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

Abstract

Estuaries are important conduits between terrestrial and marine aquatic systems and function as hot spots in the aquatic methane cycle. Eutrophication and climate change may accelerate methane emissions from estuaries, causing positive feedbacks with global warming. Boreal regions will warm rapidly in the coming decades, increasing the need to understand methane cycling in these systems. In this 3-year study, we investigated seasonal and spatial variability of methane dynamics in a eutrophied boreal estuary, both in the water column and underlying sediments. The estuary and the connected archipelago were consistently a source of methane to the atmosphere, although the origin of emitted methane varied with distance offshore. In the estuary, the river was the primary source of atmospheric methane. In contrast, in the adjacent archipelago, sedimentary methanogenesis fueled by eutrophication over previous decades was the main source. Methane emissions to the atmosphere from the study area were highly variable and dependent on local hydrodynamics and environmental conditions. Despite evidence of highly active methanogenesis in the studied sediments, the vast majority of the upwards diffusive flux of methane was removed before it could escape to the atmosphere, indicating that oxidative filters are presently still functioning regardless of previous eutrophication and ongoing climate change.

Influence of electron acceptor availability and microbial community structure on sedimentary methane oxidation in a boreal estuary

“…Our estimates for CH 4 oxidation rates are broadly similar in magnitude to rates of CH 4 production in sediments from the Pojo Bay estuary (Jilbert et al 2018;Myllykangas et al 2020). Thus, our results suggest that sedimentary CH 4 oxidation filters in this estuary and its connected archipelago are functioning efficiently and removing the vast majority of the produced CH 4 in situ, preventing its escape to the water column.…”

“…1). The non-sequential lettering for the sites is used to maintain compatibility with a companion paper detailing the wider CH 4 dynamics in the same system (Myllykangas et al 2020). The sites span a water column salinity gradient of 0-7.…”

“…This work was carried out as part of a parallel project, and data was made available to the current study. The presence of a well-defined shallow (5-15 cm) SMTZ at all muddy sites in the Pojo Bay system (Myllykangas et al 2020) facilitates qualitative comparison of the vertical structure of the microbial communities and CH 4 oxidation rates, regardless of spatial offset in sampling locations. Sediment was sliced at 2.5 cm intervals and stored in 2 mL sterilized plastic vials at -80°C.…”

Baltic Earth Assessment Report on the biogeochemistry of the Baltic Sea

“…7), increasing the likelihood that a significant fraction of accumulated Corg will degrade via methanogenesis. Similar porewater profiles have been observed in other areas of the northern Baltic Sea (Jilbert et al, 2018;Myllykangas et al, 2020a;Sawicka and Bruchert, 2017) indicating a key role for methanogenesis in lowsalinity, eutrophied regions. In contrast to sulfate, the availability of sedimentary Fe oxides is generally greater in the low salinity northern regions, as a result of high supply rates of Fe from peatland-rich catchments (Sarkkola et al, 2013), limited loss to flocculation due to the weak coastal salinity gradient (Jilbert et al, 2018), and less sulfidization of Fe in surface sediments.…”

An Unexpectedly Broad Thermal and Salinity-Tolerant Estuarine Methanogen Community

“…Interest is often primarily because, along with fjords and shallow coastal areas, estuaries are thought to contribute up to 75% of the marine global flux of methane to the atmosphere [ 22 ], despite making up only 16% of the total ocean surface area [ 23 ]. Therefore, in the future, estuaries may become more important methane source environments due to climatic and land use change enhancing nutrient and organic matter flow to them [ 24 ]. Many studies of methanogenesis indicate that the limits of methanogenic activity reach beyond those encountered under, or predicted for, in situ conditions [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ].…”