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Groundwater seepage in Eckernförde Bay (Western Baltic Sea): Effect on methane and salinity distribution of the water column
Continental Shelf Research 18 (1998) 1795 — 1806
Groundwater seepage in Eckernfo¨rde Bay (Western Baltic Sea): Effect on methane and salinity distribution of the water column I. Bussmann*, E. Suess GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany
Abstract The effluent activity from a well-known pockmark structure in Eckernfo¨rde Bay was monitored for methane, salinity, and temperature signals in the water column intermittently over three years between 1991, 1993 and 1994. Groundwater discharge from an aquifer into the brackish waters of the western Baltic, dilutes bottom water salinities to values as low as 2.9. Seasurface height and the amount of precipitation preceding sampling periods by 5 days correlated significantly with the rate of groundwater discharge. Concentrations of methane in bottom water at the pockmark site were strongly influenced by seepage intensity. At two sampling sites (control and pockmark site) distinctly lower methane concentrations were observed towards the sea surface, although the entire water body of Eckernfo¨rde Bay appears to be affected by methane seeping from the sediments. This is supported by high methane concentrations above equilibrium with atmospheric methane throughout most of the year. Maximum concentration above the equilibrium value in surface waters was 2800. Methane flux from surface waters into the atmosphere follows strong seasonal variations, with maximum values in the winter (200—400 lmol m\ d\). The study reveals the important role of coastal oceans in the global methane cycle, as an intense but variable source of methane of largely unknown magnitude. 1998 Elsevier Science Ltd. All rights reserved
1. Introduction The role of the world’s ocean in the global methane budget is considered to be minor (Cicerone and Oremland, 1988). More specific evaluations, however, revealed that shallow marine waters do contribute significantly (8—65 Tg CH /yr) to the * Corresponding author. Present address: Alfred-Wegener Institut fu¨r Polar- und Meeresforschung, Am Handelshafen 12, 27570 Bremerhaven, Germany. E-mail: [email protected] 0278—4343/98/$ — See front matter 1998 Elsevier Science Ltd. All rights reserved PII: S 0 27 8 —4 3 43 ( 9 8) 0 0 05 8 —2
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atmospheric methane pool (Hovland et al., 1993). Rivers entering the North Sea are thought to be a source of methane, at least to the coastal ocean (Scranton and McShane, 1991). For the Baltic Sea, Bange et al. (1994) demonstrated that methane concentration of surface waters of the southern and central basins were above equilibrium with atmospheric methane throughout most of the year. Active seepage of low-salinity groundwater in Eckernfo¨rde Bay has been known since the early works at the Mittelgrund site by Whiticar and Werner (Whiticar, 1978; Whiticar and Werner, 1981). As groundwater percolates up through the methane-rich Holocene sediments filling the pockmarks; it becomes enriched in methane and other dissolved pore water constituents before it reaches the sediment—water interface. The important role of groundwater as a major carrier of nutrients to embayments has previously been recognised (Milham and Howes, 1994). Recent studies estimate the groundwater input into coastal waters to be about 40% of the river water flux (Moore, 1996). These findings require a revision of terrestrial fluxes of nutrients, gases and dissolved organic matter to coastal waters, and hence to the global ocean (Church, 1996). There are numerous examples of methane seeps which cause elevated methane concentrations in the surrounding water column. However, these studies were mainly conducted in deeper waters, and little information is available on the variability of methane discharge with time (Carson et al., 1990; Dando et al., 1994; Lammers et al., 1995). One pockmark in Eckernfo¨rde Bay, being readily accessible, was chosen for monitoring the variability of seepage activity and assessing its influence on the methane budget of the coastal water column in the western Baltic Sea.
2. Study area Eckernfo¨rde Bay is a 17 km long and 3 km wide inlet of Kiel Bay situated in the western Baltic Sea (Fig. 1). Subsurface Tertiary deposits and glacial and postglacial sediments, determine the morphology of the inlet. Erosion during the last glacial period, and refilling during the Littorina transgression have formed a sediment cover (Seibold et al., 1971). Organic-rich, fine-grained mud of post-glacial age covers these sediments in the deeper part of the inlet ('20 m water depth). A sedimentation rate of 4.4$1.3 mm yr\ has been determined for post-glacial sediment of the central basin of Eckernfo¨rde Bay (Nittrouer et al., 1997). The basin is separated into two elongated troughs by the Mittelgrund, a morainic remnant which reaches up to 6 m of water depth. The Mittelgrund lies above an early Tertiary salt dome structure of Schwedeneck-Waabs (Fabian and Roese, 1962). Oil and gas are exploited at the Schwedeneck field associated with this structure (Whiticar and Werner, 1981). Pockmarks are distributed along the northern and southern shores of Eckernfo¨rde Bay and the western edge of the Mittelgrund ridge. Their sizes range from 50 to 500 m in length and from 10 to 30 m in width, with a maximum depth of 1.5 m (Khandriche and Werner, 1994). The groundwater discharge is related to a very large aquifer extending from onshore into the bay. This groundwater reservoir is the large-scale feature
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Fig. 1. The study area in Eckernfo¨rde Bay, western Baltic. Dark shaded areas are the pockmarks. ‘PM’, pockmark site; ‘C’, control site.
‘Untere Braunkohlesande’, the most important drinking water reservoir of this region (Johannsen, 1980; Liebau, 1985).
3. Methods The specific pockmark monitored is located south of the Mittelgrund (54°29.95 N; 10°2.28 E) at a water depth of 25 m (Fig. 1). It is about 1.5 m deeper than the surrounding seafloor. The position of the sampling site was located with the echo sounder system of the R.V. ‘Littorina’ (IfM Kiel) and marked with a buoy for repeated sampling. A control site (54°30.13 N; 10°2.37 E), about 350 m away from the immediate pockmark region was selected for reference (Fig. 1). During the years 1991, 1993 and 1994, the pockmark and the control site were sampled intermittently for salinity, water temperature and methane concentration. Sampling was done twice in 1991 and almost monthly in 1993 until the beginning of 1994; altogether 11 sampling campaigns were carried out.
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Temperature and salinity of the water column were determined with standard sensors prior to sampling. Difference of salinity (*S) was defined as the difference between the minimum value at the base of the pockmark and the mean salinity of the adjacent water column (20—25 m). Water samples were obtained with Niskin bottles. Bottom water and surface water samples were taken at 1 m above the seafloor and at the surface (0.5 m). Samples were degassed and analysed for dissolved methane with a combined vacuum-ultrasonication method described by Lammers and Suess (1994). This VU method has an efficiency of 62$3.8% and a reproducibility of 3.15%. Data on the meteorological conditions (air pressure, precipitation, seasurface height, wind velocity and wind direction) for the period sampled were kindly provided by the Department of Meteorology, Institut fu¨r Meereskunde, Kiel. In this data set, rainfall was recorded twice a day (7:00 and 19:00). All other data were selected accordingly. The location of the recorders were on the roof or at the pier of the institute. For further processing, the data were smoothed with a 10-point running average. Statistical analysis of the seepage intensity and meteorological data included general linear model (GLM) procedure. Multiple linear regression based on rank transformed data was performed for correlations between bottom water methane concentration and seepage intensity. Expected equilibrium concentrations of methane in seawater were calculated according to the equation of Wiesenburg and Guinasso (1979), by applying an average atmospheric mixing ratio of 1.89 ppmv for the Baltic Sea (Bange et al., 1994). Fluxes (F) of methane across the air—sea interface were computed by the following equation (F"k *c), where *c is the difference between the measured methane concentration and the calculated equilibrium concentration. The transfer coefficient k was corrected according to the Schmidt number (Sc) for methane (k"0.31 v(Sc/660)\ ) (Wanninkhof, 1992) at the actual salinity and temperature of seawater. Mean wind velocities (v) of the sampling dates were obtained at 14:30 (climate date II) in Kiel-Holtenau, at 10 m above ground (German Weather Service).
4. Results Pronounced and highly variable anomalies in salinity of the bottom water and in the water column above the pockmark were detected (Fig. 2a). The differences between the salinity of the bottom water and the water column were strongest in April 1991. Compared to the salinity of the adjacent water column (18.3), groundwater input diluted ambient salinity down to 2.9. Thus, the difference of salinity was *S"15.4. In March, July and December 1993, the differences were about *S+6. In June 1991, February, April, May 1993 and January 1994 no salinity anomaly was detected. In May 1994, again groundwater was discharged from the floor of the pockmark, judging from the salinity distribution. Methane concentrations at the same site (bottom water of the pockmark) also showed strong fluctuations (Fig. 2b). At the begining of 1993 methane concentrations were low (11.23 nmol l\ in February); increasing towards the end of April (218.48 nmol l\). Maximal values were reached in December 1993 with 441.38 nmol
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Fig. 2. (A) Difference in salinity between the base of the pockmark and the adjacent water column at the pockmark site, a positive sign indicates groundwater seepage. (B) Methane concentration of the bottom water. n.d."not determined.
l\. At the begining of 1994 methane concentrations were again low 73.47—164.78 nmol l\. No correlation to the salinity anomalies was obvious (Fig. 2b). At all sampling times, surface waters at the pockmark site showed considerably higher concentrations of methane than the control site (Fig. 3a). However, maximum values for both sites were found in winter/spring 1993/94 and comparable low values during the rest of the year. Nevertheless, methane concentrations of surface waters were above equilibrium with atmospheric methane during most of the sampling period (Table 1). The calculated saturation concentration ranged from 2.82 to 3.85 nmol l\ for both stations, measured concentrations, however, were much higher. Thus, methane concentrations above equilibrium with atmospheric methane ranged from 115 and 62% in May 1994 to about 2800 and 2400% in January 1994 for the pockmark and the control site, respectively (Table 1). The concentrations of methane in the bottom water at the pockmark site showed very strong fluctuations with maximum values of 441 nmol l\ in December 1993 and minimum values with 11 nmol l\ in February 1993 (Fig. 3b). No seasonal pattern
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Fig. 3. Methane concentrations of surface water (A) and bottom water (B) during the years 1993 and 1994, », pockmark site, 䊐 control site. The c line indicates the mean equilibrium concentration of methane (3.47 nmol l\) in seawater at the actual salinities and water temperature (calculations see text).
was evident. At the control site the bottom water concentrations of methane ranged between 8.3 and 88.5 nmol l\ in February 1993 and January 1994, respectively (Fig. 3b). Maximum values were found in early summer and again at the end of the winter/spring season. Concentrations at the pockmark site were always higher than at the control site; in April and July 1993 they were even 18—10-fold higher.
5. Discussion A pockmark in Eckernfo¨rde Bay, western Baltic was monitored for freshwater seepage and methane concentrations in the water column in the years 1991—1994. Seepage activity was defined by the difference in salinity at the base of the pockmark and the adjacent water column (20—25 m). Thus, at 6 out of 11 sampling campaigns
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Table 1 Methane concentrations of surface water at the pockmark and control sites. Equilibrium concentrations according to Wiesenburg and Guinasso (1979) were set as 100%. Flux (F) of methane across the air—sea interface were calculated according to Wanninkhof (1992) (F"k *c and k"0.31 v(Sc/660)\ ) Pockmark
Control
Date
Meas. conc. % conc. (nmol l\) above atmosp. equilibrium
Flux Meas. conc. % conc. Flux (lmol m\ d\) (nmol l\) above (lmol m\ d\) atmosp. equilibrium
04-Feb-93 30-Mar-93 22-Apr-93 07-Jul-93 29-Jul-93 06-Dec-93 25-Jan-94 04-May-94
10.79 7.22 7.35 7.89 14.65 99.77 105.50 3.87
14.73 0.95 3.75 49.04 15.24 208.96 412.62 0.17
289 188 203 272 519 2658 2815 115
8.30 6.35 6.72 5.60 7.26 n.d.* 88.4 2.08
222 165 192 194 258
9.53 0.71 3.23 26.66 5.73
2360 62
343.29 !0.42
* n.d."not determined.
seepage activity could be detected. The lighter groundwater at the base of the pockmark seemed not to influence the stability of the water column. Only the first few meters above ground showed sometimes strong salinity fluctuations (data not shown). In May 1994 it was possible to verify this seepage activity with an in situ flow measurement device. Flow rate of the freshwater at the pockmark site was measured with a benthic chamber (Linke et al., 1994). Flow rates varied on 3 consecutive days between 170 and 472 l m\ d\ (P. Linke, Geomar, unpubl. data). Submarine groundwater fluxes at the North and South Carolina Coast are reported to range between 5 and 10 l m\ d\ (Simons, 1992). Whereas flow rates at the Aleutian subduction zone lay more within our data, 240$200 l m\ d\ (Suess et al., 1997). Using statistical methods we here attempted to elucidate which of the meteorological parameters (air pressure, precipitation, seasurface height and wind) might influence the seepage activity (Table 2). Stepwise multiple regression showed that only the variables ‘seasurface height’ and ‘precipitation 5 days ago’ correlated significantly with the intensity of seepage *S () (Table 2). Together these two parameters explain 65% of the observed variance of *S. Milham and Howes (1994) showed that temporal scales of groundwater discharge are controlled by short-term modulations of tides and by seasonal cycles of recharge. Although the tidal influence upon fluctuations of seasurface height in the western Baltic is minor, levels may change considerably in the course of strong westerly winds. An analogous relationship between the occurrence of groundwater seepage and the amount of precipitation has been shown for a Spanish lake (Casamitjana and Roget, 1993). Several studies address the high methane content of the organic-rich sediments in the central parts of Eckernfo¨rde Bay and there is evidence for a pronounced bacterial
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I. Bussmann, E. Suess/Continental Shelf Research 18 (1998) 1795—1806 Table 2 Partial multiple linear regression between difference in salinity (dependent variable) and meteorological data (independent variables). P-levels (0.15 were regarded as significant as recommended by SAS (1988) Dependent variable
p'F
Partial R
Model R
Seasurface height Actual precipitation Precipitation 5 days ago Precipitation 10 days ago Precipitation 20 days ago Wind velocity Wind direction Air pressure
0.0379 n.s. 0.1362 n.s. n.s. n.s. n.s. n.s.
0.4824
0.4824
0.1709
0.6534
-
n.s."not significant.
methane production in these organic-rich sediments (Whiticar, 1982; Martens and Albert, 1994). In this study the working hypothesis was: groundwater seeping through the methane-rich sediment will increase methane concentrations in the overlying water column by flushing out the dissolved constituents. To separate the controlling factors on methane discharge, the data set (methane concentration, salinity anomaly; n"9) of the pockmark station was divided into two categories: (a) with groundwater seepage and (b) without seepage. Multiple linear regression analysis (GLM) showed a strong correlation (r"0.77) between the methane concentration of the bottom water and the intensity of the seepage — in category (a) when seepage is active and the salinity anomaly *S'0. Under non-seepage conditions (category b), flux of methane in the bottom water is dependent on the balance of methane production and methane oxidation by the natural bacterial populations. These populations, as well as the methane flux itself, in turn are influenced by strong seasonal variations of temperature (Wilson et al., 1989; King and Adamsen, 1992). With respect to the formation of methane, methanogenic bacteria compete with sulfate-reducing bacteria for their common electron donor, acetate. When sulfate-reducing bacteria are inhibited by SO depletion, methane production on the other side is increasing (Kuivila et al., 1990; Hoehler et al., 1994). The concentration of oxygen is crucial for methane oxidation (Casper, 1992; Roslev and King, 1996). In late summer, when the near-bottom water column becomes anoxic, a considerable liberation of methane into the water column was observed in nearby Kiel Harbour (Schmaljohann, 1996). Concentration of methane in surface waters at both sites showed the same seasonal variation, even though bottom water concentrations were independent of each other (Fig. 3). This may be explained by a methane plume extending from the floor of the pockmark to the surface directly above, including the surface water of the control site. In hindsight the location of the control site, being only 350 m away from the pockmark, turns out to be poor choice. But, on the other hand it permitted to show methane variability on a short spatial scale.
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The different methane concentrations of the bottom waters (119.91 and 28.93 nmol l\ median values) are reduced to similar surface values of 9.33 and 6.53 nmol l\ (median values) for the pockmark and the control site, respectively (Fig. 4). There are several explanations for this pronounced reduction of methane in a water column of only 25 m. For deeper waters it has been shown that due to microbial oxidation, a methane plume easily diminishes within a few kilometres (Angelis et al., 1993). Higher oxygen concentrations in the water compared to the sediment may lead to higher methane oxidation rates (Bussmann, 1994). The resuspension of surface sediments by currents or wave action is known to enhance heterotrophic processes (Wainright, 1987); this is also valid for methane oxidation activity (unpubl. data). Even though a notable proportion of the methane disappears in the water column presumably by oxidation and dilution, surface waters reveal methane concentration far above the equilibrium with atmospheric methane most of the time. Therefore, methane escapes continuously from the surface water into the atmosphere. During most of the year this sea—air flux ranged from minimum values around zero to about 50 lmol m\ d\. In the winter, the flux peaked at higher values of 200—400 lmol m\ d\ (Table 2). But for the stormy season in autumn and winter the flux may be underestimated as the influence of breaking waves (whitecaps) on the gas transfer is not included. Whitecaps can increase the gas flux by generating turbulence and creating bubble plumes (Farmer et al., 1993; Asher et al., 1996). Bange et al. found their data of methane fluxes in the central Baltic Sea to be grouped into low values in February (9.5—14.51 lmol m\ d\) and high values in July (101—1200 lmol m\ d\). This distribution is exactly opposite to the results of this seasonal study. The exact reasons for these strong seasonal variations of methane fluxes remains unknown. We strongly suspect that the summer maxima in methane flux reflects the increase methane formation in the sediment at higher temperatures, and that the winter maxima reported here, reflect the seepage activity.
Fig. 4. Box and Whisker plot of methane concentrations at all sampling dates for bottom and surface waters at both stations.
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6. Conclusion Our study fully supports the statements of Hovland and Bange that coastal oceans represent a source of methane (Hovland et al., 1993; Bange et al., 1994). It also becomes abundantly evident that in shelf areas, seasonal influences lead to variations in methane flux over several orders of magnitude. This seasonality is overlaid by fluctuations of seepage activity. Seepage activity increases methane concentrations in the bottom and surface waters dramatically. Hence, for calculations of the cumulative methane flux from the coastal ocean the strong fluctuations of methane seeps, as well as seasonal variations have to be taken into account. A regional study in order to estimate the relative proportion of methane release from seep sites and undisturbed sediments is badly needed. Acknowledgements We would like to thank the Dept. of Meteorology, (Institut fu¨r Meereskunde, Kiel) for providing the meteorological data, S. Lammers for methane analysis, P. Fischer for assistance in statistical treatment of the data. This study was supported by the EU (MAS2-CT92-0040) and the Deutsche Forschungsgesellschaft (SFB 313, Kiel). References Angelis, M.A.d., Liley, M.D., Olson, E.J., Baross, J.A., 1993. Methane oxidation in deep-sea hydrothermal plumes of the Endeavour Segment of the Juan da Fuca Ridge. Deep Sea Research 40 (6), 1169—1186. Asher, W.E., Karle, L.M., Higgins, B.J., Farley, P.J., 1996. The influence of bubble plumes on air-seawater gas transfer velocities. Journal of Geophysical Research 101 (C5), 12027—12041. Bange, H.W., Bartell, U.H., Rapsomanikis, S., Andrae, M.O., 1994. Methane in the Baltic and North Seas and a reassessment of the marine emissions of methane. Global Biogeochemical Cycles 8 (4), 465—480. Bussmann, I., 1994. Verteilung und Steuergro¨{en der Aktivita¨t Methan-oxidierender Bakterien in Randmeeren des Nordatlantiks. Ph.D. Thesis, University of Kiel. Carson, B., Suess, E., Strasser, J.C., 1990. Fluid flow and mass flux determinations at vent sites on the Cascadia Margin accretionary prism. Journal of Geophysical Research 95 (B6), 8891—8897. Casamitjana, X., Roget, E., 1993. Resuspension of sediment by focused groundwater in Lake Banyoles. Limnology and Oceanography 38 (3), 643—656. Casper, P., 1992. Methane production in lakes of different trophic state. Archiv fu( r Hydrobiologie, Ergebnisse der Limnologie 37, 149—154. Church, T.M., 1996. An underground route for the water cycle. Nature 380, 579—580. Cicerone, R.J., Oremland, R.S., 1988. Biogeochemical aspects of atmospheric methane. Global Biogeochemical Cycles 2, 299—327. Dando, P.R., O’Hara, S.C.M., Schuster, U., Taylor, L.J., Clayton, C.J., Baylis, S., Laier, T., 1994. Gas seepage from a carbonate-cemented sandstone reef on the Kattegat coast of Denmark. Marine and Petroleum Geology 11 (2), 182—188. Fabian, H.-S., Roese, K.-L., 1962. Das Erdo¨lfeld Schwedeneck. Erdo¨l-Zeitschrift 78, 283—294. Farmer, D.M., McNeil, C.L., Johnson, B.D., 1993. Evidence for the importance of bubbles in increasing air-sea gas flux. Nature 361, 620—623. Hoehler, T.M., Alperin, M.J., Albert, D.B., Martens, C.S., 1994. Field and laboratory studies of methane oxidation in an anoxic marine sediment: evidence for a methanogen-sulfate reducer consortium. Global Biogeochemical Cycles 8 (4), 451—463.
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Hovland, M., Judd, A.G., Burke, R.A., 1993. The global flux of methane from shallow submarine sediments. Chemosphere 26 (1—4), 559—578. Johannsen, A., 1980. Hydrogeologie von Schleswig-Holstein. Geologisches Jahrbuch, Reihe C, vol. 28, Bundesanstalt fu¨r Geowissenschaften und Rohstoffe, Hannover. Khandriche, A., Werner, F., 1994. Freshwater induced pockmarks in Bay of Eckernfo¨rde, Western Baltic. Proceedings of 3rd Marine Geological Conference of the Baltic, Sopot. King, G.M., Adamsen, A.P.S., 1992. Effects of temperature on methane consumption in a forest soil and in pure cultures of the methanotroph Methylomonas rubra. Applied and Environmental Microbiology 8 (9), 2758—2763. Kuivila, K.M., Murray, J.W., Devol, A.H., 1990. Methane production in the sulfate-depleted sediments of two marine basins. Geochemica et Cosmochemica Acta 54, 403—411. Lammers, S., Suess, E., 1994. An improved head-space analysis method for methane in seawater. Marine Chemistry 47 (2), 115—125. Lammers, S., Suess, E., Hovland, M., 1995. A large methane plume east of Bear Island (Barents Sea): Implications for the marine methane cycle. Geologische Rundschau 84 (1), 59—66. Liebau, C., 1985. Hydrogeologische Untersuchungen eines tiefen Grundwasserleiters im Gebiet von Kiel. Report from Geological-Paleontological Institute and Museum, vol. 10, University of Kiel. Linke, P., Suess, E., Torres, M., Martens, V., Rugh, W.D., Ziebis, W., Kulm, L.D., 1994. In situ measurement of fluid flow from cold seeps at active continental margins. Deep Sea Research 41, 721—739. Martens, C., Albert, D., 1994. Biogeochemical processes controlling gas production, consumption and transport in organic-rich marine sediments. In: Wever, T.E. (ed.) Proceedings of the ‘‘Gassy Mud Workshop’’, Kiel, pp. 101—107. Milham, N.P., Howes, B.L., 1994. Nutrient balance of a shallow coastal embayment: I. Patterns of groundwater discharge. Marine Ecology Progress Series 112, 155—167. Moore, W.S., 1996. Large groundwater inputs to coastal water revealed by Ra enrichments. Nature 380, 612—617. Nittrouer, C.A., Lopez, G.R., Wright, L.D., Bentley, S.J., D’Andrea, A.F., Friedrichs, C.T., Craig, N.I., Sommerfield, C.K., 1998. Oceanographic processes and the preservation of sedimentary structure in Eckernfo¨rde Bay, Baltic Sea. Continental Shelf Research 18, 1689—1714. Roslev, P., King, G.M., 1996. Regulation of methane oxidation in a freshwater wetland by water changes and anoxia. FEMS Microbiology Ecology 19, 105—115. Schmaljohann, R., 1996. Methane dynamics in the sediments and water column of Kiel Harbour (Baltic Sea). Marine Ecology Progress Series 131, 263—273. Scranton, M. I., McShane, K., 1991. Methane fluxes in the southern North Sea: the role of European rivers. Continental Shelf Research 11 (1), 37—52. Seibold, E., Exon, N., Hartmann, M., Ko¨gler, F.C., Krumm, H., Lutze, G.F., Newton, R.S., Werner, F., 1971. Marine geology of Kiel Bay. In Sedimentology of Parts of Central Europe. Verlag W. Kramer, Frankfurt a.M., pp. 209—235. Simmons, G.M., 1992. Importance of submarine groundwater discharge (SGWD) and seawater cycling to material flux across sediment/water interfaces in marine environments. Marine Ecology Progress Series 84, 173—184. Suess, E., Bohrmann, G., Huene, R.V., Linke, P., Wallmann, K., Lammers, S., Sahlig, H., Winckler, G., 1998. Fluid venting in the Aleutian subduction zone. Journal of Geophysical Research 103 (B2), 2597—2614. Wainright, S. C., 1987. Stimulation of heterotrophic microplankton production by resuspended marine sediments. Science 238, 1710—1712. Wanninkhof, R., 1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research 97 (C5), 7373—7382. Whiticar, M.J., 1982. The presence of methane bubbles in the acoustically turbid sediments of Eckernfo¨rde Bay, Baltic Sea. In: Manheim, F.T., Fanning, K.A. (Eds.), Dynamic Environment of the Ocean Floor. Lexington Books, MA, pp. 219—235. Whiticar, M.J., 1978. Relationships of interstitial gases and fluids during early diagenesis in some marine sediments. Reports Sonderforschungsbereich 95, vol. 35, University Kiel.
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Groundwater seepage in Eckernfo¨rde Bay (Western Baltic Sea): Effect on methane and salinity distribution of the water column I. Bussmann*, E. Suess GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany
Abstract The effluent activity from a well-known pockmark structure in Eckernfo¨rde Bay was monitored for methane, salinity, and temperature signals in the water column intermittently over three years between 1991, 1993 and 1994. Groundwater discharge from an aquifer into the brackish waters of the western Baltic, dilutes bottom water salinities to values as low as 2.9. Seasurface height and the amount of precipitation preceding sampling periods by 5 days correlated significantly with the rate of groundwater discharge. Concentrations of methane in bottom water at the pockmark site were strongly influenced by seepage intensity. At two sampling sites (control and pockmark site) distinctly lower methane concentrations were observed towards the sea surface, although the entire water body of Eckernfo¨rde Bay appears to be affected by methane seeping from the sediments. This is supported by high methane concentrations above equilibrium with atmospheric methane throughout most of the year. Maximum concentration above the equilibrium value in surface waters was 2800. Methane flux from surface waters into the atmosphere follows strong seasonal variations, with maximum values in the winter (200—400 lmol m\ d\). The study reveals the important role of coastal oceans in the global methane cycle, as an intense but variable source of methane of largely unknown magnitude. 1998 Elsevier Science Ltd. All rights reserved
1. Introduction The role of the world’s ocean in the global methane budget is considered to be minor (Cicerone and Oremland, 1988). More specific evaluations, however, revealed that shallow marine waters do contribute significantly (8—65 Tg CH /yr) to the * Corresponding author. Present address: Alfred-Wegener Institut fu¨r Polar- und Meeresforschung, Am Handelshafen 12, 27570 Bremerhaven, Germany. E-mail: [email protected] 0278—4343/98/$ — See front matter 1998 Elsevier Science Ltd. All rights reserved PII: S 0 27 8 —4 3 43 ( 9 8) 0 0 05 8 —2
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atmospheric methane pool (Hovland et al., 1993). Rivers entering the North Sea are thought to be a source of methane, at least to the coastal ocean (Scranton and McShane, 1991). For the Baltic Sea, Bange et al. (1994) demonstrated that methane concentration of surface waters of the southern and central basins were above equilibrium with atmospheric methane throughout most of the year. Active seepage of low-salinity groundwater in Eckernfo¨rde Bay has been known since the early works at the Mittelgrund site by Whiticar and Werner (Whiticar, 1978; Whiticar and Werner, 1981). As groundwater percolates up through the methane-rich Holocene sediments filling the pockmarks; it becomes enriched in methane and other dissolved pore water constituents before it reaches the sediment—water interface. The important role of groundwater as a major carrier of nutrients to embayments has previously been recognised (Milham and Howes, 1994). Recent studies estimate the groundwater input into coastal waters to be about 40% of the river water flux (Moore, 1996). These findings require a revision of terrestrial fluxes of nutrients, gases and dissolved organic matter to coastal waters, and hence to the global ocean (Church, 1996). There are numerous examples of methane seeps which cause elevated methane concentrations in the surrounding water column. However, these studies were mainly conducted in deeper waters, and little information is available on the variability of methane discharge with time (Carson et al., 1990; Dando et al., 1994; Lammers et al., 1995). One pockmark in Eckernfo¨rde Bay, being readily accessible, was chosen for monitoring the variability of seepage activity and assessing its influence on the methane budget of the coastal water column in the western Baltic Sea.
2. Study area Eckernfo¨rde Bay is a 17 km long and 3 km wide inlet of Kiel Bay situated in the western Baltic Sea (Fig. 1). Subsurface Tertiary deposits and glacial and postglacial sediments, determine the morphology of the inlet. Erosion during the last glacial period, and refilling during the Littorina transgression have formed a sediment cover (Seibold et al., 1971). Organic-rich, fine-grained mud of post-glacial age covers these sediments in the deeper part of the inlet ('20 m water depth). A sedimentation rate of 4.4$1.3 mm yr\ has been determined for post-glacial sediment of the central basin of Eckernfo¨rde Bay (Nittrouer et al., 1997). The basin is separated into two elongated troughs by the Mittelgrund, a morainic remnant which reaches up to 6 m of water depth. The Mittelgrund lies above an early Tertiary salt dome structure of Schwedeneck-Waabs (Fabian and Roese, 1962). Oil and gas are exploited at the Schwedeneck field associated with this structure (Whiticar and Werner, 1981). Pockmarks are distributed along the northern and southern shores of Eckernfo¨rde Bay and the western edge of the Mittelgrund ridge. Their sizes range from 50 to 500 m in length and from 10 to 30 m in width, with a maximum depth of 1.5 m (Khandriche and Werner, 1994). The groundwater discharge is related to a very large aquifer extending from onshore into the bay. This groundwater reservoir is the large-scale feature
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Fig. 1. The study area in Eckernfo¨rde Bay, western Baltic. Dark shaded areas are the pockmarks. ‘PM’, pockmark site; ‘C’, control site.
‘Untere Braunkohlesande’, the most important drinking water reservoir of this region (Johannsen, 1980; Liebau, 1985).
3. Methods The specific pockmark monitored is located south of the Mittelgrund (54°29.95 N; 10°2.28 E) at a water depth of 25 m (Fig. 1). It is about 1.5 m deeper than the surrounding seafloor. The position of the sampling site was located with the echo sounder system of the R.V. ‘Littorina’ (IfM Kiel) and marked with a buoy for repeated sampling. A control site (54°30.13 N; 10°2.37 E), about 350 m away from the immediate pockmark region was selected for reference (Fig. 1). During the years 1991, 1993 and 1994, the pockmark and the control site were sampled intermittently for salinity, water temperature and methane concentration. Sampling was done twice in 1991 and almost monthly in 1993 until the beginning of 1994; altogether 11 sampling campaigns were carried out.
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Temperature and salinity of the water column were determined with standard sensors prior to sampling. Difference of salinity (*S) was defined as the difference between the minimum value at the base of the pockmark and the mean salinity of the adjacent water column (20—25 m). Water samples were obtained with Niskin bottles. Bottom water and surface water samples were taken at 1 m above the seafloor and at the surface (0.5 m). Samples were degassed and analysed for dissolved methane with a combined vacuum-ultrasonication method described by Lammers and Suess (1994). This VU method has an efficiency of 62$3.8% and a reproducibility of 3.15%. Data on the meteorological conditions (air pressure, precipitation, seasurface height, wind velocity and wind direction) for the period sampled were kindly provided by the Department of Meteorology, Institut fu¨r Meereskunde, Kiel. In this data set, rainfall was recorded twice a day (7:00 and 19:00). All other data were selected accordingly. The location of the recorders were on the roof or at the pier of the institute. For further processing, the data were smoothed with a 10-point running average. Statistical analysis of the seepage intensity and meteorological data included general linear model (GLM) procedure. Multiple linear regression based on rank transformed data was performed for correlations between bottom water methane concentration and seepage intensity. Expected equilibrium concentrations of methane in seawater were calculated according to the equation of Wiesenburg and Guinasso (1979), by applying an average atmospheric mixing ratio of 1.89 ppmv for the Baltic Sea (Bange et al., 1994). Fluxes (F) of methane across the air—sea interface were computed by the following equation (F"k *c), where *c is the difference between the measured methane concentration and the calculated equilibrium concentration. The transfer coefficient k was corrected according to the Schmidt number (Sc) for methane (k"0.31 v(Sc/660)\ ) (Wanninkhof, 1992) at the actual salinity and temperature of seawater. Mean wind velocities (v) of the sampling dates were obtained at 14:30 (climate date II) in Kiel-Holtenau, at 10 m above ground (German Weather Service).
4. Results Pronounced and highly variable anomalies in salinity of the bottom water and in the water column above the pockmark were detected (Fig. 2a). The differences between the salinity of the bottom water and the water column were strongest in April 1991. Compared to the salinity of the adjacent water column (18.3), groundwater input diluted ambient salinity down to 2.9. Thus, the difference of salinity was *S"15.4. In March, July and December 1993, the differences were about *S+6. In June 1991, February, April, May 1993 and January 1994 no salinity anomaly was detected. In May 1994, again groundwater was discharged from the floor of the pockmark, judging from the salinity distribution. Methane concentrations at the same site (bottom water of the pockmark) also showed strong fluctuations (Fig. 2b). At the begining of 1993 methane concentrations were low (11.23 nmol l\ in February); increasing towards the end of April (218.48 nmol l\). Maximal values were reached in December 1993 with 441.38 nmol
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Fig. 2. (A) Difference in salinity between the base of the pockmark and the adjacent water column at the pockmark site, a positive sign indicates groundwater seepage. (B) Methane concentration of the bottom water. n.d."not determined.
l\. At the begining of 1994 methane concentrations were again low 73.47—164.78 nmol l\. No correlation to the salinity anomalies was obvious (Fig. 2b). At all sampling times, surface waters at the pockmark site showed considerably higher concentrations of methane than the control site (Fig. 3a). However, maximum values for both sites were found in winter/spring 1993/94 and comparable low values during the rest of the year. Nevertheless, methane concentrations of surface waters were above equilibrium with atmospheric methane during most of the sampling period (Table 1). The calculated saturation concentration ranged from 2.82 to 3.85 nmol l\ for both stations, measured concentrations, however, were much higher. Thus, methane concentrations above equilibrium with atmospheric methane ranged from 115 and 62% in May 1994 to about 2800 and 2400% in January 1994 for the pockmark and the control site, respectively (Table 1). The concentrations of methane in the bottom water at the pockmark site showed very strong fluctuations with maximum values of 441 nmol l\ in December 1993 and minimum values with 11 nmol l\ in February 1993 (Fig. 3b). No seasonal pattern
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Fig. 3. Methane concentrations of surface water (A) and bottom water (B) during the years 1993 and 1994, », pockmark site, 䊐 control site. The c line indicates the mean equilibrium concentration of methane (3.47 nmol l\) in seawater at the actual salinities and water temperature (calculations see text).
was evident. At the control site the bottom water concentrations of methane ranged between 8.3 and 88.5 nmol l\ in February 1993 and January 1994, respectively (Fig. 3b). Maximum values were found in early summer and again at the end of the winter/spring season. Concentrations at the pockmark site were always higher than at the control site; in April and July 1993 they were even 18—10-fold higher.
5. Discussion A pockmark in Eckernfo¨rde Bay, western Baltic was monitored for freshwater seepage and methane concentrations in the water column in the years 1991—1994. Seepage activity was defined by the difference in salinity at the base of the pockmark and the adjacent water column (20—25 m). Thus, at 6 out of 11 sampling campaigns
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Table 1 Methane concentrations of surface water at the pockmark and control sites. Equilibrium concentrations according to Wiesenburg and Guinasso (1979) were set as 100%. Flux (F) of methane across the air—sea interface were calculated according to Wanninkhof (1992) (F"k *c and k"0.31 v(Sc/660)\ ) Pockmark
Control
Date
Meas. conc. % conc. (nmol l\) above atmosp. equilibrium
Flux Meas. conc. % conc. Flux (lmol m\ d\) (nmol l\) above (lmol m\ d\) atmosp. equilibrium
04-Feb-93 30-Mar-93 22-Apr-93 07-Jul-93 29-Jul-93 06-Dec-93 25-Jan-94 04-May-94
10.79 7.22 7.35 7.89 14.65 99.77 105.50 3.87
14.73 0.95 3.75 49.04 15.24 208.96 412.62 0.17
289 188 203 272 519 2658 2815 115
8.30 6.35 6.72 5.60 7.26 n.d.* 88.4 2.08
222 165 192 194 258
9.53 0.71 3.23 26.66 5.73
2360 62
343.29 !0.42
* n.d."not determined.
seepage activity could be detected. The lighter groundwater at the base of the pockmark seemed not to influence the stability of the water column. Only the first few meters above ground showed sometimes strong salinity fluctuations (data not shown). In May 1994 it was possible to verify this seepage activity with an in situ flow measurement device. Flow rate of the freshwater at the pockmark site was measured with a benthic chamber (Linke et al., 1994). Flow rates varied on 3 consecutive days between 170 and 472 l m\ d\ (P. Linke, Geomar, unpubl. data). Submarine groundwater fluxes at the North and South Carolina Coast are reported to range between 5 and 10 l m\ d\ (Simons, 1992). Whereas flow rates at the Aleutian subduction zone lay more within our data, 240$200 l m\ d\ (Suess et al., 1997). Using statistical methods we here attempted to elucidate which of the meteorological parameters (air pressure, precipitation, seasurface height and wind) might influence the seepage activity (Table 2). Stepwise multiple regression showed that only the variables ‘seasurface height’ and ‘precipitation 5 days ago’ correlated significantly with the intensity of seepage *S () (Table 2). Together these two parameters explain 65% of the observed variance of *S. Milham and Howes (1994) showed that temporal scales of groundwater discharge are controlled by short-term modulations of tides and by seasonal cycles of recharge. Although the tidal influence upon fluctuations of seasurface height in the western Baltic is minor, levels may change considerably in the course of strong westerly winds. An analogous relationship between the occurrence of groundwater seepage and the amount of precipitation has been shown for a Spanish lake (Casamitjana and Roget, 1993). Several studies address the high methane content of the organic-rich sediments in the central parts of Eckernfo¨rde Bay and there is evidence for a pronounced bacterial
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I. Bussmann, E. Suess/Continental Shelf Research 18 (1998) 1795—1806 Table 2 Partial multiple linear regression between difference in salinity (dependent variable) and meteorological data (independent variables). P-levels (0.15 were regarded as significant as recommended by SAS (1988) Dependent variable
p'F
Partial R
Model R
Seasurface height Actual precipitation Precipitation 5 days ago Precipitation 10 days ago Precipitation 20 days ago Wind velocity Wind direction Air pressure
0.0379 n.s. 0.1362 n.s. n.s. n.s. n.s. n.s.
0.4824
0.4824
0.1709
0.6534
-
n.s."not significant.
methane production in these organic-rich sediments (Whiticar, 1982; Martens and Albert, 1994). In this study the working hypothesis was: groundwater seeping through the methane-rich sediment will increase methane concentrations in the overlying water column by flushing out the dissolved constituents. To separate the controlling factors on methane discharge, the data set (methane concentration, salinity anomaly; n"9) of the pockmark station was divided into two categories: (a) with groundwater seepage and (b) without seepage. Multiple linear regression analysis (GLM) showed a strong correlation (r"0.77) between the methane concentration of the bottom water and the intensity of the seepage — in category (a) when seepage is active and the salinity anomaly *S'0. Under non-seepage conditions (category b), flux of methane in the bottom water is dependent on the balance of methane production and methane oxidation by the natural bacterial populations. These populations, as well as the methane flux itself, in turn are influenced by strong seasonal variations of temperature (Wilson et al., 1989; King and Adamsen, 1992). With respect to the formation of methane, methanogenic bacteria compete with sulfate-reducing bacteria for their common electron donor, acetate. When sulfate-reducing bacteria are inhibited by SO depletion, methane production on the other side is increasing (Kuivila et al., 1990; Hoehler et al., 1994). The concentration of oxygen is crucial for methane oxidation (Casper, 1992; Roslev and King, 1996). In late summer, when the near-bottom water column becomes anoxic, a considerable liberation of methane into the water column was observed in nearby Kiel Harbour (Schmaljohann, 1996). Concentration of methane in surface waters at both sites showed the same seasonal variation, even though bottom water concentrations were independent of each other (Fig. 3). This may be explained by a methane plume extending from the floor of the pockmark to the surface directly above, including the surface water of the control site. In hindsight the location of the control site, being only 350 m away from the pockmark, turns out to be poor choice. But, on the other hand it permitted to show methane variability on a short spatial scale.
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The different methane concentrations of the bottom waters (119.91 and 28.93 nmol l\ median values) are reduced to similar surface values of 9.33 and 6.53 nmol l\ (median values) for the pockmark and the control site, respectively (Fig. 4). There are several explanations for this pronounced reduction of methane in a water column of only 25 m. For deeper waters it has been shown that due to microbial oxidation, a methane plume easily diminishes within a few kilometres (Angelis et al., 1993). Higher oxygen concentrations in the water compared to the sediment may lead to higher methane oxidation rates (Bussmann, 1994). The resuspension of surface sediments by currents or wave action is known to enhance heterotrophic processes (Wainright, 1987); this is also valid for methane oxidation activity (unpubl. data). Even though a notable proportion of the methane disappears in the water column presumably by oxidation and dilution, surface waters reveal methane concentration far above the equilibrium with atmospheric methane most of the time. Therefore, methane escapes continuously from the surface water into the atmosphere. During most of the year this sea—air flux ranged from minimum values around zero to about 50 lmol m\ d\. In the winter, the flux peaked at higher values of 200—400 lmol m\ d\ (Table 2). But for the stormy season in autumn and winter the flux may be underestimated as the influence of breaking waves (whitecaps) on the gas transfer is not included. Whitecaps can increase the gas flux by generating turbulence and creating bubble plumes (Farmer et al., 1993; Asher et al., 1996). Bange et al. found their data of methane fluxes in the central Baltic Sea to be grouped into low values in February (9.5—14.51 lmol m\ d\) and high values in July (101—1200 lmol m\ d\). This distribution is exactly opposite to the results of this seasonal study. The exact reasons for these strong seasonal variations of methane fluxes remains unknown. We strongly suspect that the summer maxima in methane flux reflects the increase methane formation in the sediment at higher temperatures, and that the winter maxima reported here, reflect the seepage activity.
Fig. 4. Box and Whisker plot of methane concentrations at all sampling dates for bottom and surface waters at both stations.
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6. Conclusion Our study fully supports the statements of Hovland and Bange that coastal oceans represent a source of methane (Hovland et al., 1993; Bange et al., 1994). It also becomes abundantly evident that in shelf areas, seasonal influences lead to variations in methane flux over several orders of magnitude. This seasonality is overlaid by fluctuations of seepage activity. Seepage activity increases methane concentrations in the bottom and surface waters dramatically. Hence, for calculations of the cumulative methane flux from the coastal ocean the strong fluctuations of methane seeps, as well as seasonal variations have to be taken into account. A regional study in order to estimate the relative proportion of methane release from seep sites and undisturbed sediments is badly needed. Acknowledgements We would like to thank the Dept. of Meteorology, (Institut fu¨r Meereskunde, Kiel) for providing the meteorological data, S. Lammers for methane analysis, P. Fischer for assistance in statistical treatment of the data. This study was supported by the EU (MAS2-CT92-0040) and the Deutsche Forschungsgesellschaft (SFB 313, Kiel). References Angelis, M.A.d., Liley, M.D., Olson, E.J., Baross, J.A., 1993. Methane oxidation in deep-sea hydrothermal plumes of the Endeavour Segment of the Juan da Fuca Ridge. Deep Sea Research 40 (6), 1169—1186. Asher, W.E., Karle, L.M., Higgins, B.J., Farley, P.J., 1996. The influence of bubble plumes on air-seawater gas transfer velocities. Journal of Geophysical Research 101 (C5), 12027—12041. Bange, H.W., Bartell, U.H., Rapsomanikis, S., Andrae, M.O., 1994. Methane in the Baltic and North Seas and a reassessment of the marine emissions of methane. Global Biogeochemical Cycles 8 (4), 465—480. Bussmann, I., 1994. Verteilung und Steuergro¨{en der Aktivita¨t Methan-oxidierender Bakterien in Randmeeren des Nordatlantiks. Ph.D. Thesis, University of Kiel. Carson, B., Suess, E., Strasser, J.C., 1990. Fluid flow and mass flux determinations at vent sites on the Cascadia Margin accretionary prism. Journal of Geophysical Research 95 (B6), 8891—8897. Casamitjana, X., Roget, E., 1993. Resuspension of sediment by focused groundwater in Lake Banyoles. Limnology and Oceanography 38 (3), 643—656. Casper, P., 1992. Methane production in lakes of different trophic state. Archiv fu( r Hydrobiologie, Ergebnisse der Limnologie 37, 149—154. Church, T.M., 1996. An underground route for the water cycle. Nature 380, 579—580. Cicerone, R.J., Oremland, R.S., 1988. Biogeochemical aspects of atmospheric methane. Global Biogeochemical Cycles 2, 299—327. Dando, P.R., O’Hara, S.C.M., Schuster, U., Taylor, L.J., Clayton, C.J., Baylis, S., Laier, T., 1994. Gas seepage from a carbonate-cemented sandstone reef on the Kattegat coast of Denmark. Marine and Petroleum Geology 11 (2), 182—188. Fabian, H.-S., Roese, K.-L., 1962. Das Erdo¨lfeld Schwedeneck. Erdo¨l-Zeitschrift 78, 283—294. Farmer, D.M., McNeil, C.L., Johnson, B.D., 1993. Evidence for the importance of bubbles in increasing air-sea gas flux. Nature 361, 620—623. Hoehler, T.M., Alperin, M.J., Albert, D.B., Martens, C.S., 1994. Field and laboratory studies of methane oxidation in an anoxic marine sediment: evidence for a methanogen-sulfate reducer consortium. Global Biogeochemical Cycles 8 (4), 451—463.
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Hovland, M., Judd, A.G., Burke, R.A., 1993. The global flux of methane from shallow submarine sediments. Chemosphere 26 (1—4), 559—578. Johannsen, A., 1980. Hydrogeologie von Schleswig-Holstein. Geologisches Jahrbuch, Reihe C, vol. 28, Bundesanstalt fu¨r Geowissenschaften und Rohstoffe, Hannover. Khandriche, A., Werner, F., 1994. Freshwater induced pockmarks in Bay of Eckernfo¨rde, Western Baltic. Proceedings of 3rd Marine Geological Conference of the Baltic, Sopot. King, G.M., Adamsen, A.P.S., 1992. Effects of temperature on methane consumption in a forest soil and in pure cultures of the methanotroph Methylomonas rubra. Applied and Environmental Microbiology 8 (9), 2758—2763. Kuivila, K.M., Murray, J.W., Devol, A.H., 1990. Methane production in the sulfate-depleted sediments of two marine basins. Geochemica et Cosmochemica Acta 54, 403—411. Lammers, S., Suess, E., 1994. An improved head-space analysis method for methane in seawater. Marine Chemistry 47 (2), 115—125. Lammers, S., Suess, E., Hovland, M., 1995. A large methane plume east of Bear Island (Barents Sea): Implications for the marine methane cycle. Geologische Rundschau 84 (1), 59—66. Liebau, C., 1985. Hydrogeologische Untersuchungen eines tiefen Grundwasserleiters im Gebiet von Kiel. Report from Geological-Paleontological Institute and Museum, vol. 10, University of Kiel. Linke, P., Suess, E., Torres, M., Martens, V., Rugh, W.D., Ziebis, W., Kulm, L.D., 1994. In situ measurement of fluid flow from cold seeps at active continental margins. Deep Sea Research 41, 721—739. Martens, C., Albert, D., 1994. Biogeochemical processes controlling gas production, consumption and transport in organic-rich marine sediments. In: Wever, T.E. (ed.) Proceedings of the ‘‘Gassy Mud Workshop’’, Kiel, pp. 101—107. Milham, N.P., Howes, B.L., 1994. Nutrient balance of a shallow coastal embayment: I. Patterns of groundwater discharge. Marine Ecology Progress Series 112, 155—167. Moore, W.S., 1996. Large groundwater inputs to coastal water revealed by Ra enrichments. Nature 380, 612—617. Nittrouer, C.A., Lopez, G.R., Wright, L.D., Bentley, S.J., D’Andrea, A.F., Friedrichs, C.T., Craig, N.I., Sommerfield, C.K., 1998. Oceanographic processes and the preservation of sedimentary structure in Eckernfo¨rde Bay, Baltic Sea. Continental Shelf Research 18, 1689—1714. Roslev, P., King, G.M., 1996. Regulation of methane oxidation in a freshwater wetland by water changes and anoxia. FEMS Microbiology Ecology 19, 105—115. Schmaljohann, R., 1996. Methane dynamics in the sediments and water column of Kiel Harbour (Baltic Sea). Marine Ecology Progress Series 131, 263—273. Scranton, M. I., McShane, K., 1991. Methane fluxes in the southern North Sea: the role of European rivers. Continental Shelf Research 11 (1), 37—52. Seibold, E., Exon, N., Hartmann, M., Ko¨gler, F.C., Krumm, H., Lutze, G.F., Newton, R.S., Werner, F., 1971. Marine geology of Kiel Bay. In Sedimentology of Parts of Central Europe. Verlag W. Kramer, Frankfurt a.M., pp. 209—235. Simmons, G.M., 1992. Importance of submarine groundwater discharge (SGWD) and seawater cycling to material flux across sediment/water interfaces in marine environments. Marine Ecology Progress Series 84, 173—184. Suess, E., Bohrmann, G., Huene, R.V., Linke, P., Wallmann, K., Lammers, S., Sahlig, H., Winckler, G., 1998. Fluid venting in the Aleutian subduction zone. Journal of Geophysical Research 103 (B2), 2597—2614. Wainright, S. C., 1987. Stimulation of heterotrophic microplankton production by resuspended marine sediments. Science 238, 1710—1712. Wanninkhof, R., 1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research 97 (C5), 7373—7382. Whiticar, M.J., 1982. The presence of methane bubbles in the acoustically turbid sediments of Eckernfo¨rde Bay, Baltic Sea. In: Manheim, F.T., Fanning, K.A. (Eds.), Dynamic Environment of the Ocean Floor. Lexington Books, MA, pp. 219—235. Whiticar, M.J., 1978. Relationships of interstitial gases and fluids during early diagenesis in some marine sediments. Reports Sonderforschungsbereich 95, vol. 35, University Kiel.
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Whiticar, M.J., Werner, F., 1981. Pockmarks: Submarine vents of natural gas or freshwater seeps? GeoMarine Letters 1, 193—199. Wiesenburg, D.A., Guinasso, N.L., 1979. Equilibrium solubilities of methane, carbon monoxide and hydrogen in water and sea water. Journal of Chemical and Engineering Data 24 (4), 356—360. Wilson, J.O., Crill, P.M., Bartlett, K.B., Sebacher, D.I., Harriss, R.C., Sass, R.L., 1989. Seasonal variation of methane emissions from a temperate swamp. Biogeochemistry 8, 55—71.