Annual variability and regulation of methane and sulfate fluxes in Baltic Sea estuarine sediments

Abstract

Abstract. Marine methane emissions originate largely from near-shore coastal systems, but emission estimates are often not based on temporally well-resolved data or sufficient understanding of the variability of methane consumption and production processes in the underlying sediment. The objectives of our investigation were to explore the effects of seasonal temperature, changes in benthic oxygen concentration, and historical eutrophication on sediment methane concentrations and benthic fluxes at two type localities for open-water coastal versus eutrophic, estuarine sediment in the Baltic Sea. Benthic fluxes of methane and oxygen and sediment pore-water concentrations of dissolved sulfate, methane, and 35 S-sulfate reduction rates were obtained over a 12-month period from April 2012 to April 2013. Benthic methane fluxes varied by factors of 5 and 12 at the offshore coastal site and the eutrophic estuarine station, respectively, ranging from 0.1 mmol m −2 d −1 in winter at an open coastal site to 2.6 mmol m −2 d −1 in late summer in the inner eutrophic estuary. Total oxygen uptake (TOU) and 35 S-sulfate reduction rates (SRRs) correlated with methane fluxes showing low rates in the winter and high rates in the summer. The highest pore-water methane concentrations also varied by factors of 6 and 10 over the sampling period with the lowest values in the winter and highest values in late summerearly autumn. The highest pore-water methane concentrations were 5.7 mM a few centimeters below the sediment surface, but they never exceeded the in situ saturation concentration. Of the total sulfate reduction, 21-24 % was coupled to anaerobic methane oxidation, lowering methane concentrations below the sediment surface far below the saturation concentration. The data imply that bubble emission likely plays no or only a minor role in methane emissions in these sediments. The changes in pore-water methane concentrations over the observation period were too large to be explained by temporal changes in methane formation and methane oxidation rates due to temperature alone. Additional factors such as regional and local hydrostatic pressure changes and coastal submarine groundwater flow may also affect the vertical and lateral transport of methane.

Legacy Effects of Eutrophication on Modern Methane Dynamics in a Boreal Estuary

“…The simultaneous presence of CH 4 and H 2 S and SO 4 2− in the shallow sediments at site L (Fig. 6) indicates a strong overlap of the diagenetic zones in the sediments, as observed previously in the northern Baltic Sea (Sawicka and Brüchert 2017;Jilbert et al 2018). This shows that methanogenesis is occurring in the upper sediments simultaneously with sulfate reduction, potentially due to the use of noncompetitive substrates by the microbial communities (Maltby et al 2018).…”

A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea indicates unprecedented deoxygenation over the 20th century

“…Although the mechanism causing a diagenetic decline in Pr / Ph in this depth interval at the present day remains speculative, a likely candidate is the excess Ph production by methanogenic microbes (Brassel et al, 1981;Venkatesan and Kaplan, 1987;Duan, 2000). Indeed, the current position of the SMTZ at our study site is likely located in the uppermost 10-20 cm below the sediment surface (see Sawicka andBrüchert, 2017 andJilbert et al, 2018, for comparable systems), below which methane concentrations are expected to increase to the millimolar range. As such, Pr / Ph may not be a direct proxy for bottom water oxygenation at our study site.…”

“…6). Due to the fact that the study site is seasonally hypoxic, rather than permanently anoxic or euxinic, the most likely mechanism for Mo enrichment is via diffusion of seawater Mo into the sediment towards the SMTZ (see Helz and Adelson, 2013), which may be ampli-fied by the shuttling of Mo associated with Mn oxides (Algeo and Lyons, 2006;Scheiderich et al, 2010;Scott and Lyons, 2012;Sulu-Gambari et al, 2017). It is thus plausible that the scavenging of Mo mostly takes place close to the sediment surface at the end of summer stratification period when the SMTZ reaches its shallowest position in the sediment column (Mogollón et al, 2011) and shuttling and refluxing of Mn is expected to be at its annual maximum.…”

High Emissions of Carbon Dioxide and Methane From the Coastal Baltic Sea at the End of a Summer Heat Wave

“…Note that the global estimates of coastal gas emissions are biased toward the many investigations focusing on estuaries, lagoons, salt marches, and tidal flats (Borges and Abril, 2012), whereas studies capturing shorelines that are less affected by river inputs are less studied. Further, CH 4 emissions in coastal areas are clearly coupled to temperature (Borges et al, 2016;Sawicka and Brüchert, 2017) and the highest rates of change are to be expected in organicrich shallower parts affected by warming water bodies overlaying marine sediments. In fact, experimental studies show that a temperature increase of only 2 • C can increase anaerobic organic matter degradation by 40% in marine sediments (Roussel et al, 2015).…”