ALBERT

Description: The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20% of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO2−) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani shelf where little NO2− accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO2− producing and consuming processes, and examining the relationship between the overall NO2− balance and active N-loss in the Arabian Sea. Based on a combination of 15N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO2− accumulated in the Central-NE Arabian Sea due to a net production via primarily active nitrate (NO3−) reduction and to a certain extent ammonia oxidation. Meanwhile, NO2− consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH4+) were hardly detectable in this region, though some loss to NO2− oxidation was predicted from modeled NO3− changes. No significant correlation was found between NO2− and N-loss rates (p〉0.05). This discrepancy between NO2− accumulation and lack of active N-loss in the Central-NE Arabian Sea is best explained by the deficiency of organic matter that is directly needed for further NO2− reduction to N2O, N2 and NH4+, and indirectly for the remineralized NH4+ required by anammox. Altogether, our data do not support the long-held view that NO2− accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO2− accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities.

Description: The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20 % of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO2−) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani Shelf where little NO2− accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO2−-producing and consuming processes, and examining the relationship between the overall NO2− balance and active N-loss in the Arabian Sea. Based on a combination of 15N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO2− accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO3−) reduction and to a certain extent ammonia oxidation. Meanwhile, NO2− consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH4+) were hardly detectable in this region, though some loss to NO2− oxidation was predicted from modeled NO3− changes. No significant correlation was found between NO2− and N-loss rates (p〉0.05). This discrepancy between NO2− accumulation and lack of active N-loss in the central-NE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO2− reduction to N2O, N2 and NH4+, and indirectly for the remineralized NH4+ required by anammox. Altogether, our data do not support the long-held view that NO2− accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO2− accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities.

Description: The free energy yield of microbial respiration reactions in anaerobic marine sediments must be sufficient to be conserved as biologically usable energy in the form of ATP. Anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SRR) has a very low standard free energy yield of ΔG∘ = −33 kJ mol−1, but the in situ energy yield strongly depends on the concentrations of substrates and products in the pore water of the sediment. In this work ΔG for the AOM–SRR process was calculated from the pore water concentrations of methane, sulfate, sulfide, and dissolved inorganic carbon (DIC) in sediment cores from different sites of the European continental margin in order to determine the influence of thermodynamic regulation on the activity and distribution of microorganisms mediating AOM–SRR. In the zone of methane and sulfate coexistence, the methane-sulfate transition zone (SMTZ), the energy yield was rarely less than −20 kJ mol−1 and was mostly rather constant throughout this zone. The kinetic drive was highest at the lower part of the SMTZ, matching the occurrence of maximum AOM rates. The results show that the location of maximum AOM rates is determined by a combination of thermodynamic and kinetic drive, whereas the rate activity mainly depends on kinetic regulation.

Description: The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20 % of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO2−) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani Shelf where little NO2− accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO2−-producing and consuming processes, and examining the relationship between the overall NO2− balance and active N-loss in the Arabian Sea. Based on a combination of 15N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO2− accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO3−) reduction and to a certain extent ammonia oxidation. Meanwhile, NO2− consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH4+) were hardly detectable in this region, though some loss to NO2− oxidation was predicted from modeled NO3− changes. No significant correlation was found between NO2− and N-loss rates (p〉0.05). This discrepancy between NO2− accumulation and lack of active N-loss in the central-NE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO2− reduction to N2O, N2 and NH4+, and indirectly for the remineralized NH4+ required by anammox. Altogether, our data do not support the long-held view that NO2− accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO2− accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities.

Description: ACC, antarctic circumpolar current, BARBI model, rossby waves, transport through Drake Passage, coastal kelvin waves, realation windtress transport. - The ACC is the largest ocean current system. This current is driven directly or indirectly by the strong westerly winds, and it is one major topic of this study to investigate the variability of the ACC transport through Drake Passage due to fluctuations in these westerly winds. It is demonstrated, that the relationship between the mean zonal windstress over the southern ocean and the transport through Drake Passage can be described by a simple linear dynamic model, which contains the barotropic and the baroclinic time scales. Another topic of this study is the propagation of Rossby waves under the influence of topography and the connection of the southern ocean with the equatorial and northern latitudes via the interaction of Rossby waves, coastal and equatorial Kelvin waves. Furthermore, the relation between the transport and the bottom pressure on the one hand, and between transport and the meridional difference of potential energy on the other hand is studied further with a channel model.