ALBERT

Description: River estuaries are responsible for high rates of methane emissions to the atmosphere. The complexity and diversity of estuaries require detailed investigation of methane sources and sinks, as well as of their spatial and seasonal variations. The Elbe river estuary and the adjacent North Sea were chosen as the study site for this survey, which was conducted from October 2010 to June 2012. Using gas chromatography and radiotracer techniques, we measured methane concentrations and methane oxidation (MOX) rates along a 60 km long transect from Cuxhaven to Helgoland. Methane distribution was influenced by input from the methane-rich mouth of the Elbe and gradual dilution by methane-depleted sea water. Methane concentrations near the coast were on average 30 ± 13 nmol L−1, while in the open sea, they were 14 ± 6 nmol L−1. Interestingly, the highest methane concentrations were repeatedly detected near Cuxhaven, not in the Elbe River freshwater end-member as previously reported. Though, we did not find clear seasonality we observed temporal methane variations, which depended on temperature and presumably on water discharge from the Elbe River. The highest MOX rates generally coincided with the highest methane concentrations, and varied from 2.6 ± 2.7 near the coast to 0.417 ± 0.529 nmol L−1 d−1 in the open sea. Turnover times varied from 3 to 〉1000 days. MOX rates were strongly affected by methane concentration, temperature and salinity. We ruled out the supposition that MOX is not an important methane sink in most of the Elbe estuary and adjacent German Bight.

Description: ABSTRACT: Salinity is an important environmental control of aerobic methane oxidation, which reduces the emission of the potent greenhouse gas methane into the atmosphere. The effect of salinity on methane oxidation is especially severe in river estuaries and adjacent coastal waters, which are important sources of methane emission and, at the same time, are usually characterized by pronounced salinity gradients. Using methane oxidation rates determined by a radiotracer technique as a measure of methanotrophic activity, we tested the effect of immediate and gradual salinity changes on pure cultures of methanotrophic bacteria, and natural freshwater (Elbe River) and natural marine (North Sea) methanotrophic populations. According to our results, Methylomonas sp. and Methylosinus trichosporium are resistant to an increase in salinity, whereas Methylovulum sp. and Methylobacter luteus are sensitive to such an increase. Natural methanotrophic populations from freshwater are more resistant to an increase in salinity than those from marine water are to a decrease in salinity. In contrast to an immediate change of salinity, gradual change (1.25 PSU d−1) can attenuate salinity stress. Experiments with the natural populations revealed different reactions to changes in salinity; thus, we assume that the initial composition of the methanotrophic population, i.e. the ratio of sensitive versus resistant strains, also governs the community response to salinity stress.Repeated experiments with the natural populations revealed different reactions to changes of salinity; thus we assume that the initial composition of the methanotrophic population, i.e. the ratio of sensitive and resistant strains, also governs the community response to salinity stress.

Description: We conducted multiple small (2011–2012) and one large sampling campaign (2013) at selected profiles along the Elbe River. With the data we were able to outline spatial and temporal variability of methane concentration, oxidation and emissions in one of the major rivers of Central Europe. The highest methane concentrations were found in human-altered riverine habitats, i.e., in a harbor (1,888 nmol L−1), in a lock and weirs (1409 ± 1545 nmol L−1), and in general in the whole “impounded” river segment (383 ± 215 nmol L−1). On the other hand, the lowest methane concentrations were found in the “lowland” river segment (86 ± 56 nmol L−1). The methane oxidation rate was more efficient in the “natural” segment (71 ± 113 nmol L−1day−1, which means a turnover time of 49 ± 83 day−1) than in the “lowland” segment (4 ± 3 nmol L−1day−1, which means a turnover time of 39 ± 45 day−1). Methane emissions from the surface water into the atmosphere ranged from 0.4 to 11.9 mg m−2 day−1 (mean 2.1 ± 0.6 mg m−2 day−1) with the highest CH4 emissions at the Meissen harbor (94 kg CH4 year−1). Such human-altered riverine habitats (i.e., harbors and similar) have not been taken into consideration in the CH4 budget before, despite them being part of the river ecosystems, they may be significant CH4 hot-spots. The total CH4 diffusive flux from the whole Elbe was estimated to be approximately 97 t CH4 year−1.

Description: Shelf sea areas are the primary oceanic source for methane release, the most abundant hydrocarbon in the atmosphere. As such, the southern North Sea’s methane concentration is mainly determined by river runoff and tidal marshes. Within such a highly variable temperate estuary, this study is the first to reveal detailed information on the in situ activity, abundance and community structure of methane oxidizing bacteria along a transect from the marine environment near Helgoland island to the riverine harbor of Hamburg, Germany. The in situ methane oxidation rate was determined with a radio tracer, and methane concentration with the head-space method. Abundance and diversity of the methanotrophic bacterial community in the water column was assessed with quantitative polymerase chain reaction for the particulate methane monooxygenase and monooxygenase intergenic spacer analysis. Median abundances ranged from 2.8 × 104 cells l−1 in the marine environment to 7.5 × 105 cells l−1 in the riverine environment. Except for salinity, no conclusive linear correlation between any environmental parameter and the abundance of methanotrophs could be determined. Relating activity with abundance of methanotrophs showed that about 70% of the population is inactive, especially in the coastal and marine environment. This study found distinct operational taxonomic unit (OTU) community compositions among the 3 environmental categories (river, coast, marine). Several identified OTUs have been reported previously and imply a wide geographic occurrence. Overall, we propose that salinity is the most important driver of differing communities in the riverine, coastal and marine environment.

Description: Rivers represent a transition zone between terrestric and aquatic environments, and between methane rich and methane poor environments. The Elbe River is one of the most important rivers draining into the North Sea and, along with the Elbe, a potential importer of high amounts of methane into the water column of the North Sea. Twelve sampling cruises from October 2010 until June 2013 were conducted from Hamburg towards the mouth of the Elbe at Cuxhaven. The dynamic of methane concentration in the water column and its consumption via methane oxidation was measured. In addition, physico-chemical parameters were used to estimate their influence on the methanotrophic activity. We observed high methane concentrations at the stations in the area of Hamburg harbor (“inner estuary”) and about 10 times lower concentrations in the outer estuary (median of 416 versus 40 nmol/L, respectively). The methane oxidation (MOX) rate mirrored the methane distribution with high values in the inner estuary and low values in the outer estuary (median of 161 versus 10 nmol/L/d, respectively). Methane concentrations were significantly influenced by the river hydrology (falling water level) and the trophic state of the water (biological oxygen demand). In contrast to other studies no clear relation to the amount of suspended particulate matter (SPM) was found. Methane oxidation rates were significantly influenced by methane concentration and to a lesser extent by temperature. Methane oxidation accounted for 41 ± 12% of the total loss of methane in the summer/fall, but for only 5 ± 3% of the total loss in the winter/spring. We applied a modified box model taking into account the residence times of each water parcel depending on discharge and tidal impact. We observed almost stable methane concentrations in the outer estuary, despite a strong loss of methane through diffusion and oxidation. Thus, we postulate that the water column undergoes a balancing out in the outer Elbe estuary due to a strong additional input of methane, which could be provided by the extensive salt marshes near the river mouth.