ALBERT

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 important rivers draining into the North Sea and with the Elbe potentially high amounts of methane could be imported into the water column of the North Sea. Twelve cruises from October 2010 until June 2013 were conducted from Hamburg towards the Elbe mouth 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). The methane oxidation (MOX) rate mirrowed 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 suspendended particulate matter (SPM) was found. Methane oxidation rates were significantly influenced by methane concentration and to a weaker extent by temperature. Methane oxidation accounted for 41 ± 12% of the total loss of methane in summer/fall, but only for 5 ± 3% of the total loss in winter/spring. We applied a modified box model taking into account the residence times of a 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 in the outer Elbe estuary a strong additional input of methane is required, which could be provided by the extensive salt marshes near the river mouth.

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: Microbial methane oxidation rates in ocean and freshwater systems reveal how much of emitted methane from the sediments is oxidized to CO2 and how much can reach the atmosphere directly. The tracer-method using 3H-CH4 provides a way to measure MOX-rates even in water with low methane concentrations without needing any specific instrumentation. We assessed this method by implementing several experiments, collecting data from various environments, and including recent literature concerning the method to identify any uncertainties that should be considered. Our assessment reveals some difficulties of the method but also reassures previous assumptions to be correct. Some of the difficulties are hardly to be avoided, such as incubating all samples at the right in-situ temperature or limiting the variability of MOX-rate measurements in water of low methanotrophic activity. Other details, e.g. quickly measuring the total radioactivity after stopping the incubation, are easy to adapt in each laboratory. And yet other details as shaking during incubation and bottle size seem to be irrelevant. With our study we hope to improve and to encourage future measurements of MOX-rates in different environments and to provide a standard procedure of MOX measurements to make data of MOX better comparable.

Description: Rivers represent a transition zone between terrestric and aquatic environments, as well as a transition zone between methane rich and methane poor environments. Methane concentrations in freshwater systems are in general higher than in marine systems. The Elbe River is one of the important rivers draining into the North Sea, as is the Lena River draining into the Laptev Sea. High methane concentrations have been observed within both rivers, and additional hot spots in the Lena Delta. However due to different stratification patterns in the mixing zones, the further fate of methane in the North Sea and the Laptev Sea is different. Methane consuming bacteria are known from both environments. However, in the transition zone between marine and limnic systems the shift in salinity imposes an osmotic stress for most organisms. In this study we want to compare the environmental data obtained in both estuaries with the methane oxidation to elucidate the efficiency of the respective methane oxidizing bacteria.

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: 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.