Pathways of methane removal in the sediment and water column of a seasonally anoxic eutrophic marine basin
Olga M. Żygadłowska, Jessica Venetz, Robin Klomp, Wytze K. Lenstra, Niels A. G. M. van Helmond, Thomas Röckmann, Anna J. Wallenius, Paula Dalcin Martins, Annelies J. Veraart, Mike S. M. Jetten, Caroline P. Slomp
Abstract
Methane (CH 4 ) is a key greenhouse gas. Coastal areas account for a major proportion of marine CH 4 emissions. Eutrophication and associated bottom water hypoxia enhance CH 4 production in coastal sediments. Here, we assess the fate of CH 4 produced in sediments at a site in a seasonally anoxic eutrophic coastal marine basin (Scharendijke, Lake Grevelingen, the Netherlands) in spring (March) and late summer (September) in 2020. Removal of CH 4 in the sediment through anaerobic oxidation with sulfate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im1"><mml:mrow><mml:mo stretchy="false">(</mml:mo><mml:mtext>S</mml:mtext><mml:msubsup><mml:mtext>O</mml:mtext><mml:mn>4</mml:mn><mml:mrow><mml:mn>2</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msubsup><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:math> is known to be incomplete in this system, as confirmed here by only slightly higher values of δ 13 C-CH 4 and δD-CH 4 in the porewater in the shallow sulfate-methane-transition zone (~5-15 cm sediment depth) when compared to deeper sediment layers. In March 2020, when the water column was fully oxygenated, CH 4 that escaped from the sediment was at least partially removed in the bottom water through aerobic oxidation. In September 2020, when the water column was anoxic below ~35 m water depth, CH 4 accumulated to high concentrations (up to 73 µmol L -1 ) in the waters below the oxycline. The sharp counter gradient in oxygen and CH 4 concentrations at ~35 m depth and increase in δ 13 C-CH 4 and δD-CH 4 above the oxycline indicate mostly aerobic water column removal of CH 4 . Water column profiles of particulate and dissolved Fe and Mn suggest redox cycling of both metals at the oxycline, pointing towards a potential role of metal oxides in CH 4 removal. Water column profiles of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im2"><mml:mrow><mml:mtext>N</mml:mtext><mml:msubsup><mml:mtext>H</mml:mtext><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im3"><mml:mrow><mml:mtext>N</mml:mtext><mml:msubsup><mml:mtext>O</mml:mtext><mml:mn>3</mml:mn><mml:mo>-</mml:mo></mml:msubsup></mml:mrow></mml:math> indicate removal of both solutes near the oxycline. Analyses of 16S rRNA gene sequences retrieved from the water column reveal the presence of aerobic CH 4 oxidizing bacteria ( Methylomonadaceae ) and anaerobic methanotrophic archaea ( Methanoperedenaceae ), with the latter potentially capable of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im4"><mml:mrow><mml:mtext>N</mml:mtext><mml:msubsup><mml:mtext>O</mml:mtext><mml:mn>3</mml:mn><mml:mo>-</mml:mo></mml:msubsup></mml:mrow></mml:math> and/or metal-oxide dependent CH 4 oxidation, near the oxycline. Overall, our results indicate sediment and water column removal of CH 4 through a combination of aerobic and anaerobic pathways, which vary seasonally. Some of the CH 4 appears to escape from the surface waters to the atmosphere, however. We conclude that eutrophication may make coastal waters a more important source of CH 4 to the atmosphere than commonly assumed.