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Hermann W. Bange - One of the best experts on this subject based on the ideXlab platform.
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Origin and fate of the secondary nitrite maximum in the Arabian Sea
Biogeosciences, 2011Co-Authors: Phyllis Lam, Hermann W. Bange, Gaute Lavik, M. M. Jensen, A. Kock, K. A. Lettmann, Y. Plancherel, Marcel M M KuypersAbstract:Abstract. 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.
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The nitrogen cycle in the Arabian Sea
Progress in Oceanography, 2005Co-Authors: Hermann W. Bange, S. Wajih A. Naqvi, L.a. CodispotiAbstract:Despite their importance for the global oceanic nitrogen (N) cycle, estimates of N fluxes in the Arabian Sea remain in considerable uncertainty. In this report, we summarize current knowledge of important processes, including denitrification, N2 fixation and nitrous oxide emissions. Additionally, we discuss anthropogenic impacts on the N cycle in the region. Existing studies suggest that the Arabian Sea is a significant source of N2O, and a major sink for fixed-N mainly due to enhanced rates of denitrification that occur in suboxic portions of the water column in the Arabian Sea. Sedimentary denitrification is small compared to water column denitrification, and additions of fixed-N via N2 fixation also are small compared to pelagic denitrification. As a consequence, the fixed-N budget of the Arabian Sea is dominated by an advective supply from the south, and by the sink arising from pelagic denitrification. Although relatively small compared to the advective supply, inputs of fixed-N from runoff and from the atmosphere may have significant impacts on surface waters and on the coastal waters of western India, and these inputs are rising because of human activities. Overall, the Arabian Sea’s nitrogen cycle is likely to respond sensitively to climate change and, in turn, have an impact on climate via its N2O and denitrification components.
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Biogeochemical ocean-atmosphere transfers in the Arabian Sea
Progress in Oceanography, 2005Co-Authors: S. Wajih A. Naqvi, Hermann W. Bange, Stuart W. Gibb, Catherine Goyet, Angela D. Hatton, Robert C. Upstill-goddardAbstract:Transfers of some important biogenic atmospheric constituents, carbon dioxide (CO2), methane (CH4), molecular nitrogen (N2), nitrous oxide (N2O), nitrate View the MathML source(NO3-), ammonia (NH3), methylamines (MAs) and dimethylsulphide (DMS), across the air–Sea interface are investigated using published data generated mostly during the Arabian Sea Process Study (1992–1997) of the Joint Global Ocean Flux Study (JGOFS). The most important contribution of the region to biogeochemical fluxes is through the production of N2 and N2O facilitated by an acute, mid-water deficiency of dissolved oxygen (O2); emissions of these gases to the atmosphere from the Arabian Sea are globally significant. For the other constituents, especially CO2, even though the surface concentrations and atmospheric fluxes exhibit extremely large variations both in space and time, arising from the unique physical forcing and associated biogeochemical environment, the overall significance in terms of their global fluxes is not much because of the relatively small area of the Arabian Sea. Distribution and air–Sea exchanges of some of these constituents are likely to be greatly influenced by alterations of the subsurface O2 field forced by human-induced eutrophication and/or modifications to the regional hydrography
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Nitrous oxide emissions from the Arabian Sea: A synthesis
Atmospheric Chemistry and Physics, 2001Co-Authors: Hermann W. Bange, Meinrat O. Andreae, Tim Rixen, Shyam Lal, Cliff S. Law, S. W. A. Naqvi, Prabir K. Patra, Robert C. Upstill-goddardAbstract:Abstract. We computed high-resolution (1o latitude x 1o longitude) Seasonal and annual nitrous oxide (N2O) concentration fields for the Arabian Sea surface layer using a database containing more than 2400 values measured between December 1977 and July 1997. N2O concentrations are highest during the southwest (SW) monsoon along the southern Indian continental shelf. Annual emissions range from 0.33 to 0.70 Tg N2O and are dominated by fluxes from coastal regions during the SW and northeast monsoons. Our revised estimate for the annual N2O flux from the Arabian Sea is much more tightly constrained than the previous consensus derived using averaged in-situ data from a smaller number of studies. However, the tendency to focus on measurements in locally restricted features in combination with insufficient Seasonal data coverage leads to considerable uncertainties of the concentration fields and thus in the flux estimates, especially in the coastal zones of the northern and eastern Arabian Sea. The overall mean relative error of the annual N2O emissions from the Arabian Sea was estimated to be at least 65%.
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Nitrous oxide emissions from the Arabian Sea: A synthesis
Atmospheric Chemistry and Physics Discussions, 2001Co-Authors: Hermann W. Bange, Meinrat O. Andreae, Shyam Lal, Cliff S. Law, S. W. A. Naqvi, Prabir K. Patra, T. Rixen, Robert C. Upstill-goddardAbstract:We computed high-resolution (1o latitude × 1o longitude) Seasonal and annual nitrous oxide (N2O) concentration fields for the Arabian Sea surface layer using a database containing more than 2400 values measured between December 1977 and July 1997. N2O concentrations are highest during the southwest (SW) monsoon along the southern Indian continental shelf. Annual emissions range from 0.33 to 0.70 Tg N2O and are dominated by fluxes from coastal regions during the SW and northeast monsoons. However, the tendency to focus on measurements in locally restricted features in combination with insufficient Seasonal data coverage leads to considerable uncertainties of the concentration fields and thus in the flux estimates, especially in the coastal zones of the northern and eastern Arabian Sea.
T. G. Prasad - One of the best experts on this subject based on the ideXlab platform.
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Structure and mechanisms of the Arabian Sea variability during the winter monsoon
Deep Sea Research Part I: Oceanographic Research Papers, 2005Co-Authors: T. G. Prasad, Motoyoshi Ikeda, Julie L. MccleanAbstract:Abstract In the southern Arabian Sea (between the Equator and 10°N), the shoaling of isotherms at subsurface levels (20 °C isotherm depth is located at ∼90 m) leads to cooling at 100 m by 2–3 °C relative to surrounding waters during the winter monsoon. The annual and interannual variations of this upwelling zone, which we call the Arabian Sea dome (ASD), are studied using results from an eddy-permitting ocean general circulation model in conjunction with hydrography and TOPEX/ERS altimeter data. The ASD first appears in the southeastern Arabian Sea during September–October, maturing during November–December to extend across the entire southern Arabian Sea (along ∼5°N). It begins to weaken in January and dissipates by March in the southwestern Arabian Sea. From the analysis of heat-budget balance terms and a pair of model control experiments, it is shown that the local Ekman upwelling induced by the positive wind-stress curl of the winter monsoon generates the ASD in the southeastern Arabian Sea. The ASD decays due to the weakening of the cyclonic curl of the wind and the westward penetration of warm water from the east (Southern Arabian Sea High). The interannual variation of the ASD is governed by variations in the Ekman upwelling induced by the cyclonic wind-stress curl. Associated with the unusual winds during 1994–1995 and 1997–1998 Indian Ocean dipole (IOD) periods, the ASD failed to develop. In the absence of the ASD during the IOD events, the 20 °C isotherm depth was 20–30 m deeper than normal in the southern Arabian Sea resulting in a temperature increase at 97 m of 4–5 °C. An implication is that the SST evolution in the southern Arabian Sea during the winter monsoon is primarily controlled by advective cooling: the shoaling of isotherms associated with the ASD leads to SST cooling.
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A numerical study of the Seasonal variability of Arabian Sea high‐salinity water
Journal of Geophysical Research, 2002Co-Authors: T. G. Prasad, Moto IkedaAbstract:[1] Using a level 2 three-dimensional turbulent closure model, the physical processes affecting the upper-ocean salinity budget in the Arabian Sea have been studied with particular focus on the Seasonal variability of the Arabian Sea High-Salinity Water (ASHSW). The volume integrated salt budget terms for four subdomains in the Arabian Sea (south of 10.5°N, north of 10.5°N, east of 62.5°E, and west of 62.5°E) shows a clear balance between the horizontal advection and evaporative freshwater loss in the annual mean. The horizontal advection dominates the Seasonal cycle in salinity. The Seasonal balance is clear, when the terms of the salt budget are integrated between 10.5°N and 2.5°N and east of 62.5°E. In these subdomains the two major episodes of horizontal advection during winter and summer are both associated with the Seasonal reversal of the Arabian Sea circulation. Model estimation of the upper-ocean salinity budget terms integrated between the surface and 100 m for the winter and summer monsoons agree with a qualitative description of the ASHSW variability. After formation in the northern Arabian Sea during the winter monsoon, the ASHSW spreads predominantly southward along the eastern boundary during the summer monsoon. Advection causes salinity to increase by 1.0 kg m−2 day−1 along these regions. At this time, the poleward advection of low-salinity water from the south (and upwelling regions) by the Somali current causes the salinity along the western Arabian Sea to decrease by 0.6 kg m−2 day−1. During the winter monsoon, westward advection of the fresher Bay of Bengal water by the North Equatorial current (NEC) causes a salinity decrease by ∼1.0 kg m−2 day−1 in the region south of 10.5°N. The combined effect of precipitation and evaporation tends to increase the salinity in the Arabian Sea, and horizontal advection is found to be important for maintaining the observed Seasonal cycle.
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Seasonal spreading of the Persian Gulf Water mass in the Arabian Sea
Journal of Geophysical Research: Oceans, 2001Co-Authors: T. G. Prasad, Motoyoshi Ikeda, S. Prasanna KumarAbstract:The characteristics of the subsurface salinity maximum associated with the Persian Gulf Water mass (PGW) are used to quantify the spreading and mixing of PGW in the thermocline of the Arabian Sea based on a bimonthly climatology of temperature and salinity. Examination of the Seasonal cycles of heat and freshwater fluxes in the Persian Gulf region indicates that PGW forms as a result of elevated evaporative cooling in conjunction with reduced insolation during winter. Maps are presented of the distributions of depth, salinity, and geostrophic flow on σθ = 26.5, which nearly coincides with the core of the PGW. After intense mixing in the Strait of Hormuz, the property fields suggest that warm (>17°C) and high-salinity (>36.2 psu) PGW enters the Arabian Sea to form a subsurface salinity extremum between 200 and 300 m. We have found variability in the distribution of PGW in the Arabian Sea associated with monsoonal changes in the Arabian Sea circulation. During the winter monsoon, there is southward spreading of PGW along the western boundary; during summer it is not present. Lateral mixing with low-salinity water from the Bay of Bengal in the region south of 10°N and along the west coast of India during winter accounts for changes in the characteristics of PGW along these paths. Associated with the Findlater Jet during summer, the entire thermohaline structure is vertically displaced along the coasts of Somalia and Arabia. Ekman convergence in the central Arabian Sea accounts for deepening of the PGW. Either lateral or vertical mixing would cause changes in PGW properties in these regions. During this time, PGW spreads predominantly southward along the central Arabian Sea, as indicated by a tongue of high salinity.
Marcel M M Kuypers - One of the best experts on this subject based on the ideXlab platform.
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benthic nitrogen loss in the Arabian Sea off pakistan
Frontiers in Microbiology, 2012Co-Authors: Sarah Sokoll, Moritz Holtappels, Phyllis Lam, Gavin Collins, Michael Schluter, Gaute Lavik, Marcel M M KuypersAbstract:A pronounced deficit of nitrogen (N) in the oxygen minimum zone (OMZ) of the Arabian Sea suggests the occurrence of heavy N-loss that is commonly attributed to pelagic processes. However, the OMZ water is in direct contact with sediments on three sides of the basin. Contribution from benthic N-loss to the total N-loss in the Arabian Sea remains largely unassessed. In October 2007, we sampled the water column and surface sediments along a transect cross-cutting the Arabian Sea OMZ at the Pakistan continental margin, covering a range of station depths from 360 to 1430 m. Benthic denitrification and anammox rates were determined by using 15N-stable isotope pairing experiments. Intact core incubations showed declining rates of total benthic N-loss with water depth from 0.55 to 0.18 mmol N m-2 d-1. While denitrification rates measured in slurry incubations decreased from 2.73 to 1.46 mmol N m-2 d-1 with water depth, anammox rates increased from 0.21 to 0.89 mmol N m-2 d-1. Hence, the contribution from anammox to total benthic N-loss increased from 7% at 360 m to 40% at 1430 m. This trend is further supported by the quantification of nirS, the biomarker functional gene encoding for cytochrome cd1-nitrite reductases of microorganisms involved in both N-loss processes. Anammox-like nirS genes within the sediments increased in proportion to total nirS gene copies with water depth. Moreover, phylogenetic analyses of nirS revealed different communities of both denitrifying and anammox bacteria between shallow and deep stations. Together, rate measurement and nirS analyses showed that anammox, determined for the first time in the Arabian Sea sediments, is an important benthic N-loss process at the continental margin off Pakistan, especially in the sediments at deeper water depths. Extrapolation from the measured benthic N-loss to all shelf sediments within the basin suggests that benthic N-loss may be responsible for about half of the overall N-loss in the Arabian Sea.
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Origin and fate of the secondary nitrite maximum in the Arabian Sea
Biogeosciences, 2011Co-Authors: Phyllis Lam, Hermann W. Bange, Gaute Lavik, M. M. Jensen, A. Kock, K. A. Lettmann, Y. Plancherel, Marcel M M KuypersAbstract:Abstract. 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.
S. Prasanna Kumar - One of the best experts on this subject based on the ideXlab platform.
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Seasonal spreading of the Persian Gulf Water mass in the Arabian Sea
Journal of Geophysical Research: Oceans, 2001Co-Authors: T. G. Prasad, Motoyoshi Ikeda, S. Prasanna KumarAbstract:The characteristics of the subsurface salinity maximum associated with the Persian Gulf Water mass (PGW) are used to quantify the spreading and mixing of PGW in the thermocline of the Arabian Sea based on a bimonthly climatology of temperature and salinity. Examination of the Seasonal cycles of heat and freshwater fluxes in the Persian Gulf region indicates that PGW forms as a result of elevated evaporative cooling in conjunction with reduced insolation during winter. Maps are presented of the distributions of depth, salinity, and geostrophic flow on σθ = 26.5, which nearly coincides with the core of the PGW. After intense mixing in the Strait of Hormuz, the property fields suggest that warm (>17°C) and high-salinity (>36.2 psu) PGW enters the Arabian Sea to form a subsurface salinity extremum between 200 and 300 m. We have found variability in the distribution of PGW in the Arabian Sea associated with monsoonal changes in the Arabian Sea circulation. During the winter monsoon, there is southward spreading of PGW along the western boundary; during summer it is not present. Lateral mixing with low-salinity water from the Bay of Bengal in the region south of 10°N and along the west coast of India during winter accounts for changes in the characteristics of PGW along these paths. Associated with the Findlater Jet during summer, the entire thermohaline structure is vertically displaced along the coasts of Somalia and Arabia. Ekman convergence in the central Arabian Sea accounts for deepening of the PGW. Either lateral or vertical mixing would cause changes in PGW properties in these regions. During this time, PGW spreads predominantly southward along the central Arabian Sea, as indicated by a tongue of high salinity.
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Mechanism of the biological response to winter cooling in the northeastern Arabian Sea
Nature, 1996Co-Authors: M. Madhupratap, M. Dileep Kumar, Seshagiri Raghukumar, K.k.c. Nair, P M A Bhattathiri, S. Prasanna Kumar, Nagappa RamaiahAbstract:THE Arabian Sea is one of the most biologically productive ocean regions1, mainly due to the upwelling of nutrients during the summer (southwest) monsoon. But the northern Arabian Sea continues to sustain fairly high biological production after the upwelling Season and during much of the winter (northeast) monsoon2–4. The processes that enable this high winter productivity have hitherto been poorly understood, being variously attributed to surface cooling effects2,3 or wind-driven changes in ocean circulation4. Here we present physical, chemical and biological data that indicate that Sea surface cooling drives convection processes that lead to the injection of nutrients up into the surface waters of the northeastern Arabian Sea during winter, and that this mechanism of nutrient supply is a dominant control on winter productivity. Observed Seasonal changes in bacterial and microzooplankton populations may provide an explanation for the Arabian Sea 'paradox'5–8 that mesozooplank-ton biomass remains more or less invariable throughout the year.
Venugopalan Ittekkot - One of the best experts on this subject based on the ideXlab platform.
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A revised nitrogen budget for the Arabian Sea
Global Biogeochemical Cycles, 2000Co-Authors: Hermann W. Bange, Tim Rixen, Anne M. Johansen, Ronald L. Siefert, Ramachandran Ramesh, Venugopalan Ittekkot, Michael R. Hoffmann, Meinrat O. AndreaeAbstract:Despite its importance for the global oceanic nitrogen (N) cycle, considerable uncertainties exist about the N fluxes of the Arabian Sea. On the basis of our recent measurements during the German Arabian Sea Process Study as part of the Joint Global Ocean Flux Study (JGOFS) in 1995 and 1997, we present estimates of various N sources and sinks such as atmospheric dry and wet depositions of N aerosols, pelagic denitrification, nitrous oxide (N2O) emissions, and advective N input from the south. Additionally, we estimated the N burial in the deep Sea and the sedimentary shelf denitrification. On the basis of our measurements and literature data, the N budget for the Arabian Sea was reassessed. It is dominated by the N loss due to denitrification, which is balanced by the advective input of N from the south. The role of N fixation in the Arabian Sea is still difficult to assess owing to the small database available; however, there are hints that it might be more important than previously thought. Atmospheric N depositions are important on a regional scale during the intermonsoon in the central Arabian Sea; however, they play only a minor role for the overall N cycling. Emissions of N2O and ammonia, deep-Sea N burial, and N inputs by rivers and marginal Seas (i.e., Persian Gulf and Red Sea) are of minor importance. We found that the magnitude of the sedimentary denitrification at the shelf might be ∼17% of the total denitrification in the Arabian Sea, indicating that the shelf sediments might be of considerably greater importance for the N cycling in the Arabian Sea than previously thought. Sedimentary and pelagic denitrification together demand ∼6% of the estimated particulate organic nitrogen export flux from the photic zone. The main northward transport of N into the Arabian Sea occurs in the intermediate layers, indicating that the N cycle of the Arabian Sea might be sensitive to variations of the intermediate water circulation of the Indian Ocean.
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Seasonal and spatial variations in settling manganese fluxes in the Northern Arabian Sea
Deep Sea Research Part I: Oceanographic Research Papers, 1999Co-Authors: T. M. Balakrishnan Nair, R. Shankar, Venkitasubramani Ramaswamy, Venugopalan IttekkotAbstract:Abstract Particulate manganese (Mn) fluxes measured with six time series sediment traps showed that the annual settling fluxes were 3–6 times higher in the west compared to those in the east and central Arabian Sea. Annual detrital Mn (Mn dt ) flux was nearly the same in the eastern and western Arabian Sea, but excess Mn (Mn ex ) fluxes were much higher (>4 times) in the western Arabian Sea. Atmospheric inputs cannot account for these high-Mn fluxes. Central and eastern Arabian Sea traps are overlain by a thick and intense denitrification layer, which may cause reductive dissolution of Mn oxides from settling particles and consequently low Mn ex fluxes. As the exchange of intermediate waters between the Arabian Sea and the rest of the Indian Ocean is confined largely to the western Arabian Sea, relatively more oxic and dynamic conditions prevail in this region. Increased oxidizing conditions coupled with higher inputs of dissolved Mn through intermediate and surface advective processes might have led to in situ oxidation of Mn, thus resulting in higher vertical fluxes of Mn ex . Mn ex fluxes in traps at ∼1000 m depth exhibited Seasonal variability with a minimum during the winter monsoon (January–February) and maximum during the pre- and early- south-west monsoon (March–June). This variation is correlated with water mass movements and bacterial abundance observed during the Joint Global Ocean Flux Study (JGOFS). The possible involvement of bacteria and the microbial loop is suggested for the concentration and vertical transport of excess Mn.
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Lithogenic fluxes to the deep Arabian Sea measured by sediment traps
Deep Sea Research Part A. Oceanographic Research Papers, 1991Co-Authors: Venkatachalam Ramaswamy, R.r. Nair, Steven J. Manganini, B. Haake, Venugopalan IttekkotAbstract:Abstract Particle fluxes measured continously for one year at three locations in the Arabian Sea using time-series sediment traps show that lithogenic sedimentation processes are strongly coupled to biological processes. The vertical flux of lithogenic matter is controlled by episodic production and fluxes of biogenic matter. Illite and quartz are the dominant clay minerals in the traps at all three locations. Smectites generally range between 2 and 8%, but show higher fluxes up to 25% in the central and eastern Arabian Sea during the southwest monsoon period. Most of the river discharge is retained on the continental shelf, and less than 5% of the annual input of lithogenic material to the Arabian Sea is deposited in the deeper part as hemipelagic sediments.