A decade of methane measurements at the Boknis Eck Time-series Station in the Eckernförde Bay (Southwestern Baltic Sea)

. Coastal areas contribute significantly to the emissions of methane (CH 4 ) from the ocean. In order to decipher its temporal variability in the whole water column, dissolved CH 4 was measured on a monthly basis at the Boknis Eck Time-series Station (BE) located in the Eckernförde Bay (SW Baltic Sea) from 2006 to 2017. BE has a water depth of about 28 m and dissolved CH 4 was measured at six water depths ranging from 0 to 25 m. In general, CH 4 concentrations increased with 10 depth, indicating a sedimentary release of CH 4 . Pronounced enhancement of the CH 4 concentrations in the bottom layer (15– 25 m) was found during February, May–June and October. CH 4 was not correlated with Chlorophyll a or O 2 over the measurement period. Unusually high CH 4 concentrations (of up to 696 nM) were sporadically observed in the upper layer (0–10 m) (e.g. in November 2013 and December 2014) and were coinciding with Major Baltic Inflow (MBI) events. Surface CH 4 concentrations were always supersaturated throughout the monitoring period, indicating that the Eckernförde Bay is an 15 intense but highly variable source of atmospheric CH 4 . We did not detect significant temporal trends in CH 4 concentrations or emissions, despite of ongoing environmental changes such as warming and deoxygenation in the Eckernförde Bay. Overall, the CH 4 variability at BE is driven by a complex interplay of various biological and physical processes. Author contributions . XM, MS, STL, and HWB designed the study and participated in the fieldwork. CH 4 measurements and data processing were done by XM, MS and STL. XM wrote the article with contributions from MS, STL and HWB.


Introducti on
Methane (CH 4 ) is an at mospheric trace gas which contributes significantly to global warming (IPCC, 2013) and the 20 evolution of stratospheric ozone (WMO, 2018). At mospheric CH 4 mole fractions have been increasing by about 150 % since the industrial revolution (IPCC, 2013).
The oceanic release of CH 4 to the atmosphere plays a minor role for the global at mospheric CH 4 budget (Saunois et al. 2016). Ho wever, coastal areas have been identified as hot spots of CH 4 emissions (see e.g. Bange et al., 1994;Upstill-Goddard et al., 2000;Borges et al., 2016). Dissolved CH 4 in coastal waters is main ly resulting fro m the interplay of (i) 25 sedimentary sources such as anaerobic methanogenesis during the decomposition of organic matter (Xiao et al., 2018;Dale et al., 2019) or seepage fro m o il and natural gas reservoirs (Bernard et al., 1976;Hovland et al., 1993;Judd et al., 2002) and (ii) microbial CH 4 consumption which occurs under oxic conditions in the water column and under anoxic conditions in the sediments (Pimenov et al., 2013;Stein le et al., 2017;Egger et al., 2018). Only recently, Weber et al. (2019) estimated the Occasional studies of the CH 4 production and consumption pathways in coastal waters and the associated CH 4 emissions to the atmosphere have received increasing attention during the last decades (Bange et al., 1994;Reeburg 2007;Naqvi et al., 2010). However, time-series measurements of CH 4 wh ich would allo w identifying short-and long-term trends in view of the ongoing environmental changes in coastal regions (such as eutrophication, warming and deoxygenation) are still sparse. In this paper we present the monthly measurements of CH 4 fro m a t ime -series station in the Eckernförde Bay (Baltic Sea) 35 during 2006-2017. Due to severe eutrophication, sediments in the Eckernförde Bay receive large amount of organic matter (Smetacek et al., 1987;Oris et al., 1996;Nittrouer et al., 1998) and thus are active sites of CH 4 formation (Sch maljohann, 1996;Whit icar, 2002;Treude et al., 2005;Maltby et al., 2018). Seasonal and inter-annual CH 4 variat ions in concentration, saturation and airsea flu x density were investigated for more than a decade. The aim o f this study was to assess the seasonal dynamics of and 40 the environmental controls on CH 4 variab ility in the Eckernförde Bay wh ich is affected by high nutrient concentrations, increasing water temperatures and ongoing loss of dissolved oxygen (Lennartz et al., 2014).

Study site
The Bokn is Eck (BE) time-series station is one of the oldest continuously conducted ma rine t ime-series stations in the world.
The first sampling took place in 1957, and has been conducted on a monthly base with only minor interruptions since then 45 (Lennartz et al., 2014). It is situated in the Eckernförde Bay in the southwestern (SW) Baltic Sea, with a depth of approximately 28 m (Fig. 1). The sediments in the Bay are characterized by h igh organic matter load and sedimentation rate (Orsi et al., 1996;Whiticar, 2002), which is closely associated with the spring and autumn algae blooms (Smetacek, 1985).
The Balt ic Sea has only a limited water exchange with the North Sea through the Kattegat, which makes this area very sensitive to climate change and anthropogenic impacts. As a result of global warming, the increasing trend for the g lobal sea 50 surface (< 75 m) temperatures (SST) was about 0.11 °C per decade (IPCC, 2013), while a net SST increase of 1.35 °C was observed in the Balt ic Sea during 1982-2006, which is one of the most rapid temperature increment in large marine ecosystems (Belkin, 2009). Lennartz et al. (2014) reported a warming t rend of up to 0.2 °C per decade at the BE time-series station for the period of 1957-2013. Nutrients in the Baltic Sea have been increasing until 1980s as a result of the intensive agricultural and industrial activit ies, and then started to decline due to effective wastewater control (HELCOM, 2018). 55 However, hypoxia and ano xia have been increasing in the Balt ic Sea during the past several decades (Conley et al., 2011;Carstensen et al., 2014). Similar trends in nutrients and O 2 were also detected at the BE t ime-series station (Lennartz et al., 2014), indicating that the Eckernförde Bay is representative for the biogeochemical setting of the SW Baltic Sea. In concert with the declining nutrient concentrations, Chlorophyll a concentrations at the BE time-series station were declining as well (Lennartz et al., 2014). 60 Located close to the bottleneck of the water exchange between the North Sea and the Baltic Sea, the BE t ime -series station is also sensitive to hydrographic fluctuations such as inflows of saline North Sea Water. There is no riverine input to the Eckernförde Bay, and thus, the saline water inflow fro m the North Sea plays a dominant role in the hydrographic setting at BE. Because the inflo wing North Sea water has a higher salinity co mpared to Baltic Sea water, a pronounced summer stratification occurs which leads to the development of a pycnocline at about 15 m water depth. The seasonal stratification 65 occurs usually fro m mid-March until mid-September. During this period, vertical mixing is restricted and bacterial decomposition of organic material in the deep layer causes pronounced hypoxia and sporadically occurring ano xia during late summer (Lennartz et al., 2014). Pronounced phytoplankton blooms occur regularly in autumn (September-November) and spring (February-March) and to a lesser extent during summer (July-August) (Smetacek et al. 1985).

Sample collection and measurement
Monthly sampling of CH 4 fro m the BE time-series station started in June 2006. Seawater was collected fro m 6 depths (1, 5, 10, 15, 20 and 25 m) with 5 L Niskin bottles mounted on a CTD rosette. 20 mL brown glass vials were filled in triplicates without any bubbles. The vials were sealed immed iately with rubber stoppers and aluminum caps. These samples were poisoned with 50 µL saturated aqueous mercury chloride (Hg Cl 2 ) solution as soon as possible, and then stored in a cool, dark 75 place until measurement. The storage time of the samples before the measurements was less than 3 months.
A static headspace-equilibriu m method was adopted for the CH 4 measurements. A 10 mL Heliu m (99.9999 %, AirLiquide, Düsseldorf, Germany) headspace was created inside the vial with a gas-tight syringe (VICI Precision Sampling, Baton Rouge, LA). The sample was vibrated with Vortex (G-560E, Scientific Industries Inc., New York, USA) for appro ximately 20 s and then left for at least 2 h to reach the CH 4 equilibriu m between air and water phases. A 9.5 mL subsample of 80 headspace was injected into a gas chromatograph equipped with a flame ionization detector (GC-FID, Hewlett-Packard 5890 Series II, Agilent Technologies, Santa Clara, CA, USA). Separation took place on a packed colu mn (SS, 1.8 m length, packed with molsieve 5A, Grace, Colu mbia, Maryland, USA). Standard gas mixtures with varying mole fractions of CH 4 in synthetic air (Deuste-Steininger Gmb H, Mühlhausen, Germany and Westfalen A G, Münster, Germany) were used daily to calibrate the response of FID before measurements. The concentrations of standard gases were ad justed for every 85 measurement to make sure that the values of the samples fall in the range of the calibration curves. The standard gas mixtu res were calibrated against NOAA primary gas standard mixtures in the laboratory of the Max-Planck-Institute for Biogeochemistry in Jena, Germany. Further details about the measurements and calculations of the d issolved CH 4 concentration can be found in Bange et al. (2010). The mean precision of the CH 4 measurements, calculated as the median of the estimated standard errors (see David, 1951) (Grasshoff et al., 1999). Secchi depth was measured with a white disk (~30 cm in d iameter). Sea levels were measured at Kiel-Holtenau, which is about 15 km away fro m the BE time-series station (http://www.boos.org/). A more co mprehensive overview of temperature, salinity, dissolved O 2 , Ch lorophyll a as well as other parameters at the BE time-series station can be found in Lennartz et al. (2014). 95

Calculation of saturation and air-sea flux density
The CH 4 saturation (S CH4 , %) was calculated as: where CH 4obs and CH 4eq are the observed and equilibriu m concentrations of CH 4 in seawater, respectively. CH 4eq was calculated with the in-situ temperature and salin ity of seawater (Wiesenburg and Gu inasso, 1979), and the dry mo le fraction 100 of atmospheric CH 4 at the time of sampling, which was derived fro m the monthly at mospheric CH 4 data measured at Mace Head, Ireland (AGAGE, http://agage.mit.edu/).
The air-sea CH 4 flux density (F CH4 , in µmol m -2 d -1 ) was calculated as: where k (in c m h -1 ) is the gas transfer velocity calculated with the equation given by Nightingale et al. (2000), as a function 105 of the wind speed and the Schmidt nu mber (Sc). Sc was co mputed with the empirical equations for the kinematic v iscosity of seawater (Siedler and Peters, 1986) and the diffusion coefficients of CH 4 in water (Jähne et al., 1987). Wind speed data were recorded at the Kiel Lighthouse (www.geo mar.de/service/wetter/), wh ich is approximately 20 km away fro m the BE t imeseries station. The wind speeds were normalized to the height of 10 m (u 10 ) with the method given by Hsu et al. (1994). Enhanced Chlorophyll a concentrations, which can be used to indicate phytoplankton blooms, were usually observed in spring or autumn, but not in every year (Fig. 2). Seasonal variat ions of Ch lorophyll a concentrations were generally 125 consistent with the annual p lankton succession reported by Smetacek (1985). During 2006-2017, h igh Ch lorophyll a concentrations were usually found in the upper layers in March (Fig. 3), which is different fro m the seasonality during 1960-2013 where on average, high concentrations occupied the whole water colu mn (Lennartz et al., 2014). Another difference is that no prevailing 'winter dormancy' of biological activ ity was observed: Chlorophyll a concentrations usually remained high throughout the autumn-spring period. In November and December 2006 and March 2012 when high Chlo rophyll a 130 concentrations were observed all over the water colu mn, nutrients and temperature were generally h igher. A lthough the overall correlat ion between Chlorophyll a and nutrients (NO 3 -, r 2 =0.01, p<0.01, n=674) or temperature (r 2 =0.02, p<0.0001, n=671) is poor, nutrients or temperature might be potential environ mental controls on Ch lorophyll a distribution. As a pro xy of water transparency, Secchi depth was lowest in March indicating a high turbid ity, coincident with the Chlorophyll a maximu m. Ch lorophyll a concentrations and Secchi depths have been decreasing over the past decades in the Baltic Sea 135 (Sandén and Håkansson, 1996;Fleming-Lehtinen and Laamanen, 2012;Lennartz et al., 2014), but this trend cannot be identified from the median slope at the BE time-series station during 2006-2017. in sedimentary CH 4 release in the coastal Baltic Sea. In -situ production in the ano xic bottom water might be a potential CH 4 source as well (Scranton and Farrington, 1977;Levipan et al., 2007). We, therefore, suggest that the accumulation of CH 4 in the bottom water in October is caused by its release fro m the sediments and in-situ production in the overlying water co lu mn in comb ination with the pronounced water column stratificat ion during autumn which prevents ventilation of CH 4 to the surface layer. the Baltic Sea (Mohrholz et al., 2015). Dissolved CH 4 concentrations in the surface North Sea were much lower than in the Eckernförde Bay (Bange et al., 1994;Rehder et al., 1998), and therefore a d irect CH 4 contribution fro m the North Sea by oxygenated waters seems unlikely. We hypothesize that this inflow substituted the lower part of the water co lu mn which had high CH 4 concentration throughout the water depth before, opposite to, e.g., an in-situ production of CH 4 at the surface being responsible for the observed concentration profile anomaly. The MBI is the third strongest event ever recorded, and an 175 unusual outflow period was detected in the Eckernförde Bay : Sea levels declined since mid-November and reached minimu m on 10 December, and then began to increase with the inflow (Fig. 5). The sampling at the BE time-series station took place on 16 December, during the main in flo w period. Ext reme weather conditions (wind speed >15 m s -1 ) were observed several days before the sampling date, and storm-generated waves and currents could have affected the sediment structures in the Eckernförde Bay (Oris et al., 1996). Currents across the seabed can result in pressure gradients that drive 180 porewater flow within the permeable sediments (Ah merka mp et al., 2015), which might be a potential CH 4 source. Sed iment resuspension might also contribute to enhanced CH 4 release, but we did not observe a significant decline in Secchi depths in December 2014 (Fig. 2). The significant decrease in sea level allev iated the static pressure on the sediments. Enhanced CH 4 release fro m the sediments, via gas bubbles or exchange fro m porewater, may have led to the accu mulation of CH 4 in the water colu mn . Similar hydrostatic pressure effects were also reported in t idal systems such as mangrove creeks and estuaries 185 (see e.g. Barnes et al. 2006;Maher et al., 2015;Sturm et al., 2017). Atmospheric pressure also contributes to the overall pressure on the sediments, but it is not recorded at the BE t ime-series station and thus was omitted. A lthough the water level fluctuation of ± 1 m (Fig. 5) seems rather small co mpared to the water depth (28m), it might exert a strong influence on the sediments. Water level fluctuation, when there was no strong wind or inflow event, was approximately ± 0.2 m in the Eckernförde Bay. Lohrberg et al. (2020) detected a change in water level (± 0.5 m) and air pressure (± 1500 Pa, equivalent to 190 approximately ± 0.15 m of water level fluctuation) during a weak storm in the fall of 2014. The fluctuation in hydrostatic pressure induced a pronounced CH 4 ebullition event in the Eckernförde Bay, and a sedimentary CH 4 flu x of 1916 μmo l m -2 d -1 was estimated (Lohrberg et al., 2020). This value is generally in good agreement with the sharp increase in the sea-to-air CH 4 flu xes in December 2014 (see section 4.3). The outflo w period of the M BI in 2014 lasted for almost a month, and bulk ebullit ions and supersaturated water with CH 4 could be anticipated. During the inflow period, large amounts of North Sea 195 water flooded into the Eckernförde Bay and presumably pushed the CH 4 -enriched water to the surface. A negative correlation was found between salinity and CH 4 concentration in the water colu mn (Fig. 4a, r  The situation in March 2014 is different. We did not find any evidence for saline water inflo w or hydrostatic pressure fluctuation, and the correlation between CH 4 concentration and salin ity is poor (Fig. 4c, r 2 =0.43, p=0.16, n=6). The occurrences of the unusual CH 4 profiles were acco mpanied by the enhanced Chlorophyll a concentrations in the upper waters. CH 4 productions by widespread marine phytoplankton have been reported and might be potential sources of surface CH 4 supersaturations (Lenhart et al, 2016;Klintzsch et al., 2019). However, spring or autumn algae blooms at the BE t ime-220 series station were often observed without CH 4 accumu lation and surface CH 4 contribution fro m phytoplankton remains to be proven. Potential sources for the enhanced CH 4 in March 2014 are still unclear.

Results and discussion
In summary, we suggest that saline water inflow and the subsequent upwelling o f water are the most potential causes for the CH 4 surface accumulation in November 2013 and December 2014. Nonetheless, the occurrence of inflow does not necessarily lead to enhanced CH 4 concentrations in the upper waters. Inflow events are relatively common, for examp le, in 225 2013, besides the inflow in Nove mber, three other events with similar estimated inflow volu mes were detected in January, February and April (Nausch et al., 2014), but no CH 4 anomaly was found during that period. The magnitude of the CH 4 anomalies might depend on the strength of the inflow events and other factors, such as storms and sediment resuspension.
Besides, there is a h igh chance that the monthly samp ling at the BE time-series station only captured few CH 4 pulses. Inflow events usually last days to weeks, but the accumulated CH 4 in the upper layers might last even shorter because of effective 230 aerobic CH 4 o xidation (Stein le et al., 2017) and strong vertical mixing in winter. The occurrences of surface CH 4 accumulations at the BE time-series station might be more frequent than been observed.

Surface saturation and flux density
Surface CH 4 saturations are directly proportional to its concentrations in the surface water (S CH4 =31.40 × [CH 4 ] + 10.29, R 2 =0.9794, n=77, p<0.0001; Fig. 6a, b), despite of the pronounced seasonal variations in temperature (Fig. 3). This indicates 235 that the net CH 4 production at BE is overriding the temperature-driven variability of the CH 4 concentrations. Excluding the extreme value fro m December 2014, surface CH 4 saturations at the BE t ime-series station varied between 129-5563 %, with an average of 615 ± 688 %. The surface layer was supersaturated with CH 4 and thus emitting CH 4 to the at mosphere throughout the sampling period.
The coastal Baltic Sea, especially the southwestern part, is a hot spot for CH 4 emissions. Area-weighted mean CH 4 240 saturations for the entire Baltic Sea (113 % and 395 % in winter and summer 1992, respectively; Bange et al., 1994) were lower than at the BE time-series station. Sch male et al. (2010) extensively investigated dissolved CH 4 distributions in the Baltic Sea, and found that surface CH 4 supersaturations were stronger in the shallow western areas.
Sea-to-air CH 4 flu x densities fluctuated between 0.3-746.3 μmo l m -2 d -1 , with an average of 43.8 ± 88.7 μmol m -2 d -1 (excluding the extreme value in December 2014, Fig. 6c). Co mparable results in saturation and flux density were observed at 245 the pockmark sites in the Eckernförde Bay (Bussmann and Suess, 1998) CH 4 emissions from coastal waters could be roughly considered as the difference between format ion and o xidation of CH 4 in the water column and sediments. Although sediments are substantial CH 4 sources, most CH 4 is consumed before evading to the atmosphere (Martens et al., 1999;Treude et al., 2005;Steinle et al., 2017). Treude et al. (2005) co mpared the potential and field rates of anaerobic o xidation of methane (AOM) in the sediments of the Eckernförde Bay and suggested that the 255 AOM-mediat ing organisms are capable of fast response to changes in CH 4 supply. Steinle et al. (2017) reported that 70-95 % of dissolved CH 4 were effectively removed in the water column during summer stratificat ion. Apart fro m MBI-driven Moreover, methanogenesis in the sediments of the Eckernförde Bay is sufficient fo r CH 4 bubble format ion (Whit icar, 2002).
Hydrostatic pressure fluctuations associated with saline water inflow could have triggered CH 4 seepage and gas bubble plumes fro m the seafloor to the at mosphere (Wever et al., 2006;Lohrberg et al., 2020). Gas ebullition sites were usually 275 found accompanied by pockmark structures (Schneider von Deimling et al., 2011) and Jackson et al. (1998) provided sonar evidences for CH 4 ebullit ion in the Eckernförde Bay. Ho wever, recently Lohrberg et al. (2020) reported a widespread CH 4 ebullit ion event in the Eckernförde Bay and found no direct linkage between pockmarks and ebullitions. They estimated the bubble-driven CH 4 flu x during a weak storm in the fall of 2014 was 1916 μmo l m -2 d -1 . These findings point to the fact that ebullit ion might be an important, but highly variable, additional CH 4 efflu x to the atmosphere. However, our measurements 280 did not capture gas bubbles and, thus, the estimate of the overall CH 4 emissions resulting fro m the M BI might be too low. In this case, a time-series monitoring of saline inflo ws and sea level variations, comb ined with a continuous observation of CH 4 variability, especially in winter, are essential in quantifying CH 4 emissions from the Eckernförde Bay.

Comparison with other time-series measurements
Besides this study, time -series measurements of CH 4 have also been reported from Saanich Inlet (SI), British Co lu mbia, 285 Canada (Capelle et al., 2019) and ALOHA station in the North Pacific Subtropical Gyre (Wilson et al., 2017).
Located in a seasonally anoxic fjord, the time-series station in SI has a similar hydrographic setting compared to BE, but a deeper water depth (230 m, Capelle et al., 2019). Surface CH 4 saturations at SI fell in the lower end of the range observed here for BE (Fig. 7). Despite the fact that the mean surface saturation in SI was h igher, CH 4 flu x densities were much lower than at BE. Since the air-sea exchange approach of Nightingale et al. (2000) was used in both studies, the discrepancy is 290 resulting fro m the higher wind speeds at BE. CH 4 saturations from A LOHA were only slightly supersaturated (close to the equilibriu m saturation) and the flu x densities were consequently low as well, which is resulting fro m the fact that ALOHA is a deep water (~4800 m) station located in the o ligotrophic open ocean where potential strong CH 4 sources such as sedimentary release or methanogenesis under low O 2 in the water column are negligible (Wilson et al., 2017). Wilson et al. (2017) analy zed the time-series CH 4 data fro m A LOHA during 2008-2016 and observed a decline in the 295 surface CH 4 concentrations since 2013. They attributed the potential decrease in CH 4 production to fluctuations in phosphate concentrations. Capelle et al. (2019) also detected a significant decline of CH 4 concentrations in the upper water co lu mn over time at SI and proposed a link with the shoaling of the boundary of the hypoxic layer. However, no significant trend was detected in CH 4 concentrations or flu x densities at the BE t ime-series station (Fig. 6), despite of the relatively long observation period. The different situations can be explained by the shallow water depth in the Eckernförde Bay, wh ich 300 ma kes the CH 4 distribution sensitive to the variability of its sedimentary release and events such as MBI and wind-driven upwelling.

Conclusions
The CH 4 measurements at the BE t ime-series station showed a strong temporal variab ility and variations with depths. A pronounced enhancement of the CH 4 concentrations was usually found in the bottom layer (15-25 m) during February, 305 May-June and October which indicates that the release from the sediments is the major source of CH 4 . Organic matter and dissolved O 2 are usually considered as the main controlling factors for CH 4 production and consumption pathways, but we did not detect correlations of CH 4 with Chlorophyll a or O 2 during 2006-2017.
Obviously non-biological processes such as local wind-driven-upwelling and the inflo w o f saline No rth Sea waters play a significant role for the observed variability of CH 4 at BE. However, these phenomena, which occur on relatively short time 310 scales of day or weeks, were not frequently detected; most probably due to the monthly sampling frequency. The surface layer at BE was always supersaturated with CH 4 and therefore, BE was a persistent and strong, but highly variable, source of CH 4 to the atmosphere. We did not detect significant temporal trends in CH 4 concentrations or emissions, despite of ongoing environmental changes (warming, deo xygenation) in the Eckernförde Bay. Overall, the CH 4 variability at BE is driven by a complex interplay of various biological (i.e. methanogenesis, oxidation) and physical (i.e. upwelling, in flo w events) 315 processes. Continuous observations at the BE time-series station, with an emphasis on the period when upwelling and saline inflow usually occur is therefore, of great importance in quantifying CH 4 variability and the associated emissions as well as for predicting future CH 4 variability in the SW Baltic Sea. and MEMENTO (the MarinE MethanE and NiTrous Oxide database, https://memento.geomar.de (Kock and Bange, 2015).
Author contributions. XM, MS, STL, and HWB designed the study and participated in the fieldwork. CH 4 measurements and data processing were done by XM, MS and STL. XM wrote the article with contributions from MS, STL and HWB.

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Competing interests. The authors declare that they have no conflict of interest.
Acknowledgements. The authors thank the captain and crew of the RV Littorina and Polarfuchs as well as many colleagues and numerous students who helped with the sampling and measurements of the BE t ime-series through various projects.