We investigated dissolved methane distributions along a 6 km transect
crossing active seep sites at 40 m water depth in the central North Sea.
These investigations were done under conditions of thermal stratification in
summer (July 2013) and homogenous water column in winter (January 2014).
Dissolved methane accumulated below the seasonal thermocline in summer with a
median concentration of 390 nM, whereas during winter, methane
concentrations were typically much lower (median concentration of 22 nM).
High-resolution methane analysis using an underwater mass-spectrometer
confirmed our summer results and was used to document prevailing
stratification over the tidal cycle. We contrast estimates of methane
oxidation rates (from 0.1 to 4.0 nM day
Methane is, after water vapor and CO
In general, oceans are a minor source of methane to the atmosphere,
accounting for 2–10 % of the global emissions (Bange et al., 1994). The
main oceanic source (75 %) is thought to originate from estuarine, shelf,
and coastal areas (Bange, 2006; Bange et al., 1994). The European coastal
areas were found to emit 0.46–1 Tg yr
Although continental margins account for only 10 % of the total ocean area and 20 % of the marine primary production (Killops and Killops, 1993), more than 90 % of all organic carbon burial occurs in sediment deposits on deltas, continental shelves, and upper continental slopes (Berner, 1989). At these locations, which are also characterized by high sedimentation rates, organic carbon is rapidly buried beneath the sulfate reduction zone and becomes available to methanogens (e.g., Cicerone and Oremland, 1988). Methane is also generated by thermal breakdown at high temperature and pressure. A significant fraction of the methane is oxidized in anaerobic and aerobic sediments (e.g., Boetius et al., 2000; Jørgensen and Kasten, 2006; King, 1992; Niewöhner et al., 1998). At cold seep sites, methane escaping microbial oxidation may be transported into the overlying water either dissolved in upwardly advecting pore waters or, in case of oversaturation, in the form of gas bubbles. Because methane is undersaturated in seawater, rising methane bubbles partially dissolve during ascent through the water column (McGinnis et al., 2006), where the dissolved methane may be further consumed by microbial oxidation. Only if this methane survives transport to the mixed layer may it be transferred to the atmosphere.
Because of processes consuming methane in the water column, shallow seeps are
more likely to contribute to the atmospheric methane pool. However, even at
shallow sites, density stratification may limit vertical transport. For
example, at the 70 m deep Tommeliten area in the North Sea, a summer
thermocline constrains methane transport to the atmosphere and numerical
modeling showed that during this season less than
The study site is situated in an area of active gas venting above a shallow
gas reservoir in the central North Sea south of Dogger Bank, a sandbank that
is 20 m shallower than the surrounding seabed (Fig. 1). The gas vents are
located in the Netherlands sector, license block B13 in a shallow
(
Location of the study area in the central North Sea. The main currents are shown following Howarth (2001). The map was drawn using GeoMapApp with 40 m contours.
In this region, water masses from the north (Atlantic Water) and south (Straits of Dover) meet (Kröncke and Knust, 1995) and the general anticlockwise circulation along the coasts of the North Sea becomes weak and varied (Fig. 1, Howarth, 2001). Tides have the strongest influence on the currents in this region, with wind forcing becoming secondary (Howarth, 2001; Otto et al., 1990; Sündermann and Pohlmann, 2011).
Seasonal temperature stratification, common to this and other shelf seas, separates high-light and low-nutrient surface water from low-light and high-nutrient bottom water. Even though in some shelf areas, the tidal energy is sufficient to overcome stratification, Pingree and Griffiths (1978) and Holt and Umlauf (2008) have shown that our study area is situated east of the tidal front that bifurcates Dogger bank. Consequently, the water column above the Dogger sandbank is well mixed throughout the year, whereas the deeper waters that surround the bank become stratified during spring and summer through the course of a tidal cycle.
All data used in this study were collected during two cruises with RV
Hydroacoustic data were collected only during the winter cruise, using a Kongsberg EM710 multibeam echosounder to map active gas emissions (Fig. 2). For the precise localization of individual flares, i.e., bubble streams in an echogram, the water column data were post-processed using the Fledermaus tools FMMidwater, DMagic, and the 3D Editor (©QPS). The origin of individual flares was identified as the point of highest amplitudes near the seafloor. The coordinates of these points were extracted using the FMGeopicker and subsequently plotted on top of the bathymetry using ArcGIS 10.2 (©ESRI).
For visualization of flare deflections and bubble rising heights, selected flares were extracted from the water column data as point data and edited using the 3D Editor of DMagic. The processed flares were plotted over the bathymetry data in a 3D view (Fig. 2).
To identify the size and magnitude of the dissolved methane plume generated
by the bubble discharge, seawater was sampled along a hydrocast transect that
crossed the active gas emission sites (Fig. 2). The transect extends 3 km to
the east and 3 km to the west from the main bubbling location denoted as
cluster 1 in Fig. 2a and c (4
We used a rosette equipped with 12 5 L Niskin bottles mounted on a frame that holds a Sea-Bird SBE 911 plus conductivity, temperature, and depth (CTD) sensors and an SBE 43 oxygen sensor for online monitoring of salinity, temperature, pressure, and dissolved oxygen. The data are archived in PANGAEA (doi:10.1594/PANGAEA.824863 and doi:10.1594/PANGAEA.832334). Twelve different water depths were sampled at each station for quantification of the methane concentration and five water depths for methane oxidation rates. Additional casts were conducted to recover sufficient water for molecular analyses.
For methane concentration analysis, samples were collected in 60 mL
crimp-top glass bottles, flushed with 2 volumes of sample water and filled
completely to eliminate bubbles. Bottles were immediately capped with butyl
rubber stoppers and crimp sealed. After adding 0.2 mL of 10 M NaOH to stop
any microbial activity, a 5 mL headspace of pure N
Methane oxidation (MOx) rates were determined from ex situ incubations of
water samples in 100 mL serum vials. Sample collection and incubation were
performed as described in Mau et al. (2013). Briefly, duplicate samples were
collected and 50
MOx rates were calculated assuming first-order kinetics (Reeburgh et al.,
1991; Valentine et al., 2001):
In addition, control samples were frequently taken and poisoned immediately
after the addition of the tracer. The mean (
The composition of the bacterioplankton assemblages was examined using
denaturing gradient gel electrophoresis (DGGE) based on the 16S rRNA gene as
described in Mau et al. (2013). In short, immediately after sampling, 8 L of
water were filtered and the bacterial cells were concentrated on Nuclepore
filters (0.2
The presence of methane-oxidizing bacteria was checked by searching for genes
encoding the particulate methane monooxygenase (
In addition to the conventional methane analysis, in situ methane
concentrations were quantified with an UWMS during the summer 2013 cruise
(Inspectr200-200, Bell et al., 2007; Gentz et al., 2013; Schlüter and
Gentz, 2008; Short et al., 2001; Wenner et al., 2004). The fast sampling
frequency (
The UWMS was deployed above the central gas seeps (cluster 1, Fig. 2) on
21 July 2013 (16:31–22:32 UTC) at five different water depths: just above the
seafloor, 35, 28, 25, and 10 m. When the system had reached the respective
depth, the research vessel moved slowly along a rectangular track
(
Advection, horizontal and vertical turbulent diffusion, sea–air flux, and microbial oxidation rates were quantified for the upper (0–30 m) and lower water column (30–40 m) during summer stratification (July 2013) and for the entirely mixed water column (0–40 m) in winter (January 2014).
The advective flux (ADV) was calculated by multiplying methane
concentration ([CH
If advective transport were to be uniform then it would simply displace
methane, but differences in current velocity and direction with depth lead to
turbulent mixing, i.e., eddy diffusion (DIF). The strength of small-scale
motions that act to smooth out concentration gradients can be parameterized
by the eddy diffusivity
The sea–air flux (SAF) was calculated as
The oxidative loss (OL) was calculated by depth integration of the MOx
rates:
Echosounder data collected during the winter survey indicate bubble emission in the area of the sampled transect (Fig. 2). The center station was located at a known gas bubble emission site or flare cluster, where several bubble streams occur in close proximity to each other. We observed four additional flare clusters near the western sector of the transect, similar in seepage intensity as those from the central seep denoted as cluster 1 (Fig. 2a and c). In contrast, no additional flares were found in the area of the eastern sector. Although echosounder data point to bubbles rising to, or close to, the sea surface, no bubbles were visually identified at the sea surface due to the rough sea state. Seepage intensity showed no obvious variation related to tidal cycles, i.e., pressure variations due to high or low tides. The seeps were found to be active during all survey crossings. No echosounder data were collected in summer 2013; nonetheless, surfacing gas bubbles were visually documented when the sea was calm.
In summer (July 2013) a seasonal thermocline separated surface (0–30 m)
from bottom waters (30–42 m; Fig. 3). The surface water consisted of a 10 m
thick mixed layer below which the temperature decreased stepwise from 17.5 to
7
Depth profiles of potential temperature, salinity, density (sigma theta), and oxygen for all stations in both summer and winter field programs.
In winter (January 2014) the entire water column was mixed (Fig. 3). The
water had a temperature of 7
Modeled regional current data provided by the BSH indicate a dominant
northwest transport throughout the water column with surface speeds ranging
between 0.06 and 0.27 m s
Consistent with the two-layer structure observed on the hydrographic data,
the methane concentration in summer 2013 also show a two-layer distribution,
with higher values in the bottom water (Fig. 4a, Supplement 4). Methane
concentrations in the surface water range from 4 to 518 nM with a median of
33 nM. Methane concentrations in the bottom water range between 40 and
1628 nM with a median of 391 nM. Highest concentrations in the surface
water were found near cluster 1 (170 nM) and generally decreased towards the
outermost stations (to the west to 96 nM and to the east to 13 nM).
Similarly, in the bottom water the highest methane concentrations were found
at cluster 1 (600–700 nM), and concentrations decreased unevenly towards
the outmost stations (200–300 nM). In both layers the methane
concentrations exceeded the background concentration of
Much lower methane concentrations were found in winter 2014 (Fig. 4b, Supplement 4). Highest values were observed only at one station near cluster 1 with concentrations reaching 657 nM. Such elevated values decreased rapidly horizontally (within 1 km) and were not encountered during repeated hydrocasts at the same location. The median of all methane concentration measurements along the transect was 22 nM, which is only slightly above the regional background concentration. In general, methane concentrations indicate a patchy spatial distribution as expected in an active seep area.
The UWMS was deployed in the vicinity of flare cluster 1 in summer 2013,
covering an area of 125 m by 150 m during instrument tow (Fig. 2c).
Therefore, the hydrocast data (described in Sect. 3.3) cover a much larger
spatial scale (6 km) than sampled during the UWMS tows. When the UWMS was
towed close to bubble streams, it recorded methane concentrations that range
over 3 orders of magnitude from
Methane concentrations recorded by UWMS on 21 July 2013 in the vicinity of flare cluster 1 (Fig. 2c) at different water depths. The detection limit of the instrument is 16 nM, and all measurements below this value are recorded as 0. Apart from temporal and spatial elevations most likely due to bubble streams, the background value is elevated throughout the recording time in 30 and 42 m water depth.
UWMS and hydrocasts were deployed during different tidal phases to check the
persistence of higher methane concentrations in the bottom water as tidal
pressure changes can affect methane seepage (Boles et al., 2001). The UWMS
tows were conducted during ebbing tides, when water levels fell from 0.18 to
Similar to the distribution of methane and co-located oceanographic data, the
MOx rates calculated using Eq. (1) show a two-layer pattern in summer
2013 but are uniform throughout the water column during the winter 2014
survey (Fig. 6a). In summer, MOx rates in surface waters ranged between
0.04 and 9.2 nM day
Methane oxidation rates versus water depth measured with
Molecular samples taken in summer 2013 show also a difference between surface
and deep waters, whereas winter 2014 samples indicate a homogeneous spatial
distribution of microorganisms (Fig. 7, Table 1). In summer 2013, different
DGGE banding patterns reveal changes in microbial communities with depth. The
surface water samples showed two strong bands (Fig. 7, bands 6, 7) that could
be affiliated with the
DGGE profile of 16S rRNA gene fragments of samples from different depth and stations in the central North Sea. Numbers on the lines indicate excised and successfully sequenced DGGE bands, whose phylogenetic assignment is listed in Table 1.
Neither the summer nor the winter bacterial communities exhibited known
methanotrophic bacteria, even though the samples originate from an actively
gas venting area. The absence of methanotrophic bacteria was further
supported by the negative results of the
Classification of partial 16S rRNA gene sequences (Fig. 7) to bacterial taxa performed with the SILVA classifier (Pruesse et al., 2012). The confidence value (0–1) for assignment at the level of class and genus is given in parentheses.
The echosounder and visual observations at the central North Sea sites
document gas emissions that in some cases reach the sea surface. This
fraction of methane that is transported directly to the atmosphere by bubbles
and released upon bursting might be significant, as was shown for example at
the shallow seep field Coal Oil Point in California (
Our highest dissolved methane concentrations, measured in the bottom water during the summer survey, reach magnitudes similar to those observed at other shallow seep sites (Table 2). Our highest value of 1628 nM is comparable to measurements downfield of the Coal Oil Point seep field (up to 1900 nM; Mau et al., 2012), although orders of magnitude less than measurements in the immediate vicinity of the bubble plumes (Clark et al., 2003). Our highest value is higher than methane concentrations reported for seep locations in the Tommeliten, North Sea (268 nM; Schneider von Deimling et al., 2011), and offshore Svalbard, west of Prins Karls Forland (524 nM; Gentz et al., 2013).
Comparison of highest methane concentrations, methane oxidation rates, and sea–air fluxes from different locations.
Even though gas bubbles were observed at the sea surface during the summer survey, the dissolved methane appears trapped beneath the seasonal thermocline (Fig. 4a). This observation is similar to those at the Tommeliten site, where the dissolved methane plume was restricted beneath the seasonal thermocline (Schneider von Deimling et al., 2011) although gas flares were imaged to rise within 10 m of the sea surface. Elevated methane concentrations at other vent sites have also been reported beneath a thermocline or halocline that hamper further ascent of dissolved methane to the mixed layer. The dissolved methane plume originating from the 245 m deep seeps offshore Prins Karls Forland was confined to water depths beneath a local halocline (Gentz et al., 2013). In the Baltic Sea, summer stratification also leads to accumulation of methane below the thermocline (Gülzow et al., 2013). At all these sites, an enhanced release of methane to the atmosphere is thought to occur upon erosion of stratification. In contrast, the dissolved methane plume originating from seeps situated between 5 and 70 m at the Coal Oil Point is dispersed within the mixed layer above the thermocline (Mau et al., 2012), and as such it is not controlled by seasonal stratification patterns.
Trapping and accumulation of dissolved methane beneath a thermocline also is well documented in lakes and freshwater reservoirs, where thermal stratification separates methane-poor surface water from the methane-rich, but anoxic, bottom water in, e.g., a shallow floodplain lake in southeastern Australia (Ford et al., 2002), in a polyhumic lake in southern Finland (Kankaala et al., 2007), in the subtropical Lake Kinneret in Israel (Eckert and Conrad, 2007), and in eight freshwater reservoirs in India (Narvenkar et al., 2013). In these locations, the accumulated methane is released to the atmosphere at the onset of water column mixing in response to enhanced wind forcing and lower temperatures.
Our results show that in a seasonal stratified system, methane accumulation
does not occur in winter, when the water column is well mixed (Fig. 4b).
Methane concentrations were found to deviate only due to bubble ascent and
were otherwise low and constant throughout the water. The median winter
concentration of 22 nM is similar to the background values of 20 nM
reported by Grunwald et al. (2009) for the German Bight but is elevated
relative to water originating from the Atlantic Ocean, which carries
2.5–3.5 nM of methane (Rehder et al., 1998) and to the methane background
concentrations of
The observed difference between summer and winter dissolved methane concentrations also may be due to changes in seepage rate. The visual observation of gas bubbles during the summer, the sub-bottom profiler recording of gas plumes in the water column in August 2002 by Schroot et al. (2005), and our acoustic records of gas flares in the winter (Fig. 2b) indicate that seepage occurred during both seasons. Notwithstanding these observations, we recognize that we have insufficient temporal data coverage and that bubble release frequency, bubble size, and initial methane content could vary between our surveys, causing the difference in overall methane concentrations (Greinert and McGinnis, 2009; Leifer and Clark, 2001; McGinnis et al., 2006). However, even when a change in seepage regimes could affect the overall methane concentration, it would not explain the difference in the shape of the methane profiles observed between summer and winter surveys.
Discrete sampling bias and current variability also explain some fraction of
the difference observed between summer and winter dissolved methane
concentrations. The currents had a strong westward component during summer
sampling with small north/south deviation throughout the water column
(Supplement 2), and thus the easternmost profiles are likely to be less
influenced from direct bubble seepage (Fig. 4a). However, the profiles still
show elevated methane concentration in the bottom water and lower
concentrations in the shallow samples, consistent with methane trapping below
the seasonal thermocline. We considered whether the low observed
concentrations during winter may be due to the fact that during this survey
we only partially sampled isolated plumes. Although the east–west transect
directly crosses the cluster 1 flares (Fig. 2) and was oriented in direction of
the tidal movement in that area, the stronger northward component of the
current in winter (Supplements 2 and 3) displaced methane plumes more rapidly
than in summer. The elevated methane concentrations at the central seep site
and along the western transect (although with much lower methane
concentrations) suggest that we indeed sampled methane plumes (Fig. 4b). We
note that the horizontal concentration gradients in surface water were 0.01 to
0.02 nM m
To summarize, even when methane concentrations may appear biased by discrete sampling, current differences, and seepage rate, our data analyses suggest that the seasonal differences are real. Even if the total magnitudes may be questioned, we are confident that the methane distribution pattern is the result of seasonal stratification.
Measured MOx rates at our study site (Fig. 6a) lie at the upper end of
MOx rates previously reported at sites elsewhere, which span over 6
orders of magnitude from 0.001 to 1000 nM day
In spite of the reported high MOx values, our data reveal an overall low
activity of methane oxidizing microorganisms based on the values obtained for
the rate constant
We note that seven data points collected in summer near flare cluster 1 (stations
12 and 13) had
The general low activity of methane oxidizing microorganisms is further
supported by molecular analysis of filtered matter from seawater.
Consistently, DGGE and
Even though during summer stratification methane is trapped beneath the
seasonal thermocline, the resulting higher methane concentrations do not
appear to enhance the activity of methane oxidation microbes. The residence
time of central North Sea water is about 1.5–2 years (Prandle, 1984; Ursin
and Andersen, 1978) and thermal stratification prevails for 4 months, which
may provide sufficient time to establish a methanotrophic community. However,
microbial turnover times in bottom water samples are consistently low and we
were not able to identify methanotrophic organisms in the water column.
Doubling times of planktonic marine methanotrophs are not known to the
authors, but if we assume a doubling time of
In summary, even though total MOx rates are necessary to constrain overall
methane budgets and carbon cycles, to better characterize microbial activity
among different ecosystems it is necessary to also report data on the
microbial turnover rates at each site. The low turnover rates measured here
are consistent with molecular analyses that failed to identify methanotrophic
bacteria or
When methane enters the water column, either directly from the seep or by dissolution/gas exchange from ascending bubbles, it is transported by ocean currents and spreads by horizontal and vertical eddy diffusion. Methane oxidizing microorganisms can consume dissolved methane in the water column, and methane will be transferred into the atmosphere if its concentration in the mixed layer is higher than saturation.
As a first-order evaluation of the relative importance of these transport and
loss processes, we estimated the advective transport, the horizontal and
vertical eddy diffusion, sea–air flux, and integrated the MOx rates (see
methods and Mau et al., 2012). Summer fluxes for the bottom (30–43 m) and
surface waters (0–30 m) were estimated using data collected in July 2013,
and winter fluxes were derived for the entire unstratified water column
(0–42 m) using data from January 2014. All fluxes were estimated in units
of nmol m
The results shown in Fig. 8 revealed that in both summer and winter seasons, horizontal advection and eddy diffusion are the dominant processes transporting and diluting the emitted methane. The loss processes, i.e., sea–air flux and microbial oxidation, are more than 4 orders of magnitude lower than physical horizontal transport processes.
Sketch of transport and loss terms estimated for the study area in
nmol m
Vertical mixing due to internal waves resulting from proximity to the
elevation of the Dogger Bank cannot be ruled out. Estimates of
Not surprisingly, the sea–air flux removes more methane from the water column during winter due to increased wind speed and storm sparging (Shakhova et al., 2013). More unexpectedly, our flux estimates revealed that within our study area the amount of methane that is transported in summer via vertical diffusion into the surface water is of similar magnitude to the loss by oxidation in the bottom water, even water stratification leads to enhanced methane concentrations at depth. When lower wind speeds prevail, methane oxidation was estimated to be of similar magnitude as the gas transfer to the atmosphere. However, our estimates do not include potential transport to the atmosphere as bottom water reached topographic highs such as the Dogger Bank or areas with no stratification.
Our findings are similar to those reported by Scranton and McShane (1991) for
the Southern Bight of the North Sea. They found methane oxidation
(0.00023–0.3 nM day
Observations at a shallow gas seep site in the central North Sea document elevated
methane concentrations below the thermocline during summer stratification. In contrast,
regional background methane concentrations were observed throughout the water column in
the winter, when the water column is well mixed. At our study site, physical transport processes always out-compete microbial methane
oxidation. Horizontal advection and diffusion of methane are consistently higher than vertical
transport, even within order of magnitude uncertainties. During periods of high wind speed
(fall and winter), more methane reaches the atmosphere than is oxidized in the water; in summer
the loss to the atmosphere and the oxidation terms are of similar magnitude. We show that MOx rates alone cannot be used to characterize the ecosystem microbial
activity, as these values are scaled to the methane concentration. We instead propose to include
interpretation of Our results demonstrate that trapping of methane below a seasonal thermocline does not necessarily
lead to enhance microbial oxidation. Further research is needed to elucidate why stratification over
a summer season of 4 months does not enhance methanotrophy enough to significantly hamper methane release to the atmosphere upon water column mixing.
S. M. designed the study, measured methane concentrations and methane oxidation rates, calculated the fluxes, and wrote the manuscript. T. G., R. M., and M. S. deployed the UWMS and post-processed the data; J.-H. K., M. R., H. S., and P. W. collected and post-processed hydroacoustic data; M. T. interpreted the methane oxidation rate data and edited the manuscript; E. H. implemented and interpreted molecular analyses.
We are indebted to the captain, crew, and scientific research party of the
research vessel