁₀varied with lake morphometry and was higher for shallow lakes. We conclude that global climate change and the resulting shifts in temperature will strongly influence lake CH₄fluxes across the boreal biome, but these climate effects may be altered by regional patterns in lake morphometry, nutrient status, and browning.
Here we examine the patterns in carbon dioxide partial pressure (pCO2) measured in a number of small boreal streams (<5 km in length) in the northwestern boreal region of Québec during the ice‐free ...season and compare these to the patterns found in a major river (Eastmain River) and in a tributary in the same region. All systems were consistently supersaturated in CO2 (range 450 to 5000 μatm) streams having both higher (mean 1850 μatm) and more variable pCO2 than that of rivers (range 550 to 800 μatm). Stream pCO2 was positively related to DOC concentration and stream segment length, both suggesting a direct influence of the surrounding landscape. Calculated stream water‐air CO2 fluxes ranged from 700 to over 3000 mg C m−2 d−1, up to 2 orders of magnitude higher than those measured in large rivers and lakes of the same region. Small streams, despite their extremely reduced areal coverage (1% of the aquatic surface), accounted for 25% of the total aquatic C emissions, and the resulting areal stream fluxes were comparable to those measured in different soils or wetlands in the region.
Methanogenesis is traditionally considered as a strictly anaerobic process. Recent evidence suggests instead that the ubiquitous methane (CH4) oversaturation found in freshwater lakes is sustained, ...at least partially, by methanogenesis in oxic conditions. Although this paradigm shift is rapidly gaining acceptance, the magnitude and regulation of oxic CH4 production (OMP) have remained ambiguous. Based on the summer CH4 mass balance in the surface mixed layer (SML) of five small temperate lakes (surface area, SA, of 0.008–0.44 km2), we show that OMP (range of 0.01 ± 0.01 to 0.52 ± 0.04 μmol L–1 day–1) is linked to the concentrations of chlorophyll-a, total phosphorus, and dissolved organic carbon. The stable carbon isotopic mass balance of CH4 (δ13C-CH4) indicates direct photoautotrophic release as the most likely source of oxic CH4. Furthermore, we show that the oxic CH4 contribution to the SML CH4 saturation and emission is an inverse function of the ratio of the sediment area to the SML volume in lakes as small as 0.06 km2. Given that global lake CH4 emissions are dominated by small lakes (SA of <1 km2), the large contribution of oxic CH4 production (up to 76%) observed in this study suggests that OMP can contribute significantly to global CH4 emissions.
Collectively, reservoirs constitute a significant global source of C‐based greenhouse gases (GHGs). Yet, global estimates of reservoir carbon dioxide (CO2) and methane (CH4) emissions remain ...uncertain, varying more than four‐fold in recent analyses. Here we present results from a global application of the Greenhouse Gas from Reservoirs (G‐res) model wherein we estimate per‐area and per‐reservoir CO2 and CH4 fluxes, by specific flux pathway and in a spatially and temporally explicit manner, as a function of reservoir characteristics. We show: (a) CH4 fluxes via degassing and ebullition are much larger than previously recognized and diffusive CH4 fluxes are lower than previously estimated, while CO2 emissions are similar to those reported in past work; (b) per‐area reservoir GHG fluxes are >29% higher than suggested by previous studies, due in large part to our novel inclusion of the degassing flux in our global estimate; (c) CO2 flux is the dominant emissions pathway in boreal regions and CH4 degassing and ebullition are dominant in tropical and subtropical regions, with the highest overall reservoir GHG fluxes in the tropics and subtropics; and (d) reservoir GHG fluxes are quite sensitive to input parameters that are both poorly constrained and likely to be strongly influenced by climate change in coming decades (parameters such as temperature and littoral area, where the latter may be expanded by deepening thermoclines expected to accompany warming surface waters). Together these results highlight a critical need to both better understand climate‐related drivers of GHG emission and to better quantify GHG emissions via CH4 ebullition and degassing.
Plain Language Summary
By damming rivers, humans have created millions of reservoirs, which, collectively, constitute an important greenhouse gas source, especially for methane, a particularly potent greenhouse gas. Using observed relationships between reservoir characteristics and greenhouse gas emissions, we show that much more methane either bubbles out of reservoirs or is emitted just downstream from reservoirs than was previously known. This is important because it may be possible to reduce methane emissions from downstream of reservoirs by selectively withdrawing water from near the surface of reservoirs, which tends to be methane‐poor, rather than from greater depths, where methane often accumulates. We also found that on a per‐area basis reservoirs are a more potent source of greenhouse gases than previously recognized, and that the highest rates of emissions occur in the tropics and subtropics. Finally, we show that estimates of reservoir greenhouse gas emissions are quite sensitive to climate‐related factors like temperature.
Key Points
This is the most comprehensive global analysis of reservoir methane and CO2 emissions to‐date, and the first to estimate methane degassing
Although diffusive CH4 fluxes are somewhat lower than previously believed, CH4 fluxes via degassing and ebullition are much larger
The highest reservoir greenhouse gas emissions globally occur in the tropics and subtropics, with CH4 degassing and ebullition as dominant flux paths
We performed a series of gas exchange measurements in 12 diverse aquatic systems to develop the direct relationship between near-surface turbulence and gas transfer velocity. The relationship was ...log-linear, explained 78% of the variation in instantaneous gas transfer velocities, and was valid over a range of turbulent energy dissipation rates spanning about two orders of magnitude. Unlike wind-based relationships, our model is applicable to systems ranging in size from less than 1 km² to over 600 km². Gas fluxes measured with our specific model of floating chambers can be grossly overestimated (by up to 1000%), particularly in low-turbulence conditions. In high-turbulence regimes, flux overestimation decreases to within 50%. Direct measurements of turbulent energy dissipation rate provide reliable estimation of the associated gas transfer velocity even at short temporal and spatial scales.
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