WP4 - deliverable No. 2: Methane emission through sediment-water and sea-air interfaces
Volker Brüchert, Timothy G. Ferdelman, Henrik Fossing, Wanda Gülzow, Laura Lapham, Gregor Rehder, Nguyen Thanh Manh, Jens Schneider von Deimling, Torben Gentz, Michael Schlüter
Synopsis
An array of sampling and direct measurement methods were applied to determine fluxes of methane from the sediment to the bottom water and across the sea surface in coastal and open-sea Baltic waters. Benthic fluxes and fluxes to the sediment-water interface were derived from porewater gradients of methane at 85 stations and direct benthic flux investigations at selected stations. Sea-air exchange was quantified from data of an autonomous measurement system that was installed on the ferry M/S FINNMAID in November 2009 to measure methane and carbon dioxide concentration in the surface water. The ferry regularly commutes between Travemünde (Germany), Gdynia (Poland) and Helsinki (Finland) (Gülzow et al., 2011). The continuous measurements yielded surface water methane concentration data at a frequency of 2 to 3 days and provided first insights into seasonal and small-scale variability. These data were combined with atmospheric data on temperature, pressure, and wind speed to calculate sea-air fluxes. Direct sea-air fluxes of methane were also determined with floating chambers in near-shore areas of the Southern Stockholm archipelago and the Himmerfjärden estuary.
Sediment-water-fluxes
Whole core incubations of sediment cores from Himmerfjärden and data from pore water measurements of methane indicate that in most areas of the Baltic, diffusive fluxes of methane to the sediment surface control transport rates. The data indicate that methane oxidation (aerobic and anaerobic) generally removes more than 90% of the upward methane flux entering the water column from the sediment. Benthic diffusive fluxes in the whole Baltic Sea basin ranged from values as low as 2.7 µmol m-2 d-1 in the central Bothnian Bay to 1187 µmol m-2 d-1 in the eutrophied inner Himmerfjärden estuary. Benthic fluxes in the anoxic basins, e.g., the Gotland Basin, were comparable to inshore fluxes and as high as 566 µmol m-2 d-1.
Based on acoustic measurements using Merian’s multibeam echosounder system (see WP 3.3), fluxes were estimated at the active seep areas, e.g., Bothnian Bay and Mecklenburg Bight. Based on estimated rise velocities and bubble frequency in these areas, bubble flux may account for a transport of about 6500 µmol CH4 per day to the water column.
Sea-to-air fluxes
Water measurements in the offshore areas of the Baltic Sea generally show high methane concentrations in the basins at depth (up to 820nM, Gotland Basin) and lower concentrations close to the sea surface. Large areas of the surface water are oversaturated in methane (up to 250%, max. values 550%) with respect to atmospheric equilibrium. Storm and upwelling events had a strong effect on methane concentrations. Additional surface water methane measurements during the cruises Pos392 and MSM16 revealed hotspots of methane emission in the Lübeck Bight, the Arkona Basin, the western Gulf of Riga, and the Åland basin. First flux estimations of selected areas of the Baltic Sea show large seasonal and regional variability. In the open water of the Gulf of Finland fluxes ranged from 0.7 µmol m-2 d-1 during April to 14 µmol m-2 d‑1 in October, with generally lower fluxes during spring and summer. The lowest fluxes were determined in the Gotland Basin ranging from 0.7 during April to 2.5 µmol m-2 d-1 in October.
Figure 1. Seasonal variation in sea surface fluxes determined for different regions of the Baltic along the path of the Finnlines vessel M/S FINNMAID (Gülzow et al., to be subm.)
Direct measurements of the sea-air fluxes in the inshore areas on the Swedish coast in the southern Stockholm archipelago focused on the Himmerfjärden estuary and neighbouring small fjords and basins. Sea-to-air fluxes in water depths from 3 to 75 m depth measured in June 2011 and October 2011 ranged from 6 to 566 µmol CH4 m-2 d-1, with an average of 119 ± 19 µmol CH4 m-2 d-1. Methane fluxes showed a weak negative correlation with water depth. The highest flux (556 µmol m-2 d-1) was measured at the edge of densely vegetated shore areas. Bubble shield experiments at 4 shallow sites in depths less than 5 meters indicated that a substantial portion of the total flux (up to 84%) can be attributed to bubble flux. Owing to the discrete nature of the measurement method, intermittent bubble emissions with significantly higher emissions are implicated from the results, but could not be proven directly. Inshore measurements in the eutrophied inner Himmerfjärden revealed clear methane surface maxima, which are likely due to discharge of methane from a local sewage treatment. These concentrations were significantly higher than concentrations measured bottom waters over a whole summer-fall measuring campaign and suggest that a significant part of the methane is not derived from benthic emissions. Lateral advection and surface production of methane are alternative sources.
Figure 2. Chamber-derived methane fluxes (µmol m-2 d-1) determined in June and October 2011 from the inner part of the Himmerfjärden estuary (H6) to the outermost area (H2) (Brüchert et al., to be submitted).
Comparison of the benthic flux data with data of sea-to-air fluxes determined in this study and with literature data imply that most of the benthic methane is oxidized in the water column by aerobic and to some extent, anaerobic methane oxidation. This is also true for the anoxic Gotland Basin and Landsort Deep. Secondly, for most time periods, the sea-to air flux is not proportional or spatially or temporally related to the benthic diffusive flux. The same holds true for most inshore waters. Notable exceptions are very shallow, densely vegetated littoral areas where the bubble sea-air flux is close to the benthic efflux and the few areas of the Baltic in the Gulf of Finland, the Bothnian Bay, the Gdansk Basin, and the Eckernförde Bight, where active bubble escape occurs. Thirdly, the existing continuous measurements point to the strong control of sea-air fluxes by atmospheric conditions. Storm events and resulting mixing of the water column lead to a temporally significant enhancement of the sea-to-air flux.
The present data confirm the need for continuous long-term measurements at various sites to establish the temporal and spatial variability of methane emissions. The inner coastal zone with its high natural and anthropogenic carbon load remains insufficiently covered – both temporally and spatially. Here, continuous measurement systems that determine the benthic and atmospheric flux of methane need to be installed in the future.
Summary including management aspects
- Continuous measurements detected local hotspots of potential methane emission from surface water methane supersaturation.
- Inshore shallow water areas are also potential hotspots for methane emission; these emissions are partly associated with treated sewage discharge rather than benthos-derived;
- Seep-associated areas are rare in the Baltic proper and do not significantly contribute to the total benthic and atmospheric flux.
- There are strong temporal and spatial variations in the flux from the sea surface that are possibly associated with bubble emissions.
- Future monitoring of methane requires the installation of more ship-based and coast-based continuous measurement systems to extend areal and temporal coverage
- In-shore monitoring stations need to be installed to unravel the potentially large temporal variation
Literature
Gülzow, W., Rehder, G., Schneider v. Deimling, J., Seifert, T., Toth, Z. (to be subm) One year of continuous measurements constraining methane emissions from the Baltic Sea to the atmosphere using a ship of opportunity.
Brüchert, V., Bastviken, D., Gentz, T., Schlueter, M., Nguyen, T.M., Ferdelman, T.G. (to be subm.) Benthic sea-to-air fluxes of methane in a brackish coastal estuary in the Baltic Sea.