Methane Concentration In Water Research Articles

CH4 emissions from runoff water of Alaskan mountain glaciers

Recent studies have observed high methane concentrations in runoff water and the ambient air at various glacier sites, including the Greenland Ice Sheet, the glacier forefield in Svalbard, and the ice cap in Iceland. This study extends these findings to smaller mountain glaciers in Alaska. Methane and carbon dioxide concentrations in the ambient air near the meltwater outlet, fluxes of these gases at the surface of runoff water and riverbank sediments, and dissolved methane content in the runoff water were measured at four glaciers. Three of the four glaciers showed conspicuous signals of methane emissions from runoff water, with the Castner Glacier terminus exhibiting a methane concentration three times higher than background levels, along with elevated dissolved methane levels in the runoff water. This study marks the detection of significant methane emissions from small mountain glacier runoff, contributing to the understanding that mountain glaciers also release methane into the atmosphere.

Development of a fast‐response system with integrated calibration for high‐resolution mapping of dissolved methane concentration in surface waters

AbstractDissolved gas concentrations in surface waters can have sharp gradients across marine and freshwater environments, which often prove challenging to capture with analytical measurement. Collecting discrete samples for laboratory analysis provides accurate results, but suffers from poor spatial resolution. To overcome this limitation, water equilibrators and gas membrane contactors (GMCs) have been used for the automated underway measurement of dissolved gas concentrations in surface water. However, while water equilibrators can provide continuous measurements, their analytical response times to changes in surface water concentration can be slow, lasting tens of minutes. This leads to spatial imprecisions in the dissolved gas concentration data. Conversely, while GMCs have proven to have much faster analytical response times, often lasting only a few minutes or less, they suffer from poor accuracy and thus require routine calibration. Here we present an analytical system for the high accuracy and high precision spatial mapping of dissolved methane concentration in surface waters. The system integrates a GMC with a cavity ringdown spectrometer for fast analytical response times, with a calibration method involving two Weiss‐style equilibrators and discrete measurements in vials. Data from both the GMC and equilibrators are collected simultaneously, with discrete vial samples collected periodically throughout data collection. We also present a mathematical algorithm integrating all data collected for the routine calibration of the GMC dataset. The algorithm facilitates comparison between the GMC and equilibrator datasets despite the substantial differences in response times (0.7–2.1 and 4.1–17.6 min, respectively). This measurement system was tested with both systematic laboratory experiments and field data collected on a research cruise along the US Atlantic margin. Once calibrated, this system identified numerous sharp peaks of dissolved methane concentration in the US Atlantic margin dataset that would be poorly resolved, or outright missed with previous measurement techniques. Overall, the precision and accuracy for the technique presented here were determined to be 11.2% and 10.4%, respectively, the operating range was 0–1000 ppm methane, and the e‐folding response time to changes in dissolved methane concentration was 0.7–2.1 min.

Spatial Distribution of Methane Concentration in Baikal Surface Water in the Spring Period

Spatial Distribution of Methane Concentration in Baikal Surface Water in the Spring Period

Methane Distribution in Lake Baikal Water

Data on the distribution of methane concentration in water in the open water area and the northern end of Lake Baikal and in the source of the Angara R. collected in September 2016 and 2019 are analyzed. To apply correlation analysis, in addition to methane, various hydrochemical characteristics were also determined, including the temperature, pH, the concentrations of О2, suspended matter, Corg, Norg, Porg, mineral compounds of nitrogen and phosphorus. Methane concentration in Baikal water in 2016 varied within 0.44–3.41 µL/dm3 (on the average, 0.80 µL/dm3); in 2019, within 0.20–5.19 µL/dm3 (on the average, 1.22 µL/dm3). The maximal methane concentration was recorded in the water mass of the deepest central depression of the lake, and the minimal, in its southern depression. Among shallow areas, minimal methane concentrations were recorded in the coastal zone of Listvenichnyi Bay, and its maximal concentrations, in the northern part of the lake, into which many rivers empty, as well as in Selenginskoe shallows. Most stations, either deep-water or shallow, showed a peak of subsurface maximum of methane concentration at depths of 25–50 m (thermocline zone), after which its concentrations commonly dropped, reaching their minimal values either in the intermediate water mass or in bottom layers. Shallow areas, in addition to higher methane concentrations compared to deep-water areas, also featured a higher contrast in its vertical distribution in the water mass. The analysis of correlation relationships between the examined hydrochemical characteristics revealed significant direct correlations of methane concentration with the concentrations of Corg and Norg and an inverse correlation with O2 concentrations.

ГЛУБИННОЕ СТРОЕНИЕ ЗЕМНОЙ КОРЫ ЮЖНОЙ ЧАСТИ ТАТАРСКОГО ТРОГА И РАСПРЕДЕЛЕНИЕ ГАЗОГЕОХИМИЧЕСКИХ АНОМАЛИЙ

Deep structural features of the structures in the southern part of the Tatar trough were determined based on structural, density and petromagnetic modeling. It was revealed that the morphostructure has a riftogenic nature. The distribution of anomalous methane concentrations in water and the upper part of the sedimentary cover above these objects was studied. It is shown that the sialic crust in the area adjacent to the rift structures has undergone characteristic changes. Possible ways of delivering thermogenic methane to the upper part of the sedimentary cover and the water column were determined.

Monitoring of Shallow-Water Methane Seeps at Cape Fiolent (Black Sea)

During the period from 2019 to 2021, complex studies of new shallow-water methane bubble gas emission sites were carried out in the coastal zone near Cape Fiolent (Southwest Coast of Crimea). The studies included determining the hydrocarbon and isotopic composition of bubble gas, measuring the concentration of methane and nutrients in the water in the areas of gas emissions, estimating the value of bubble flows, and measuring hydrophysical parameters over the sip sites compared to background areas. The seasonal type of Cape Fiolent methane seeps was noted, its active phases of gas emissions differed in duration in different years. The increased pore water silica concentration at the seep sites and their localization in the vicinity of freshwater slope springs may indicate its association with submarine freshwater discharge in the area. However, no significant desalination of both pore water and the bottom water layer above the siphons was recorded. Dissolved methane concentrations in pore water at seep sites were two orders of magnitude higher compared to background areas and reached 448 μmol/L. Also high values were obtained for surface water directly above the bubble gas emission points (maximum 353 nmol/L). Multi-hour monitoring of hydrophysical parameters above the active seeps showed a dissolved oxygen decrease compared to the background sites. The maximum difference in O2 concentrations was 3 mg/l. The carbon isotopic composition of bubble gas methane δ13C-CH4 (–62.84…38.27‰) and сarbon dioxide δ13C-CO2 (–16.83…–10.17‰) was corresponded to a mixture of isotopically heavy gas and near-surface isotopically light gas of microbial origin. The question remains open: what are the reasons for the change in the summer active and the cold season passive gas emission phases?

Space and Time Variations of Hydroenvironmental Characteristics of the Ivankovo Reservoir in Years with Different Weather Conditions

The article gives the results of studies of the hydroecological characteristics (water temperature, dissolved oxygen, chemical characteristics, production, methane concentration and emission, and the abundance of phytoplankton and benthos) in the Ivankovo Reservoir in August 2020–2021. Data of integrated surveys were used to show that the specific weather conditions and flowage have led to difference in these closely interrelated characteristics. The hot weather and the low flowage in 2021 contributed to an increase in oxygen consumption by bed soil, intensification of exchange processes at the water–bottom sediment interface, an increase in mineral phosphorus flux, more active blooming of the water body, decay of dead organic matter at higher temperature of the bottom layer. The role of bioturbation in changes in the rate of exchange processes between bottom water layer and sediments is demonstrated. The changes in the combination of processes in the water body caused an increase in methane flux into the atmosphere in 2021 compared with the previous year.

Autonomous methane seep site monitoring offshore western Svalbard: hourly to seasonal variability and associated oceanographic parameters

Abstract. Improved quantification techniques of natural sources are needed to explain variations in atmospheric methane. In polar regions, high uncertainties in current estimates of methane release from the seabed remain. We present unique 10- and 3-month time series of bottom water measurements of physical and chemical parameters from two autonomous ocean observatories deployed at separate intense seabed methane seep sites (91 and 246 m depth) offshore western Svalbard from 2015 to 2016. Results show high short-term (100–1000 nmol L−1 within hours) and seasonal variation, as well as higher (2–7 times) methane concentrations compared to previous measurements. Rapid variability is explained by uneven distribution of seepage and changing ocean current directions. No overt influence of tidal hydrostatic pressure or water temperature variations on methane concentration was observed, but an observed negative correlation with temperature at the 246 m site fits with hypothesized seasonal blocking of lateral methane pathways in the sediments. Negative correlation between bottom water methane concentration (and variability) and wind forcing, concomitant with signs of weaker water column stratification, indicates increased potential for methane release to the atmosphere in fall and winter. We present new information about short- and long-term methane variability and provide a preliminary constraint on the uncertainties that arise in methane inventory estimates from this variability.

Influence of water storage on deflagration characteristics of methane in confined space

Comparative experiments were conducted under different water level heights and methane concentration conditions using a self-designed explosion experiment pipeline. The results showed that, in comparison with the scenario without water storage, when the water level was 2 cm and methane concentration was 6.5–12.5%, there was a dual effect including a pressure decrease caused by the endothermic cooling of liquid water and a pressure increase caused by the expansion of water vapour. These effects caused the pressure time history curve to exhibit a double-peak or multi-peak structure, and the average decrease in the peak deflagration pressure was 23.76%. The heat of vaporisation absorbed by the stored water and barrier effect of water vapour on the transfer of the heat slowed down the increase in the deflagration temperature. The average decrease in the peak deflagration temperature of methane was 13.82%, and the time to reach the peak deflagration temperature was extended as a whole, with an average delay of 0.22 s. Water storage also changed the shape of the deflagration flame front, which exhibited ‘knife’, ‘V’, and ‘crescent’ structures. Moreover, the flame propagation speed was significantly reduced, with the peak and average flame propagation speeds decreased by 83.3% and 83.6%, respectively. The research results can provide a certain reference for preventing gas explosions in typical confined spaces, and also help to explore new anti-explosion methods, which can be applied to marine equipment such as ships.

Identifying Drivers Behind Spatial Variability of Methane Concentrations in East Siberian Ponds

Waterbody methane emissions per area are negatively correlated with the size of the emitting waterbody. Thus, ponds, defined here as having an area smaller than 8 · 104m2, contribute out of proportion to the aquatic methane budget compared to the total area they cover and compared to other waterbodies. However, methane concentrations in and methane emissions from ponds show more spatial variability than larger waterbodies. We need to better understand this variability to improve upscaling estimates of freshwater methane emissions. In this regard, the Arctic permafrost landscape is an important region, which, besides carbon-rich soils, features a high pond density and is exposed to above-average climatic warming. We studied 41 polygonal-tundra ponds in the Lena River Delta, northeast Siberia. We collected water samples at different locations and depths in each pond and determined methane concentrations using gas chromatography. Additionally, we collected information on the key properties of the ponds to identify drivers of surface water methane concentrations. The ponds can be categorized into three geomorphological types with distinct differences in drivers of methane concentrations: polygonal-center ponds, ice-wedge ponds and larger merged polygonal ponds. All ponds are supersaturated in methane, but ice-wedge ponds exhibit the highest surface water concentrations. We find that ice-wedge ponds feature a strong stratification due to consistently low bottom temperatures. This causes surface concentrations to mainly depend on wind speed and on the amount of methane that has accumulated in the hypolimnion. In polygonal-center ponds, high methane surface concentrations are mostly determined by a small water depth. Apart from the influence of water depth on mixing speed, water depth controls the overgrown fraction, the fraction of the pond covered by vascular plants. The plants provide labile substrate to the methane-producing microbes. This link can also be seen in merged polygonal ponds, which furthermore show the strongest dependence on area as well as an anticorrelation to energy input indicating that stratification influences the surface water methane concentrations in larger ponds. Overall, our findings underpin the strong variability of methane concentrations in ponds. No single driver could explain a significant part of the variability over all pond types suggesting that more complex upscaling methods such as process-based modeling are needed.

Characterization of marine microbial communities around an Arctic seabed hydrocarbon seep at Scott Inlet, Baffin Bay

Seabed hydrocarbon seeps present natural laboratories for investigating responses of marine ecosystems to petroleum input. A hydrocarbon seep near Scott Inlet, Baffin Bay, was visited for in situ observations and sampling in the summer of 2018. Video evidence of an active hydrocarbon seep was confirmed by methane and hydrocarbon analysis of the overlying water column, which is 260 m at this site. Elevated methane concentrations in bottom water above and down current from the seep decreased to background seawater levels in the mid-water column >150 m above the seafloor. Seafloor microbial mats morphologically resembling sulfide-oxidizing bacteria surrounded areas of bubble ebullition. Calcareous tube worms, brittle stars, shrimp, sponges, sea stars, sea anemones, sea urchins, small fish and soft corals were observed near the seep, with soft corals showing evidence for hydrocarbon incorporation. Sediment microbial communities included putative methane-oxidizing Methyloprofundus, sulfate-reducing Desulfobulbaceae and sulfide-oxidizing Sulfurovum. A metabolic gene diagnostic for aerobic methanotrophs (pmoA) was detected in the sediment and bottom water above the seep epicentre and up to 5 km away. Both 16S rRNA gene and pmoA amplicon sequencing revealed that pelagic microbial communities oriented along the geologic basement rise associated with methane seepage (running SW to NE) differed from communities in off-axis water up to 5 km away. Relative abundances of aerobic methanotrophs and putative hydrocarbon-degrading bacteria were elevated in the bottom water down current from the seep. Detection of bacterial clades typically associated with hydrocarbon and methane oxidation highlights the importance of Arctic marine microbial communities in mitigating hydrocarbon emissions from natural geologic sources.

Low Abundance of Methanotrophs in Sediments of Shallow Boreal Coastal Zones With High Water Methane Concentrations.

Coastal zones are transitional areas between land and sea where large amounts of organic and inorganic carbon compounds are recycled by microbes. Especially shallow zones near land have been shown to be the main source for oceanic methane (CH4) emissions. Water depth has been predicted as the best explanatory variable, which is related to CH4 ebullition, but exactly how sediment methanotrophs mediates these emissions along water depth is unknown. Here, we investigated the relative abundance and RNA transcripts attributed to methane oxidation proteins of aerobic methanotrophs in the sediment of shallow coastal zones with high CH4 concentrations within a depth gradient from 10–45 m. Field sampling consisted of collecting sediment (top 0–2 cm layer) from eight stations along this depth gradient in the coastal Baltic Sea. The relative abundance and RNA transcripts attributed to the CH4 oxidizing protein (pMMO; particulate methane monooxygenase) of the dominant methanotroph Methylococcales was significantly higher in deeper costal offshore areas (36–45 m water depth) compared to adjacent shallow zones (10–28 m). This was in accordance with the shallow zones having higher CH4 concentrations in the surface water, as well as more CH4 seeps from the sediment. Furthermore, our findings indicate that the low prevalence of Methylococcales and RNA transcripts attributed to pMMO was restrained to the euphotic zone (indicated by Photosynthetically active radiation (PAR) data, photosynthesis proteins, and 18S rRNA data of benthic diatoms). This was also indicated by a positive relationship between water depth and the relative abundance of Methylococcales and pMMO. How these processes are affected by light availability requires further studies. CH4 ebullition potentially bypasses aerobic methanotrophs in shallow coastal areas, reducing CH4 availability and limiting their growth. Such mechanism could help explain their reduced relative abundance and related RNA transcripts for pMMO. These findings can partly explain the difference in CH4 concentrations between shallow and deep coastal areas, and the relationship between CH4 concentrations and water depth.

Methane oxidation dynamics in a karst subterranean estuary

Chemical gradients between fresh, brackish and saline waters shape biogeochemical reactions and organic matter transformation within subterranean estuaries. In the Yucatán Peninsula’s karst subterranean estuary (KSE), methane and dissolved organic matter generated during the anaerobic decomposition of tropical forest vegetation are transported into flooded cave networks where microbial consumption greatly reduces their concentrations in the groundwater. To test the hypothesis that chemoclines associated with salinity gradients of the KSE are sites of methane oxidation, we obtained methane concentration and δ13C profiles of unprecedented vertical resolution from within a fully-submerged cave system located 6.6 km inland from the coastline using the ‘OctoPiPi’ (OPP) water sampler. Along a 12–24 cm thick low-salinity-halocline at ∼4.5 m water depth, salinity increased from fresh to brackish (0.2–1.8 psu), methane concentrations decreased, and δ13C values increased, as expected for microbial methane oxidation. The underlying brackish water had elevated oxygen concentrations compared to the always anoxic freshwater, suggesting that aerobic methane oxidation is the dominant process facilitating methane consumption. By contrast, as salinity increased from 1.8 to 36 psu through a 24–36 cm thick high-salinity-halocline between the meteoric lens and the saline groundwater at ∼20 m water depth, methane concentrations and δ13C values were constant. Conservative mixing and kinetic isotope models incorporating the methane data confirm a hotspot for microbial methane oxidation at the low-salinity-halocline. At least 98% of methane originating in the anoxic freshwaters was removed before its transport via channelized flow towards the coastline. These findings provide novel insight into the spatial constraints of methane dynamics within a karst subterranean estuary.

Population dynamics of methanogens and methanotrophs along the salinity gradient in Pearl River Estuary: implications for methane metabolism.

Methane, a major greenhouse gas, plays an important role in global carbon cycling and climate change. Methanogenesis is identified as an important process for methane formation in estuarine sediments. However, the metabolism of methane in the water columns of estuaries is not well understood. The goal of this research was to examine the dynamics in abundance and community composition of methanogens and methanotrophs, and to examine whether and how they take part in methane metabolism in the water columns from the lower Pearl River (freshwater) to the coastal South China Sea (seawater). Quantitative PCR (qPCR) and high-throughput sequencing results showed that the abundance of methanogens decreased with increasing salinity, suggesting that growth of these methanogens in the Pearl River Estuary may be influenced by high salinity. Also, the methane concentration in surface waters was lower than that in near-bottom waters at most sites, suggesting sediment methanogens are a likely source of methane. In the estuarine mixing zone, significantly high methane concentrations existed with the presence of salt-tolerant methanogens (e.g., Methanomicrobiaceae, Methanocella, Methanosaeta and Methanobacterium) and methanotrophs (e.g., Methylocystis and Methylococcaceae), which were found in brackish habitats. Furthermore, a number of methanotrophic OTUs (from pmoA gene sequence data) had specific positive correlations with methanogenic OTUs (from mcrA gene sequence data), and some of these methanogenic OTUs were correlated with concentrations of particulate organic carbon (POC). The results indicate that methanotrophs and methanogens may be intimately linked in methane metabolism attached with particles in estuarine waters.

Potential effects of the invasive bivalve Corbicula fluminea on methane cycling processes in an urban stream

Lotic methane production, consumption, and flux depends on biogeochemical conditions that vary at fine scales within streams. The invasive bivalve, Corbicula fluminea, is known to affect stream biogeochemistry and may influence methane cycling via bioturbation and respiration. Response of methanogenesis rate, potential rate of methanotrophy, and net methane flux to Corbicula density was tested using intact sediment core incubations. Potential methanotrophy decreased and net methane flux increased with Corbicula density in sediment cores. However, the magnitude of this effect decreased at the highest Corbicula densities tested here. Response of sediment pore water methane concentration to Corbicula density was tested using in situ cage enclosures in a randomized block design over a 50 m stream reach. Block effect was the most significant predictor of in situ pore water methane concentration, and Corbicula density had a positive, marginally significant, interaction with the block effect, indicating that Corbicula’s effect on methane concentration varies with in situ conditions. Our study shows that Corbicula influences methane cycling processes in the laboratory, apparently via their impact on dissolved oxygen, but this effect depends on stream conditions in the field. Further study is needed to evaluate mechanisms of this relationship more fully.

Dredging Influence on Methane Concentration in the Taganrog Bay

The effects of dredging and dumping on methane concentration in water and bottom sediments of the Taganrog Bay (the Sea of Azov) are studied. It is shown that dredging operations are accompanied by the inflow of methane deposited in soil to the water column. This inflow is associated with the resuspension of soils as a result of their loosening, lifting, and loading by the dredger to the barge as well as during their discharge to underwater piles and further erosion and separation of dumping material by waves and seabed currents. In addition to the increase in methane emission to the atmosphere, this is accompanied by a negative impact on the oxygen regime due to the microbial oxidation of methane and other organic and inorganic substances which arrived during soil resuspension. However, due to a relatively small area of the seabed affected by dredging and occupied by underwater dumps as well as due to the active hydrodynamics of waters in general, their influence on methane concentration in water and bottom sediments of the Taganrog Bay is insignificant.

Seasonal Dynamics of Microbial Processes in Bottom Sediments of the Sevastopol Coastal Area

A seasonal study of microbial methane oxidation (MO) and sulfate reduction (SR) processes in bottom sediments was carried out at five stations in the estuarian Sevastopol coastal area, which differed in their sedimentation and geochemical conditions. High methane concentrations in bottom sediments, as well as accordingly high MO rates, were obtained at stations marked by high organic carbon (Corg) concentrations. The dependence of these values was found to be exponential, whereas for SR, the amount of organic carbon was not the limiting factor. For the studied stations, the integral SR rates ranged from 2.9 to 11.3 mmol m–2 day–1. The trends of the SR rate have opposite directions for the open and closed parts of the sea: SR decreased during study period at the center of the bay, while in the open sea area it increased. The MO rates varied in a wide range from 0.0001 to 3.6 mmol m–2 day–1; the highest were obtained in Corg-rich sediments of the central part of the bay. Only at one station, where the mean annual Corg content in sediments reached 3.39%, did the methane concentration in sediment pore water exceed the calculated solubility.

Temporal variation in infaunal community structure of chemosynthetic and non-chemosynthetic environments in Barkley Hydrates, British Columbia, Canada

Increasing evidence points to greater temporal variation in deep-sea ecosystems than previously thought. In cold seeps, most available evidence focuses on successional stages of megafauna, with few studies on temporal variability in infaunal communities. We present a temporal study of infaunal communities and sedimentary organic matter characteristics from Barkley Hydrates outcrop, a chemosynthetic environment that receives organic matter from chemosynthetic and photosynthetic (phytodetritus) origins. In order to help interpret results from the outcrop, we contrast temporal variability in outcrop communities during two sampling periods with corresponding variation in an adjacent background sedimentary habitat (both habitats at ~900 m depth, offshore of British Columbia, Canada), that receives food input only from a photosynthetic source (phytodetritus). Comparisons with the background community help in differentiating chemosynthetic from photosynthetic food effects at the outcrop. Our results show contrasting temporal patterns for each community. First, lower (though not significantly) total density at the outcrop site in May 2014 corresponded to a significant change in species characteristic of that location (i.e., oligochaetes, dorvilleid polychaetes, and bivalves) in the previous sampling period (i.e., September 2013). Background communities did not change significantly in total density or species composition. Changes at the outcrop coincided with higher values of δ13C in sediment layers in May 2014, indicating reduced contributions of chemosynthetic organic matter to the total organic carbon pool relative to September 2013, and low bottom water methane concentrations. Second, although not significantly different, we detected more recruits in September 2013 for multiple species (Bipalponephtys cornuta, Cossura longocirrata, ampharetid polychaetes, Leitoscoloplos sp., and Levinsenia sp.) at the background site, following increased surface primary productivity in summer (June-August 2013). The presence of recruits of ampharetid polychaetes at both sites in September 2013 suggests that, although most individuals from the hydrate site depend on chemosynthetic food sources and therefore track temporal variation of organic matter chemosynthesised from methane, some outcrop site species also track phytodetritus pulses, much as observed in the background community. Our results demonstrate the utility of temporal studies in identifying the primary food source(s) fueling seafloor communities, and the dynamic nature of deep-sea infaunal communities from contrasting sites in Barkley Hydrates.

Flotsam samples can help explain the δ13C and δ15N values of invertebrate resting stages in lake sediment

The stable isotopic composition of chitinous remains of Cladocera (water fleas) and freshwater Bryozoa (moss animals) preserved in lake sediment records can provide supporting insights into past environmental and ecosystem changes in lakes. Here we explore whether analyses of these remains isolated from lake flotsam can provide information on the driving variables affecting the isotopic composition of these remains. We collected flotsam in 53 lakes and found enough material in 33 lakes to measure the stable carbon and nitrogen isotope ratios (expressed as δ13C and δ15N values, respectively) of resting stages. These values were compared with lake characteristics, water chemistry measurements, and the isotopic composition of sedimentary organic matter (SOM) in the lakes. Mean δ13C values of cladoceran ephippia and SOM were correlated and both were also negatively correlated with deep water methane concentrations and indicators of lake stratification. This supports the findings of previous studies that methane-derived carbon can provide a significant proportion of carbon entering planktonic food webs. Mean δ15N values of bryozoan statoblasts and SOM were correlated, suggesting that both reflect the δ15N values of phytoplankton. Our results provide information on how environmental variables in lakes can influence the δ13C and δ15N values in resting stages, but flotsam samples can also potentially be used to assess seasonal stable isotope variability of resting stages. Both types of information are important to improve palaeoenvironmental interpretations of stable isotope records based on these remains in lake sediments.

Methane and nitrous oxide distributions in coastal and open ocean waters of the Northeast Subarctic Pacific during 2015–2016

Abstract We measured depth profiles of CH 4 and N 2 O concentrations at six stations along a coastal - oceanic transect in the Northeastern Subarctic Pacific Ocean in winter, spring and summer of 2015 and 2016. In the offshore region, we observed mixed layer CH 4 concentrations that were close to atmospheric equilibrium with no evidence of a near-surface CH 4 maximum. Sub-surface waters of the oceanic region exhibited persistent CH 4 under-saturation, suggesting in situ biological consumption below the mixed layer. Methane concentrations in coastal waters were significantly more variable, with maximum super-saturation of up to 600% on the Vancouver Island shelf and slope, likely due to supply from seeps and gas hydrates. In both coastal and open ocean waters, upper water column concentrations of N 2 O were close to atmospheric equilibrium, while subsurface waters exhibited a strong N 2 O maximum coincident with the depth of the oxygen minimum at ~1000 m. The relationship between ∆N 2 O, N* and apparent oxygen utilization (AOU) revealed likely signatures of both nitrification and denitrification as N 2 O sources, suggesting a possible contribution of lateral advective supply. Sea-air fluxes for both N 2 O and CH 4 were low in the open ocean and increased towards the coast, with the highest values for N 2 O observed during the summer upwelling season. Our results provide new information on the distribution of CH 4 and N 2 O in the Northeast Subarctic Pacific, and the potential underlying mechanisms driving the production and consumption of these gases. Continued time-series measurements will provide valuable information on climate-driven changes in the biogeochemical cycling of CH 4 and N 2 O.