Spatio-temporal variability and controls on methane and nitrous oxide in the Guadalquivir Estuary, Southwestern Europe

Estuaries are significant methane (CH4) and nitrous oxide (N2O) emitters, although dynamics of both greenhouse gases in these ecosystems are regulated by complex processes. In this work, we aimed at characterizing the spatio-temporal distribution of CH4 and N2O in the Guadalquivir river estuary (SW Spain), the southernmost European estuary. During eight sampling cruises conducted between 2016 and 2017, surface water CH4 and N2O concentrations were measured along the salinity gradient of the estuary by using static-head space equilibration gas chromatography. The CH4 and N2O saturation ranges over the estuarine transect were 520–30,800% (average 2285%) and 40–390% (average 183%), respectively and air–water fluxes ranged from 13 to 1000 µmol m− 2 day− 1(average 66.2 µmol m− 2 day− 1) for CH4 and from − 7 to 35 µmol m− 2 day− 1 (average 8.5 µmol m− 2 day− 1) for N2O. A slight increase in the emissions was detected upstream and no seasonal trends were observed. Mixing between freshwater and oceanic waters influenced biogeochemistry of estuarine waters, affecting CH4 and N2O fluxes. In order to identify potential sources of CH4 and N2O, biogeochemical parameters involved in the formation pathways of both gases, such as salinity, dissolved oxygen, nutrients and organic matter were analyzed. Results suggested that sulfate inhibition and microbial oxidation played a relevant role in dissolved CH4 accumulation in the water column whereas associations found between N2O, nitrate and oxygen indicated that nitrification was a major source of this gas. Therefore, the influence of the tidal-fluvial interaction on ecosystem metabolism regulates trace gas dynamics in the Guadalquivir estuary.This research was funded by the project 1539/2015 from the Spanish Ministry for Agriculture, Food and Environment.Peer reviewe

Rapid Sediment Accumulation Results in High Methane Effluxes from Coastal Sediments.

Globally, the methane (CH4) efflux from the ocean to the atmosphere is small, despite high rates of CH4 production in continental shelf and slope environments. This low efflux results from the biological removal of CH4 through anaerobic oxidation with sulfate in marine sediments. In some settings, however, pore water CH4 is found throughout the sulfate-bearing zone, indicating an apparently inefficient oxidation barrier for CH4. Here we demonstrate that rapid sediment accumulation can explain this limited capacity for CH4 removal in coastal sediments. In a saline coastal reservoir (Lake Grevelingen, The Netherlands), we observed high diffusive CH4 effluxes from the sediment into the overlying water column (0.2-0.8 mol m-2 yr-1) during multiple years. Linear pore water CH4 profiles and the absence of an isotopic enrichment commonly associated with CH4 oxidation in a zone with high rates of sulfate reduction (50-170 nmol cm-3 d-1) both suggest that CH4 is bypassing the zone of sulfate reduction. We propose that the rapid sediment accumulation at this site (~ 13 cm yr-1) reduces the residence time of the CH4 oxidizing microorganisms in the sulfate/methane transition zone (< 5 years), thus making it difficult for these slow growing methanotrophic communities to build-up sufficient biomass to efficiently remove pore water CH4. In addition, our results indicate that the high input of organic matter (~ 91 mol C m-2 yr-1) allows for the co-occurrence of different dissimilatory respiration processes, such as (acetotrophic) methanogenesis and sulfate reduction in the surface sediments by providing abundant substrate. We conclude that anthropogenic eutrophication and rapid sediment accumulation likely increase the release of CH4 from coastal sediments