Pelagic Denitrification Research Articles

Southern Ocean sourced waters modulate Eastern Equatorial Pacific denitrification during the Mid-Pleistocene transition

The Eastern tropical Pacific holds extensive oxygen minimum zone (OMZ) areas where pelagic denitrification occurs. Sedimentary nitrogen isotopes are believed to record their denitrification history, which provides indirect evidence for the past extent of the upper ocean oxygen depletion. We present a new high-resolution sedimentary δ15N record of ODP Site 1242 located in the Eastern Equatorial Pacific during the Mid-Pleistocene Transition. The sedimentary record shows increased (decreased) denitrification during interglacial (glacial) periods and a substantial 41 kyr power. Comparing the sedimentary nitrogen signal with proxies for organic carbon supply and paleoxygenation indicates a decoupling of the denitrification from local productivity and bottom water oxygenation. We claim that this feature is a response to the extratropical ventilation of the subthermocline waters. Increased glacial AAIW influence might have provided the EEP with oxygenated and nutrient-rich waters, explaining the decreased glacial denitrification rates observed in Site 1242 and other OMZs records. Qualitative inferences about the vertical oxygen structure depict an extensive deoxygenated glacial EEP. This scenario might have contributed to the storage of the carbon captured during the glacials of the MPT.

Pelagic denitrification and methane oxidation in oxygen-depleted waters of the Louisiana shelf

Anthropogenic nutrient inputs fuel eutrophication and hypoxia ([O2] < 2 mg L−1), threatening coastal and near shore environments across the globe. The world’s second largest anthropogenic coastal hypoxic zone occurs annually along the Louisiana (LA) shelf. Springtime loading of dissolved inorganic nitrogen (DIN) from the Mississippi River, combined with summertime stratification and increased water residence time on the shelf, promotes establishment of an extensive hypoxic zone that persists throughout the summer. We investigated the patterns of pelagic denitrification and methane (CH4) oxidation along the LA shelf. Microbial activity rates were determined along with concentrations of dissolved nutrients and greenhouse gases, nitrous oxide (N2O) and CH4, during summer in 2013, 2015, and 2016. We documented denitrification rates up to 1900 nmol N L−1 d−1 and CH4 oxidation rates as high as 192 nmol L−1 d−1 in hypoxic waters characterized by high concentrations of N2O (range: 1 to 102 nM) and CH4 (range: 3 to 641 nM). Ecosystem scaling estimates suggest that pelagic denitrification could remove between 0.1 and 47% of the DIN input from the Mississippi River, whereas CH4 oxidation does not function as an effective removal process with CH4 escaping to the atmosphere. Denitrification and CH4 oxidizing bacteria within the LA shelf hypoxic zone were largely unable to keep up with the DIN and CH4 inputs to the water column. Rates were variable and physiochemical dynamics appeared to regulate the microbial removal capacity for both nitrate/nitrite and CH4 in this ecosystem.

Multiobjective Calibration of a Global Biogeochemical Ocean Model Against Nutrients, Oxygen, and Oxygen Minimum Zones

AbstractGlobal biogeochemical ocean models rely on many parameters, which govern the interaction between individual components, and their response to the physical environment. They are often assessed/calibrated against quasi‐synoptic data sets of dissolved inorganic tracers. However, a good fit to one observation might not necessarily imply a good match to another. We investigate whether two different metrics—the root‐mean‐square error to nutrients and oxygen and a metric measuring the overlap between simulated and observed oxygen minimum zones (OMZs)—help to constrain a global biogeochemical model in different aspects of performance. Three global model optimizations are carried out. Two single‐objective optimizations target the root‐mean‐square metric and a sum of both metrics, respectively. We then present and explore multiobjective optimization, which results in a set of compromise solutions. Our results suggest that optimal parameters for denitrification and nitrogen fixation differ when applying different metrics. Optimization against observed OMZs leads to parameters that enhance fixed nitrogen cycling; this causes too low nitrate concentrations and a too high global pelagic denitrification rate. Optimization against nutrient and oxygen concentrations leads to different parameters and a lower global fixed nitrogen turnover; this results in a worse fit to OMZs. Multiobjective optimization resolves this antagonistic effect and provides an ensemble of parameter sets, which help to address different research questions. We finally discuss how systematic model calibration can help to improve models used for projecting climate change and its effect on fisheries and climate gas emissions.

Variations in Denitrification and Ventilation Within the Arabian Sea Oxygen Minimum Zone During the Holocene

AbstractThe continental slope of India is exposed to an intense perennial oxygen minimum zone (OMZ) supporting pelagic denitrification. Sediments that are presently in contact with the lower boundary of the denitrification zone indicate marked changes in the intermediate and bottom waters ventilation of OMZ during the past 9,500 years. The δ15N of sediment suggests that the OMZ waters were less ventilated during the early Holocene (between 9.5 and 8.5 ka BP) resulting in intensified denitrifying conditions with an average δ15N value of 7.8‰, while at the same time stable Mo isotope composition (average δ98Mo of −0.02‰) indicates that the bottom waters that were in contact with the sediments were better oxygenated. By the mid‐Holocene OMZ became more oxygenated suppressing denitrification (average δ15N of 6.2‰), while bottom waters gradually became less oxygenated (average δ98Mo of 1.7‰). The mid‐Holocene reduction in denitrification coincided with a global decrease in atmospheric N2O as inferred from ice core records, which is consistent with a decreased contribution from the Arabian Sea. Since ~5.5 ka BP OMZ waters have again been undergoing progressive deoxygenation accompanied by increasing denitrification.

Influence of Natural Oxygenation of Baltic Proper Deep Water on Benthic Recycling and Removal of Phosphorus, Nitrogen, Silicon and Carbon

At the end of 2014, a Major Baltic Inflow (MBI) brought oxygenated, salty water into the Baltic proper and reached the long-term anoxic Eastern Gotland Basin (EGB) by March 2015. In July 2015, we measured benthic fluxes of phosphorus (P), nitrogen (N) and silicon (Si) nutrients and dissolved inorganic carbon (DIC) in situ using an autonomous benthic lander at deep sites (170-210 m) in the EGB, where the bottom water oxygen concentration was 30-45 μM. The same in situ methodology was used to measure benthic fluxes at the same sites in 2008-2010, but then under anoxic conditions. The high efflux of phosphate under anoxic conditions became lower upon oxygenation, and turned into an influx in about 50% of the flux measurements. The C:P and N:P ratios of the benthic solute flux changed from clearly below the Redfield ratio (on average about 70 and 3-4, respectively) under anoxia to approaching or being well above the Redfield ratio upon oxygenation. These observations demonstrate retention of P in newly oxygenated sediments. We found no significant effect of oxygenation on the benthic ammonium, silicate and DIC flux. We also measured benthic denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) rates at the same sites using isotope-pairing techniques. The bottom water of the long-term anoxic EGB contained less than 0.5 μM nitrate in 2008-2010, but the oxygenation event created bottom water nitrate concentrations of about 10 μM in July 2015 and the benthic flux of nitrate was consistently directed into the sediment. Nitrate reduction to both dinitrogen gas (denitrification) and ammonium (DNRA) was initiated in the newly oxygenated sediments, while anammox activity was negligible. We estimated the influence of this oxygenation event on the magnitudes of the integrated benthic P flux (the internal P load) and the fixed N removal through benthic and pelagic denitrification by comparing with a hypothetical scenario without the MBI. Our calculations suggest that the oxygenation triggered by the MBI in July 2015, extrapolated to the basin-wide scale of the Baltic proper, decreased the internal P load by 23% and increased the total (benthic plus pelagic) denitrification by 18%.

Modelling nutrient retention in the coastal zone of an eutrophic sea

Abstract. The Swedish Coastal zone Model (SCM) was used at a test site, the Stockholm archipelago, located in the northern part of the central Baltic Sea, to study the retention capacity of the coastal filter on nitrogen (N) and phosphorus (P) loads from land and atmosphere. The efficiency of the coastal filter to permanently retain nutrients determines how much of the local nutrient loads actually reach the open sea. The SCM system is a nutrient–phytoplankton–zooplankton–detritus-type model coupled to a horizontally integrated, physical model in particular suitable for estuaries. In this study the Stockholm Archipelago, consisting of 86 sub-basins, was divided into three sub-areas: the inner, the intermediate and the outer archipelago. An evaluation of model results showed that the modelled freshwater supply agrees well with observations. The nutrient, salinity and temperature dynamics simulated by the SCM are also found to be in good or acceptable agreement with observations. The analysis showed that the Stockholm Archipelago works as a filter for nutrients that enter the coastal zone from land, but the filter efficiency is not effective enough to retain all the supplied nutrients. However, at least 65 and 72 % of the P and N, respectively, are retained during the studied period (1990–2012). A major part of the retention is permanent, which for P means burial. For N, almost 92 % of the permanent retention is represented by benthic denitrification, less than 8 % by burial, while pelagic denitrification is below 1 %. Highest total amounts of P and N are retained in the outer archipelago, where the surface area is largest. The area-specific retention of P and N, however, is highest in the smaller inner archipelago and decreases towards the open sea. A reduction scenario of the land loads of N and P showed that the filter efficiencies of N and P increase and the export of N from the archipelago decreases. About 15 years after the reduction, the export of P changes into an import of P from the open sea to the archipelago.

MOPS-1.0: towards a model for the regulation of the global oceanic nitrogen budget by marine biogeochemical processes

Abstract. Global models of the oceanic nitrogen cycle are subject to many uncertainties regarding the representation of the relevant biogeochemical processes and of the feedbacks between nitrogen sources and sinks that determine space- and timescales on which the global nitrogen budget is regulated. We investigate these aspects using a global model of ocean biogeochemistry that explicitly considers phosphorus and nitrogen, including pelagic denitrification and nitrogen fixation as sink and source terms of fixed nitrogen, respectively. The model explores different parameterizations of organic matter sinking speed, oxidant affinity of oxic and suboxic remineralization, and regulation of nitrogen fixation by temperature and different stoichiometric ratios. Examination of the initial transient behavior of different model setups initialized from observed biogeochemical tracer distributions reveal changes in simulated nitrogen inventories and fluxes particularly during the first centuries. Millennial timescales have to be resolved in order to bring all biogeochemical and physical processes into a dynamically consistent steady state. Analysis of global properties suggests that not only particularly particle sinking speed but also the parameterization of denitrification determine the extent of oxygen minimum zones, global nitrogen fluxes, and hence the oceanic nitrogen inventory. However, the ways and directions in which different parameterizations of particle sinking, nitrogen fixation, and denitrification affect the global diagnostics are different suggesting that these may, in principle, be constrained independently from each other. Analysis of the model misfit with respect to observed biogeochemical tracer distributions and fluxes suggests a particle flux profile close to the one suggested by Martin et al. (1987). Simulated pelagic denitrification best agrees with the lower values between 59 and 84 Tg N yr−1 recently estimated by other authors.

Copepod carcasses as microbial hot spots for pelagic denitrification

AbstractCopepods are exposed to a high non‐predatory mortality and their decomposing carcasses act as microniches with intensified microbial activity. Sinking carcasses could thereby represent anoxic microenvironment sustaining anaerobic microbial pathways in otherwise oxic water columns. Using non‐invasive O2 imaging, we document that carcasses of Calanus finmarchicus had an anoxic interior even at fully air‐saturated ambient O2 level. The extent of anoxia gradually expanded with decreasing ambient O2 levels. Concurrent microbial sampling showed the expression of nitrite reductase genes (nirS) in all investigated carcass samples and thereby documented the potential for microbial denitrification in carcasses. The nirS gene was occasionally expressed in live copepods, but not as consistently as in carcasses. Incubations of sinking carcasses in 15 amended seawater demonstrated denitrification, of which on average 34% ± 17% (n = 28) was sustained by nitrification. However, the activity was highly variable and was strongly dependent on the ambient O2 levels. While denitrification was present even at air‐saturation (302 μmol L−1), the average carcass specific activity increased several orders of magnitude to ∼ 1 nmol d−1 at 20% air‐saturation (55 μmol O2 L−1) at an ambient temperature of 7°C. Sinking carcasses of C. finmarchicus therefore represent hotspots of pelagic denitrification, but the quantitative importance as a sink for bioavailable nitrogen is strongly dependent on the ambient O2 level. The importance of carcass associated denitrification could be highly significant in O2 depleted environments such as Oxygen Minimum Zones (OMZ).

Evidence for changes in subsurface circulation in the late Eocene equatorial Pacific from radiolarian‐bound nitrogen isotope values

AbstractMicrofossil‐bound organic matter represents an important archive of surface ocean environmental information. Sedimentary nitrogen (N) isotope reconstructions of surface nitrate consumption and nitrogen source changes are made using fossil diatom (autotrophs) and planktic foraminiferal (heterotrophs)‐bound organic matter with success. However, because diatoms and planktic foraminifera are poorly preserved and sedimentary organic matter content is near zero during the late Eocene, our ability to examine nutrient dynamics across this important climate transition is limited. Here we present new data exploring the use of N isotope records from radiolarian tests. A comparison of surface ocean nitrate and core top bulk and radiolarian N isotope values (as δ15N) from the equatorial Pacific indicates that radiolarian‐N records δ15N variability with fidelity but that a significant offset exists between bulk sedimentary and diatom δ15N values and those measured from radiolarians (~7.1 ± 1.1‰). A downcore profile of radiolarian δ15N values is compared to siliceous microfossil assemblage changes across the Eocene‐Oligocene boundary. Average of radiolarian‐bound δ15N values is 0.5 ± 2.0‰, which, when corrected using the offset derived from the modern surface samples, suggests that the mean nitrogen isotopic composition of the early Cenozoic eastern Pacific was not significantly different from today. The overall trend, of decreasing δ15N values with decreasing export productivity, is consistent with either a regional decline in pelagic denitrification or a large‐scale change in nutrient sources to the eastern equatorial Pacific (EEP), both linked to the cooling climate and changing intermediate water circulation. Decreasing/low δ15N values cooccur with high radiolarian species turnover at ~35.5 and 34 Ma, suggestive of a significant ecological change in the EEP, consistent with cooling and water mass distribution changes. The preliminary results suggest that radiolarian‐bound organic nitrogen represents another promising archive and underscores the fact that the different microfossil fractions must be separated to ensure robust results.

Partitioning between benthic and pelagic nitrate reduction in the Lake Lugano south basin

We evaluated the seasonal variation of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) rates in the sediments and the integrative N (and O) isotopic signatures of dissolved inorganic nitrogen (DIN) compounds in the overlying water column of the monomictic Lake Lugano south basin. Denitrification was the dominant NO−3 reduction pathway, whereas the contribution of anammox and DNRA to total benthic NO−3 reduction was &lt; 6% and &lt; 12%, respectively. Sedimentary denitrification rates were highest (up to 57.2 ± 16.8 µmol N m−2 h−1) during fully oxic bottom water conditions. With the formation of seasonal bottom water anoxia, NO−3 reduction was partitioned between water column and sedimentary processes. Total benthic NO−3 reduction rates determined in 15N‐label experiments and sediment—water interface N2 fluxes as calculated from water column N2 : Ar gradients revealed that sedimentary denitrification still accounted for ∼ 40% of total N2 production during bottom water anoxia. The partitioning between water column and sedimentary denitrification was further evaluated by the natural abundance stable N isotope composition of dissolved NO−3 in the water column. With anaerobic bottom water conditions, water column NO−3 concentrations gradually decreased, paralleled by an increase in δ15N— and δ18O— NO−3 from approximately 7‰ to 20‰ and from 2‰ to 14‰, respectively. Using a closed‐system (Rayleigh) model, the N and O isotope effects associated with community NO−3 consumption were 15ε ≈ 13.7‰ and 18ε ≈ 11.3‰, respectively. With the assumptions of a relatively low net N isotope effect associated with sedimentary denitrification (i.e., 15εsed = 1.5–3‰) vs. a fully expressed biological N isotope fractionation during water column denitrification (i.e., 15εwater = 20–25‰), our results confirm that 36–51% of reduction occurred within the sediment. The general agreement between the indirect (isotopic) approach and the flux and rate measurements suggests that water column nitrate isotope measurements can be used to distinguish between benthic and pelagic denitrification quantitatively.

Expansion of pelagic denitrification during early Pleistocene cooling

Bioavailable nitrogen is removed from the oceans in oxygen-deficient benthic and pelagic environments by denitrification. Future warming is predicted to reduce ocean oxygenation and to cause hypoxic regions to expand, potentially accelerating denitrification. A compilation of high-resolution sedimentary nitrogen isotope (δN15) records from the eastern tropical Pacific, North Pacific, and the Arabian Sea, and a global multi-site survey are presented as evidence for weak pelagic denitrification at the end of the Pliocene warm period. Mean δN15 values increased in the major oxygen minimum zones (OMZs) between 2.1 and 1.5 Ma. Pelagic denitrification strengthened during a period of long term global cooling, despite solubility driven increases in initial oxygen contents of Antarctic intermediate and Subantarctic mode waters ventilating the OMZs. This trend is opposite to the predicted mean trend for a cooling ocean as well as to the observed glacial–interglacial variation. Several alternatives to explain the shift are proposed, including a rise in net respiration, a progressive increase in the ventilation age of the deep ocean associated with million year scale, secular cooling, and a shoaling of the remotely ventilated thermocline to shallow depths corresponding to the zone of peak subsurface respiration. Given no evidence for a net increase in production, we assert that large-scale, climate-driven changes in ocean circulation regulate long timescale variations in the extent of pelagic denitrification. Additional data and modeling are required to fully explain the observations.

Pelagic nitrification and denitrification rates in an Arctic fjord during early spring

This study addresses factors governing nitrification and denitrification rates, along with the abundance of the bacterial groups likely involved in these activities, in Kongsfjorden, an Arctic fjord at Ny-Alesund, Svalbard. The fjord was sampled three times during the month of March 2008 as day length and direct solar radiation increased. Although initially well mixed, cooler and more saline, the fjord became stratified, warmer and less saline during late March. The concentrations of NH4 + (4.4 ± 1.6 to 6 ± 1.6 μM) and NO2 − (1 ± 0.3 to 1.2 ± 0.4 μM) increased progressively with the decrease in NO3 − (6.1 ± 1.3 to 3.8 ± 1.5 μM), reflecting the onset of primary productivity. Nitrification rates and the culturable population of nitrifiers decreased significantly from 1.6 ± 0.9 to 0.4 ± 0.1 ng at NH4 +-N l−1 h−1 and 5.1 ± 0.3 × 102 to 29 ± 14 cells l−1, respectively. In contrast, denitrification rates increased (2.4 ± 0.5 to 4.6 ± 1.3 ng-at NO3 −-N l−1 h−1), although the abundance of culturable denitrifiers did not vary significantly. A significant correlation of nitrifiers with NO3 − during early March (p < 0.01, r = 0.51) indicated that nitrifiers may play an important role in regulating the NO3 − pool and thereby in controlling the abundance of denitrifiers. However, the contribution of nitrification to the total NO3 − pool decreased with time. Experimental simulations were also set up to understand the impact of change in duration of light and progressive increase in temperature on these processes. The application of 24 h light inhibited nitrification, suggesting that during peak Arctic summer the contribution of nitrification to the nitrate pool is minimal. It was also observed that a brief exposure to light (≤6 h) was enough to hamper nitrification rates. Experimental simulations suggested that a gradual increase in temperature in the fjord may enhance the magnitude of nitrification and denitrification in the fjord.

Denitrifying Alphaproteobacteria from the Arabian Sea That Express nosZ , the Gene Encoding Nitrous Oxide Reductase, in Oxic and Suboxic Waters

Marine ecosystems are significant sources of the powerful greenhouse gas nitrous oxide (N2O). A by-product of nitrification and an intermediate in the denitrification pathway, N2O is formed primarily in oxygen-deficient waters and sediments. We describe the isolation of a group of alphaproteobacteria from the suboxic waters of the Arabian Sea that are phylogenetically affiliated with Labrenzia spp. and other denitrifiers. Quantitative PCR assays revealed that these organisms were very broadly distributed in this semienclosed ocean basin. Their biogeographical range extended from the productive, upwelling region off the Omani shelf to the clear, oligotrophic waters that are found much further south and also included the mesotrophic waters overlying the oxygen minimum zone (OMZ) in the northeastern sector of the Arabian Sea. These organisms actively expressed NosZ (N2O reductase, the terminal step in the denitrification pathway) within the OMZ, an established region of pelagic denitrification. They were found in greatest numbers outside the OMZ, however, and nosZ mRNAs were also readily detected near the base of the upper mixed layer in nutrient-poor, oxic regions. Our findings provide firm molecular evidence of a potential sink for N2O within well-ventilated, oceanic surface waters in this biogeochemically important region. We show that the Labrenzia-like denitrifiers and their close relatives are habitual colonizers of the pseudobenthic environment provided by Trichodesmium spp. We develop the conjecture that the O2-depleted microzones that occur within the colonies of these filamentous, diazotrophic cyanobacteria might provide unexpected niches for the reduction of nitrogen oxides in tropical and subtropical surface waters.

Denitrification in the water column of the central Baltic Sea

Removal of fixed nitrogen in the water column of the eastern Gotland Basin, central Baltic Sea, was studied during two cruises in September 2008 and August 2010. The water column was stratified with anoxic sulfidic bottom water meeting oxic nitrate containing water at the oxic–anoxic interface. Anammox was never detected whereas denitrification was found in all incubations from anoxic depths and occurred immediately below the oxic–anoxic interface. Sulfide (H2S+HS−+S2−) was in most cases the only electron donor for denitrification but, in contrast to previous findings, denitrification was in some situations driven by organic matter alone. Nitrous oxide (N2O) became an increasingly important product of denitrification with increasing sulfide concentration and was >80% of the total N gas formation at 10μM sulfide. The potential rates of denitrification measured in incubations at elevated NO3- or sulfide concentrations were converted to in situ rates using the measured water column concentrations of NO3- and sulfide and the actual measured relations between NO3- and sulfide concentrations and denitrification rates. In situ denitrification ranged from 0.24 to 15.9nMN2h−1. Assuming that these rates were valid throughout the anoxic NO3- containing zone, depth integrated in situ denitrification rates of 0.06–2.11mmol Nm−2d−1 were estimated. The thickness of this zone was generally 3–6m, which is probably what can be maintained through regular turbulent mixing induced by internal waves at the oxic–anoxic interface. However, layers of up to 55m thickness with low O2 water (<10μM) were observed which was probably the result of larger scale mixing. In such a layer nitrification may produce NO3- and once the O2 has been depleted denitrification will follow resulting in enormous rates per unit area. Even with an active denitrification layer of 3–6m thickness the pelagic denitrification per unit area clearly exceeded sediment denitrification rates elsewhere in the Baltic Sea. When extrapolated to the entire Baltic Proper (BP) denitrification in the water column was in the range of 132–547ktonNyr−1 and was thus at least as important as sediment denitrification which has recently been estimated to 191ktonNyr−1. With a total external N-input of 773ktonNyr−1 it is clear that denitrification plays a significant role in the N-budget of the BP.

Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific

Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the δ15N and δ18O of deep ocean nitrate. A depth profile of nitrate isotopes from the Hawaii Ocean Time‐series Station ALOHA shows less of an increase in δ18O than in δ15N as one ascends from abyssal waters into the denitrification‐impacted mid‐depth waters. A box model of the ocean nitrate N and O isotopes indicates that this is the effect of the low latitude nitrate assimilation/regeneration cycle: organic N sinking out of the surface spreads the high‐δ15N signal of pelagic denitrification into waters well below and beyond the suboxic zone, whereas the nitrate δ18O signal of denitrification can only be transmitted by circulation in the interior.

Carbon and nitrogen elemental and stable isotopic compositions of surficial sediments from the Pakistan margin of the Arabian Sea

Abstract The factors controlling sedimentary organic matter distributions are the subject of ongoing debate. We report here the carbon and nitrogen contents and stable isotopic compositions for surface sediments and downcore profiles from sites ranging in water depth from 140 to 3200 m, located across the Indus margin of the Arabian Sea. The sites spanned the intense midwater oxygen minimum zone (OMZ) and showed dramatic differences in bottom-water redox conditions, benthic communities, and degrees of bioturbation. Five primary stations were studied, at 140, 300, 940, 1200 and 1850 m water depth, with sampling in March–May (“intermonsoon”) and August–October (“late-to-postmonsoon”) 2003. High-resolution sampling was also conducted across the upper (140–275 m) and lower OMZ (600–1200 m) boundaries. The sampling strategy was selected to help clarify the roles of oxygen availability and other potential environmental factors in determining sediment organic matter content and composition, the extent of organic matter cycling within the sediments under contrasting depositional conditions, and the fate of organic matter deposited during the southwest monsoon. As previously noted on this and other OMZ margins, there was a roughly inverse spatial relationship between bottom-water oxygen and sedimentary organic matter concentrations. However, variability in organic carbon content within the OMZ that was unrelated to oxygen, and the fact that maximal organic carbon contents occurred under relatively oxidizing conditions towards the lower OMZ boundary, rather than at the OMZ core, indicate that factors other than oxygen contribute significantly as controls on carbon distributions. Despite a 3-fold range in organic carbon content, carbon-to-nitrogen ratios showed no systematic trends across the OMZ, with values reflecting uniform (predominantly marine) organic matter inputs. Nitrogen isotopic signatures (δ15N values) in the range of 6.5–8.5‰ also did not show clear cross-OMZ trends, and are attributed to a 14N-depleted nitrate pool created by pelagic denitrification. There was little downcore change in organic matter content at any site, suggesting either that organic matter decay occurs largely in the water column, or that decay within the sediments (at oxygenated sites) is masked by bioturbation. Seasonal variability in organic matter content and composition was restricted to the most surficial sediments, and changes were ephemeral, suggesting rapid turnover of organic matter deposited during the southwest monsoon, on timescales of weeks. Stable C isotopic compositions trended towards more negative values in surficial sediments at hypoxic sites within the OMZ core. This is attributed to a chemosynthetic bacterial imprint.

Anaerobic denitrification in fungi from the coastal marine sediments off Goa, India

Denitrification is a microbial process during which nitrate or nitrite is reduced under anaerobic condition to gaseous nitrogen. The Arabian Sea contains one of the major pelagic denitrification zones and in addition to this, denitrification also takes places along the continental shelf. Prokaryotic microorganisms were considered to be the only players in this process. However recent studies have shown that higher microeukaryotes such as fungi can also adapt to anaerobic mode of respiration and reduce nitrate to harmful green house gases such as NO and N 2O. In this study we examined the distribution and biomass of fungi in the sediments of the seasonal anoxic region off Goa from two stations. The sampling was carried out in five different periods from October 2005, when dissolved oxygen levels were near zero in bottom waters to March 2006. We isolated mycelial fungi, thraustochytrids and yeasts. Species of Aspergillus and thraustochytrids were dominant. Fungi were isolated under aerobic, as well as anaerobic conditions from different seasons. Four isolates were examined for their denitrification activity. Two cultures obtained from the anoxic sediments showed better growth under anaerobic condition than the other two cultures that were isolated from oxic sediments. Our preliminary results suggest that several species of fungi can grow under oxygen deficient conditions and participate in denitrification processes.

Coastal versus open-ocean denitrification in the Arabian Sea

Abstract. The Arabian Sea contains one of the three major open-ocean denitrification zones in the world. In addition, pelagic denitrification also occurs over the inner and mid-shelf off the west coast of India. The major differences between the two environments are highlighted using the available data. The perennial open-ocean system occupies two orders of magnitude larger volume than the seasonal coastal system, however, the latter offers more extreme conditions (greater nitrate consumption leading to complete anoxia). Unlike the open-ocean system, the coastal system seems to have undergone a change (i.e., it has intensified) over the past few decades presumably due to enhanced nutrient loading from land. The two systems also differ from each other with regard to the modes of nitrous oxide (N2O) production: In the open-ocean suboxic zone, an accumulation of secondary nitrite (NO2−) is invariably accompanied by depletion of N2O whereas in the coastal suboxic zone high NO2− and very high N2O concentrations frequently co-occur, indicating, respectively, net consumption and net production of N2O by denitrifiers. The extents of heavier isotope enrichment in the combined nitrate and nitrite (NO3−+NO2−) pool and in N2O in reducing waters appear to be considerably smaller in the coastal region, reflecting more varied sources/sinks and/or different isotopic fractionation factors.

The nitrogen cycle in the Arabian Sea

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, N 2 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 N 2O, 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 N 2 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 N 2O and denitrification components.

A revised nitrogen budget for the Arabian Sea

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.