The Baltic Sea is a semi-enclosed brackish water body consisting of a set of major relatively deep basins, shallower bays, gulfs, and archipelagoes. The most recent digital bathymetric model by the European Marine Observation and Data Network (EMODnet Bathymetry Consortium, 2018) delineates the various sub basins of the Baltic Sea that are used throughout this study and indicate the average depth of the Baltic Sea with 53 m (Jakobsson et al., 2019). After the Black Sea, the central Baltic Sea is the second largest anoxic coastal water body in the world. This is due to water residence time of ∼30 years, a strong vertical salinity gradient (halocline) at 60–90 m water depth, and ongoing eutrophication (Wulff et al., 2007).

The cruise was conducted with Stockholm University’s Research Vessel (RV) Electra with the aim of obtaining a comprehensive and representative picture of the distribution of CO₂ and CH₄ concentrations in surface water across the Baltic Sea, covering a broad depth range, at the end of the heat wave during the summer of 2018. We covered archipelago-type coastal seascapes of Sweden and Finland, the Northern Baltic Proper, the Åland Sea, and the Gulf Finland (Figure 1). In fact, these shallower coastal seascapes consisting of hundreds to thousands of semi-enclosed bays and basins make up a huge proportion of the Baltic Sea, i.e., 25% of the Northern Baltic Proper, 70% of the Åland Sea, and 60% of the Gulf of Finland are shallower than 40 m depth (Jakobsson et al., 2019). The 40 m boundary used in this study to discriminate shallower from deeper sites is chosen because it separates two concentration regimes in the Baltic Sea and the shallower parts show much more elevated concentration of CO₂ and CH₄. In general, very little is known about sea-air gas exchange in these shallow areas of the Baltic Sea, and hardly any data are available with only a few exceptions for the Gulf of Finland, i.e., the harbor areas of St. Petersburg and Helsinki (Schneider et al., 2014). For the northern boreal part of the Baltic Sea consisting of the Bothnian Bay, the Quark, the Bothnian Sea, the Gulf of Finland, and the Northern, Western and Eastern Baltic Proper the total area shallower than 40 m is some 71,000 km² (Jakobsson et al., 2019). For perspective, impressive data sets on water-air CO₂ and CH₄ exchange are available (Juutinen et al., 2009; Humborg et al., 2010) for Sweden and Finland, where total lake area is about 33,000 km² and 30,000 km², respectively (the Swedish lake area was estimated excepting the largest deep lakes Vättern and Vänern where no CH₄ and CO₂ exchange estimate exists).

The long-term temperature record from the TZS coastal monitoring site (lat: 59.85548N, lon: 23.26033E) covers August 1926 to December 2018. A moving average was adopted to smooth the short-term fluctuations in the temperature data by taking the average value of a 1-year length and shifting forward, i.e., for each average annual value calculation, the first number of the 1-year data series was excluded and the next value was included. Note that the time interval of each temperature record varied slightly, from 10 days in the years before 1997 down to 4–7 days after 1998 and finally daily records in 2018. Thus, the 1-year length used in this study is fixed from a day in a year to the same day in the year after regardless of the time interval between two specific temperature records.

The partial pressures of atmospheric and dissolved CO₂ and CH₄ were measured using the Water Equilibration Gas Analyzer System (WEGAS) coupled to cavity ring-down spectrometer (CRDS). This system consists of three major components: (i) a water handling system comprised of a showerhead equilibrator (1 L headspace volume) fed by the seawater intake, an E&H electrode probe and a thermosalinograph (Seabird TSG 45); (ii) a gas handling system with circulation pumps for the showerhead and ambient air from the bow of the ship; and (iii) CRDS gas analyzers for CO₂ and CH₄ concentrations (model G2131-i, Picarro Inc.); and A detailed description of the WEGAS systems and its performance can be found in the Supplementary Material.

Measurements of surface water CO₂ and CH₄ concentrations were performed using the seawater intake located just below the sea surface. Water was pumped through spray nozzles into the open headspace equilibrator at ∼4.5 L min^–1. By creating a fine spray of droplets, the exchange surface between showerhead and water was maximized and an optimal equilibration is achieved. The ambient pressure was maintained through a ambient air-fed vent flow that was monitored continuously during the measurements. The gas equilibrated in the showerhead was measured using the CRDS analyzer. For the standard CO₂ and CH₄ analysis routine, ambient air is measured for 8 min followed by the gas measurements in the showerhead of the equilibrator for 12 min, i.e., one complete analysis cycle is 20 min. Cycles are performed continuously during the cruise including the station time when RV Electra is not steaming.

Continuous CO₂ and CH₄ measurements in both ambient air and surface water were conducted during September 18–23, 2018; in total, 80,393 data points for both CO₂ and CH₄ in air and water were recorded. The recorded data were filtered by removing data taken during the transition period between ambient air and water measurements (see detailed description in Supplementary Material) and collected during improper functioning of WEGAS (e.g., low water flow, exceptional values due to air contamination by the ship) and thereafter the data were also corrected for the vent flow to the equilibrator that was induced while WEGAS was running. Any remaining data with CO₂ concentrations above 450 ppm were also removed. After filtering, atmospheric CO₂ averaged 401.3 ± 2.08 ppm (2σ). As a result of spatial normalization and excluding data that could have been contaminated by the ship, here we present 15,733 data points for both air and water concentrations.

For reporting the CO₂ content in water, the ppm value obtained by the cavity ring down spectrometer (CRDS) were converted to pressure units (pCO2 in μatm). We assumed ambient atmospheric pressure because water was taken at depth of around half a meter below the surface and the pressure in the showerhead is equal to ambient air as achieved by the vent flow taken at the bow of the ship.

For reporting the CH₄ concentrations in water, the ppm value obtained by CRDS were converted to molar units (i.e., nmol/L) by Henry’s law (Equation 1), because the achieved equilibration means that the gas content in the showerhead is equivalent to that in the water phase, i.e., CH₄ measured by CRDS is considered as the real-time concentration of CH₄ in water. This makes Henry’s law applicable for dealing with gas/liquid equilibria and the Henry’s constant for molar concentration and partial pressure conversion is chosen in this case. As the showerhead in the equilibrator is maintained at ambient pressure and the water temperature is measured by thermosalinograph right before the water reaches the equilibrator, Henry’s law constant is corrected for each calculation by the measured temperature at atmospheric pressure.

(1) C = p × K H

where C is the concentration of CH₄ (mol/L), p is the partial pressure in unit of atm (1 atm = 10⁶ ppm) of CH₄. K_H is the Henry’s law constant that is dependent on temperature and is corrected in this study following the van ’t Hoff equation (Sander, 2015):

(2) K H = K H * × exp ⁢ [ - Δ sol ⁢ H R × ( 1 T - 1 T * ) ]

Where KH* is the Henry’s law constant at the reference temperature, 298 K, T^* is the reference temperature, 298 K and T is the measured temperature in water (K). The temperature dependence of the equilibrium constant, -Δsol⁢HR does not change much with temperature and is tabulated (Sander, 2015). K_H also varies with pressure, but in this study, pressure was assumed to be atmospheric pressure because water was taken at depth of around half a meter.

In the Baltic Sea both CO₂ and CH₄ concentrations span an order of magnitude and the highest concentrations reported are >2500 μatm of CO₂ (Fransner et al., 2019) and >1200 nM of CH₄ (Jakobs et al., 2014).

The sea-air flux (F) of CO₂ and CH₄ is commonly expressed by the following equation (Liss and Slater, 1974):

(3) F = k × ( C sea - C air )

where k is the gas transfer velocity and C_sea and C_air are concentrations of gases in the sea and the overlying air, respectively. This is also frequently written as Equation 4

(4) F = k × K 0 × ( p sea - p air )

where K₀ is the solubility of CO₂ or CH₄, p_sea and p_air are the measured partial pressures of the gases in equilibrium with surface water and in the air, respectively. k is the gas transfer velocity that has been optimized as Equation 5 (Wanninkhof, 2014):

(5) k = 0.251 × U 2 × ( S ⁢ c 660 ) - 0.5

where U is wind speed (m/s) and Sc is the Schmidt number, which is highly dependent on temperature and gas. Sc in our calculations was corrected for the corresponding temperature that was measured simultaneously with partial pressures of CH₄ and CO₂ (pCH₄ and pCO₂).

Combining Equations 3, 4 and 5, the flux of CO₂ and CH₄ in this study is finally calculated as Equation 6:

(6) F = 0.251 × U 2 × ( S ⁢ c 660 ) - 0.5 × K 0 × ( p sea - p air )

Surface water temperature data observation in the open Northern Baltic Proper from the automatic buoy Huvudskär (lat: 58.9333 N, lon: 19.1667 E; 100 m water depth) were retrieved from the Swedish Hydrological and Meteorological Service². Wind data observation from the Raasepori Jussarö (lat: 59.82076N, lon: 23.57309E) weather station and wave height observations from the buoy Raassepori Hästö Busö (lat: 59.82583N, lon: 23.30783E), which are located some 15 km and 5 km from the TZS, respectively, were retrieved from Finnish Meteorological Institute³.

Using the surface CO₂ and CH₄ concentration in inshore and offshore surface waters and sea-air flux data and the information about the deepening of the upper mixed surface layer after the storm event, we can construct a budget evaluating the sources and sinks of CO₂ and CH₄ (Figure 2). The conservation of mass for the surface layer under the assumptions mentioned is:

(7) h after ⁢ C ⁢ O 2 ⁢ a ⁢ f ⁢ t ⁢ e ⁢ r - h before ⁢ C ⁢ O 2 ⁢ b ⁢ e ⁢ f ⁢ o ⁢ r ⁢ e = C ⁢ O 2 ⁢ o ⁢ f ⁢ f ⁢ s ⁢ h ⁢ o ⁢ r ⁢ e ⁢ ( h after - h before ) - F CO 2 ⁢ Δ ⁢ t + I CO 2 ⁢ Δ ⁢ t

(8) h after ⁢ C ⁢ H 4 ⁢ a ⁢ f ⁢ t ⁢ e ⁢ r - h before ⁢ C ⁢ H 4 ⁢ b ⁢ e ⁢ f ⁢ o ⁢ r ⁢ e = C ⁢ H 4 ⁢ o ⁢ f ⁢ f ⁢ s ⁢ h ⁢ o ⁢ r ⁢ e ⁢ ( h after - h before ) - F CH 4 ⁢ Δ ⁢ t + I CH 4 ⁢ Δ ⁢ t

Where h is the mixed layer depth before and after the storm, respectively, F atmospheric flux and I an internal source term. The source water for the advection term, i.e., CO_2offshore and CH_4offshore are assumed to have properties close to what was observed offshore on September 19 and 20.

The acoustic water column data presented here were continuously collected with a Simrad EK80 wideband split-beam scientific echo sounder with a center frequency at 70 kHz that was installed on RV Electra. The vertical range resolution is approximately 2 cm and the ping rate was about 2.5 Hz. The sonar produces a linear frequency modulated acoustic signal which provides improved signal-to-noise ratio and a higher vertical range resolution compared to similar narrow-band systems (Stanton and Chu, 2008). The dataset collected with the EK80 was match filtered with an ideal replica signal using a MATLAB software package version R2017a provided by the system manufacturer, Kongsberg Maritime (Lars Anderson, personal communication). Position and altitude information were provided to the sonar by a Seapath 330 + motion sensor. Ranges from the transducer were calculated using the cumulative travel times through sound speed profile layers based on the nearest (in time) CTD profile (SEA-Bird SBE 911 plus), following the approach by Stranne et al. (2017).

Temperatures of up to 23.04°C (daily mean) were recorded in the upper surface layer in the open Northern Baltic Proper on 6 August at the autonomous measuring stations (Figure 3A). A similar picture appears when evaluating the coastal monitoring stations near TZS. Since 2017, water temperatures have been continuously recorded using in situ instrumentation, allowing us to follow the effects of the summer 2018 heat wave in even greater detail. The highest bottom-water temperatures (32 m depth) of 20.53°C were recorded on 30 July at this coastal monitoring station (Figure 3B) and these are the highest temperatures ever recorded at this depth over the past 93 years. A second warming event in bottom waters above the sediments was recorded in early September 2018 just before our sampling campaign with temperatures reaching nearly 17°C (Figure 2B). The record-high bottom temperatures measured in the TZS area in 2018 follow an increasing trend observed over the past century until today. In Figure 3C, the long-term temperature record (moving average) between 1926 and 2018 is presented showing a mean water temperature of 4°C above the sediments between the 1920s and the 1980s. Thereafter, we see a dramatic increase in bottom-water temperatures starting in the 1990s to about 6°C in the 2010s.

Surface-water temperatures were between 14 and 18°C throughout the transect (Figure 4A) which is somewhat cooler than at the peak of the heat wave during July/August, when temperatures of up to 23°C were recorded in the surface layer (Figure 3A). Both pCO₂ and CH₄ concentrations were closer to equilibrium with the atmosphere in the central basin of the Northern Baltic Proper, but they were elevated in coastal areas (Figures 4C,D). However, off the Swedish coast at 70–100 m water depth, we encountered an upwelling event indicated by low temperatures and high salinity at the water surface (Figures 4A,B) and with elevated pCO₂ of up to >800 μatm (Figure 4C). CH₄ showed only moderately elevated concentrations of 12 nmol/L (Figure 4D). Toward the Finnish coast, both CO₂ and CH₄ gradually increased to concentrations of ∼700 μatm for CO₂ and over 30 nmol/L for CH₄. Partial pressure and concentration vs. seafloor depth plots reveal that in coastal areas, with water depths <40 m, both CO₂ and CH₄ (Figures 5A,B) show more elevated pCO₂ and CH₄ concentrations in the upper mixed layer compared to the open Northern Baltic Proper.

At the easternmost end of the transect, we investigated parts of Finnish Archipelago Sea near the TZS in more detail between September 19–23, 2018. pCO₂ and CH₄ concentrations followed a salinity gradient with higher values toward the freshwater end-member (Figure 6) that is formed by the small river Karjaanjoki (runoff 0.59 km³ y^–1, Räike et al., 2012) that runs into the Pojo Bay and further into the Storfjärden Bay (black dashed box in Figure 8A). The highest pCO₂ (1583 μatm) was measured at the northernmost part of the Storfjärden where salinities were <5 psu. A similar pattern was recorded for CH₄, which reached its highest concentration of 70 nmol/L a bit south from the position where CO₂ peak concentrations were found. An interesting pattern appeared when plotting pCO₂ and CH₄ concentrations against seafloor depth (Figure 7). pCO₂ in surface water were lowest at the shallowest seafloor depth and continuously increased from 751 μatm at <5 m seafloor depth to 1020 μatm at 20–25 m seafloor depth (Figure 7A). In contrast, the high CH₄ concentration of 44 nmol/L (median) was recorded at the lowest seafloor depth, while concentrations ranged between 24 and 40 nmol/L at deeper depths between 5 and 25 m (Figure 7B). Note, that all median pCO₂ values and CH₄ concentrations were calculated before the wind event described in the next section.

Figures 8B,D show the temporal development of pCO₂ and CH₄ concentrations inshore and offshore the TZS coastal area. Elevated pCO₂ of some 800 μatm were recorded by the WEGAS system on September 19 and 20 within the inshore waters (indicated with the black dashed box in Figure 8A) and the offshore pCO₂ (indicated with the red dashed box in Figure 8A) were also at the same elevated range as the inshore concentrations (median of 740–856 μatm; Figure 8B). During sampling on September 21, the wind gradually increased from 5 m sec^–1 to some 10 m sec^–1 and pCO₂ in the inshore waters increased to 1,015 μatm, although this increase is not significant when taking the variability of the measurements in the inshore waters that day (between 700 and 1 300 μatm; Figure 8B) into account. Wave height increased simultaneously from 0.6 to 1.6 m. Wind speeds peaked at 20 m sec^–1 during September 22 and decreased again down to 3 m sec^–1 during the evening hours of September 23 (Figure 8C). During the same time peak wave height of 1.9 m was recorded at the monitoring buoy near TZS, which decreased to 0.5 m. Thus, during the entire period between September 19–23 the average wind was between 8 and 10 m sec^–1 (0.5–0.8 m wave height), disrupted by the peak wind event on September 22 with wave heights of up to 1.9 m. CO₂ sea-air fluxes were greatest during this peak wind event with 274 mmol m^–2 d^–1 (Figure 8C), whereas at the end of the wind event, CO₂ was undersaturated in the upper surface layer and even negative sea-air fluxes were calculated. However, during the entire wind event, the pCO₂ decreased sharply from a 2–3 times oversaturation (median of 1015 μatm) to a slight undersaturation (median of 321 μatm recorded at September 23). Thus, the wind event rather effectively depleted the water column with respect to pCO₂.

In contrast to the expected depletion of CO₂ from oversaturation caused by the wind event, CH₄ remained oversaturated before, during, and also after the wind event. Elevated CH₄ concentrations of 46 – 47 nmol/L (median; Figure 8D) were recorded by the WEGAS system on September 19 and 20 within the inshore waters, whereas for the offshore waters CH₄ concentrations were also supersaturated, but less so, with median concentrations between 13 and 15 nmol/L. During and after the wind event during September 22, CH₄ concentrations decreased in the inshore waters to a median value of 20–24 nmol/L, however, remained oversaturated by 5–8 times. Accordingly, the CH₄ sea-air flux calculations (Figure 8E) revealed the highest CH₄ average fluxes between 2.8 – 3.3 mmol m^–2 d^–1 before the wind event and lower, but still elevated, fluxes between 1.0 – 2.4 mmol m^–2 d^–1 during and after the wind event.

Although wind is directly related to the gas transfer velocity (k) in Equation 3, wind speed could only partly explain the variation in both CO₂ and CH₄ fluxes. However, the highest CH₄ fluxes were observed at relatively low winds (12 m sec^–1), whereas the highest CO₂ fluxes were observed at peak wind speeds (Figures 8C,E). Accordingly, the CO₂ and CH₄ fluxes show positive relationship (not shown), however, the highest CH₄ fluxes were recorded at very low CO₂ flux regimes, which indicates that CH₄ flux events are decoupled from CO₂ flux events during low wind regimes (9–12 m sec^–1).

As indicated in Figures 8C,E, the wind increased significantly after September 21 from less than 10 to nearly 20 m sec^–1. Simultaneously, CTD data collected between September 21 (10:25 UTC) and September 23 (14:00 UTC) show a deepening of the surface mixed layer after the wind event (Figure 9). Note that the temperature (Figure 9B) before and after the wind event (September 22) remained more or less constant at 11°C and that salinity (Figure 9C) decreased only slightly from 6.5 to 6.4 psu, whereas the upper mixed layer depth increased from roughly 10–25 m water depth. Thus, the surface mixed layer increased over these 2 days in depth from about 10 m to nearly 25 m and this was mainly caused by a downwelling event triggered by south-southeasterly winds pressing water masses into the Finnish inner coastal waters rather than by vertical mixing. Thereafter, the mixed layer depth remained at the same depth for the remainder of the cruise.

The concentrations, mixed layer depths, and atmospheric fluxes used to estimate the internal sink/source terms are given in Table 1. We assume that the transition occurs during 24 h corresponding to approximately the time of the storm event and the interval between the two CTD profiles mentioned above. The results of the budget calculation indicate a net sink of CO₂ in the water column, however, the sink could be somewhat overestimated because the offshore concentrations decreased due to the massive outgassing. Further, the budget indicates a CH₄ source of about 2.5 mmol m^–2 d^–1 from below the mixed surface layer.

A possible explanation for the deviating patterns of CO₂ vs. CH₄ fluxes can be found in the geophysical variables investigated and continuously recorded during the cruise. The acoustic mid-water mapping revealed vigorous and widespread gas flares from the sediments in the Finnish coastal waters as exemplified in Figure 10 where some 10–15 major flares and numerous trains of individual bubbles are visible along a 3 km transect, indicating intense bubble formation and ebullition at the sea floor. Further, gas bubbles are visible throughout the entire water column up to the water surface, penetrating into the mixed surface layer that was situated at around 10–15 m water depth (based on CTD data, Figure 9). The mixed layer is also discernable in the echogram, although vaguer than observations made in Arctic Ocean coastal waters (Stranne et al., 2017).