Suboxic Waters Research Articles

Foraminiferal assemblages and geochemistry for interpreting the incidence of Early Toarcian environmental changes in North Gondwana palaeomargin (Traras Mountains, Algeria)

The Early Toarcian was characterised by important environmental changes and a mass extinction event usually related to a global oceanic anoxic event. The analysis of ecostratigraphic fluctuations of foraminiferal morphogroups, elemental geochemical proxies and C and O stable isotopes of the Mellala section (Tlemcen Domain, North Algeria) makes it possible to determine the incidence of the anoxic event in this sector of the north Gondwana palaeomargin. The end of the Pliensbachian is characterised by a diverse foraminiferal assemblage with equilibrium species suggesting good oxygen and nutrient availability. The beginning of the Toarcian (Polymorphum Zone) evidences major changes in foraminifera with the disappearance of species, decreasing proportions of epifauna and shallow infauna, and fluctuations in diversity and dominance of Lenticulina toarcense and Lingulina tenera confirming a perturbation in the palaeoecological conditions in the sea-bottom. Redox proxies (Co/Al, Cr/Al and V/Al) with local maximum values suggest a decrease in oxygenation degree. A negative excursion of δ13C is recorded right at the Polymorphum/Levisoni Zone boundary, and the subsequent disappearance of epifauna, decreasing diversity and abundance of foraminifera (foram/100g) would be related to the accentuation of stressing conditions. Also at the Polymorphum/Levisoni Zone boundary, suboxic waters at the sea-bottom indicate the maximum values of redox proxies (Co/Al, Cu/Al, Cr/Al and V/Al). The upper part of the Levisoni Zone is more calcareous, with increasing diversity of shallow infauna and a decrease in potentially deep infauna related to more favourable conditions. The incidence of the Toarcian Oceanic Anoxic Event in this context was very low in comparison with the Saharan Basin (Raknet El Kahla section, Saharan Atlas), where a benthic barren interval, higher total organic carbon and redox proxies are recorded. The low incidence of the biotic crisis and the rapid recovery of assemblages in the Tlemcen Domain is compared with the high incidence and delayed recovery in the Saharan Basin, where the palaeogeography determined restricted water circulation between the Saharan Craton and the Oran Massif.

Nitrate elimination and regeneration as evidenced by dissolved inorganic nitrogen isotopes in Saanich Inlet, a seasonally anoxic fjord

In this study, we used natural abundance isotope measurements of dissolved inorganic nitrogen (N) species to evaluate the effect of different oxygenation regimes on N transformation and elimination in Saanich Inlet, a seasonally anoxic fjord in British Columbia, Canada. We analyzed dissolved nutrient concentrations and the N (and O) isotope composition of nitrate (NO3−) and ammonium (NH4+) at different depths throughout the water column near the mouth of the inlet between April 2008 and April 2009. A gradual increase in both the NO3− δ15N and δ18O, associated with a decrease in NO3− concentration and an increase in biological excess N2, was observed after bottom water renewal events in August–October 2008, indicating NO3− consumption by denitrifying bacteria in an expanding suboxic water column. An increase in the δ15N of NH4+ with depth toward the suboxic/hypoxic transition, indicated net consumption of NH4+, most likely by micro-aerobic or anaerobic NH4+ oxidation and dissimilatory consumption by microorganisms. Deviations from a 1:1 correlation between the NO3− δ15N and δ18O (Δ(15,18)) that appears characteristic for both assimilatory and dissimilatory NO3− consumption in the ocean, were observed in surface waters and close to the hypoxic/suboxic transition. Lowered Δ(15, 18) values can most plausibly be explained by aerobic nitrification of newly remineralized NH4+ and/or low δ15N–NO3− inputs from atmospheric precipitation in the surface mixed layer, and NO3− regeneration through NH4+ oxidation and/or the reoxidation of nitrite (NO2−) in deeper waters. Closed and open system model-derived N isotope effects for NO3− consumption calculated from time-series samples collected near the sediments in anoxic bottom waters were significantly lower (as low as ~11‰) than the biological N isotope effects of ~20–30‰ for water column denitrification reported in other studies. We argue that the reduced N isotope effect is mainly due to the combined effects of water column and sediment denitrification, the latter occurring with a highly suppressed N isotope fractionation at the ecosystem level. We estimated that ~40–60% of the denitrification occurs within the sediments of the inlet.

Marine denitrification rates determined from a global 3-D inverse model

Abstract. A major impediment to understanding long-term changes in the marine nitrogen (N) cycle is the persistent uncertainty about the rates, distribution, and sensitivity of its largest fluxes in the modern ocean. We use a global ocean circulation model to obtain the first 3-D estimate of marine denitrification rates that is maximally consistent with available observations of nitrate deficits and the nitrogen isotopic ratio of oceanic nitrate. We find a global rate of marine denitrification in suboxic waters and sediments of 120–240 Tg N yr−1, which is lower than many other recent estimates. The difference stems from the ability to represent the 3-D spatial structure of suboxic zones, where denitrification rates of 50–77 Tg N yr−1 result in up to 50% depletion of nitrate. This depletion reduces the effect of local isotopic enrichment on the rest of the ocean, allowing the N isotope ratio of oceanic nitrate to be achieved with a sedimentary denitrification rate about 1.3–2.3 times that of suboxic zones. This balance of N losses between sediments and suboxic zones is shown to obey a simple relationship between isotope fractionation and the degree of nitrate consumption in the core of the suboxic zones. The global denitrification rates derived here suggest that the marine nitrogen budget is likely close to balanced.

Establishment of euxinic conditions in the Holocene Black Sea

The paleoenvironmental evolution of the Black Sea is closely linked to the ingression of Mediterranean seawater over the Bosporus sill after the Last Glacial Maximum. We have reconstructed the temporal and spatial development of the Black Sea suboxic chemocline, which divides oxic surface water from anoxic, sulfi dic (euxinic) deep water. By combining high-resolution geochemical records of bulk parameters (carbonate, total organic carbon, sulfur), trace metals (Cu, Mo, V), and an isotopic proxy (δ 56 Fe) from seven sediment cores in the Black Sea, we generated a single composite geochemical core log that serves as a reference archive for the entire basin. Our proxy records refl ect the changing depositional and redox conditions of the Black Sea and permit us to estimate the infl ow budget of Mediterranean seawater throughout the Holocene. Our data indicate a gradual rise of the chemocline until ca. 5.3 ka, when suboxic waters fl ooded the shelf for the fi rst time. Trace metal and isotopic inventories document one major descent of the chemocline since the onset of brackish/ marine conditions before the present stable situation was established.

Oxygen and indicators of stress for marine life in multi-model global warming projections

Abstract. Decadal-to-century scale trends for a range of marine environmental variables in the upper mesopelagic layer (UML, 100–600 m) are investigated using results from seven Earth System Models forced by a high greenhouse gas emission scenario. The models as a class represent the observation-based distribution of oxygen (O2) and carbon dioxide (CO2), albeit major mismatches between observation-based and simulated values remain for individual models. By year 2100 all models project an increase in SST between 2 °C and 3 °C, and a decrease in the pH and in the saturation state of water with respect to calcium carbonate minerals in the UML. A decrease in the total ocean inventory of dissolved oxygen by 2% to 4% is projected by the range of models. Projected O2 changes in the UML show a complex pattern with both increasing and decreasing trends reflecting the subtle balance of different competing factors such as circulation, production, remineralization, and temperature changes. Projected changes in the total volume of hypoxic and suboxic waters remain relatively small in all models. A widespread increase of CO2 in the UML is projected. The median of the CO2 distribution between 100 and 600m shifts from 0.1–0.2 mol m−3 in year 1990 to 0.2–0.4 mol m−3 in year 2100, primarily as a result of the invasion of anthropogenic carbon from the atmosphere. The co-occurrence of changes in a range of environmental variables indicates the need to further investigate their synergistic impacts on marine ecosystems and Earth System feedbacks.

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.

Transcriptionally active heterotrophic diazotrophs are widespread in the upper water column of the Arabian Sea

Pelagic nitrogen fixation makes an important contribution to the fixed nitrogen budget of the world's oceans. Filamentous and unicellular cyanobacteria are significant players in this process but less is known of the potential activity of heterotrophic diazotrophs, although they are present and can be quite numerous in the nitrogen-deplete surface waters of the tropical and sub-tropical oceans. In this study we focused on the potential activity of several clades of heterotrophic nitrogen-fixers identified by phylogenetic analysis of 44 non-Trichodesmium-related, nifH (encoding the Fe-subunit of nitrogenase) clones from the Arabian Sea. Specific Northern slot blot protocols were developed to quantify nifH mRNAs from each clade and showed that two groups of Gammaproteobacteria, including the previously characterized UMB clade, and a third, novel phylotype affiliated with cluster III anaerobes, were actively expressing nitrogenase in the equatorial waters of this region. Transcripts (nifH mRNAs) from the latter clade were particularly abundant and were also detected in the suboxic waters of the oxygen minimum zone further north. Like the gammaproteobacterial groups, nifH expression by these organisms appeared to be insensitive to combined nitrogen concentrations and was readily detected in the nutrient-replete waters below the upper mixed layer as well as at shallower depths.

Evidence of an extensive spread of hydrothermal dissolved iron in the Indian Ocean

Ocean scale transect observations covering the entire water depth enable a comprehensive picture of the chemistry including the circulation and biogeochemical cycling of elements in seawater. The large-scale investigation of dissolved iron (Fe) took place through the Japanese-GEOTRACES study and here we report a basin-scale full-depth section profile of dissolved Fe in the Indian Ocean, from the Arabian Sea to the Southern Ocean. The data clearly shows the hydrothermal Fe distributed over 3000km distance in the deep layer centered at a depth of approximately 3000m, around the Central Indian Ridge segment, and a large part of the dissolved Fe from the hydrothermal sources was in the real soluble fraction rather than the colloidal fraction. In the intermediate water in the north Arabian Sea, another dissolved Fe rich water mass existed where Fe was enriched by remineralization processes from settling particles and/or adjacent reducing sediments, and preserved in the suboxic water. The basin-scale section profile indicates that there are several sources supplying dissolved Fe to deep waters, such as the hydrothermal sources and terrestrial Fe input with a persistent condition in the oxygen minimum zone (OMZ), between the northern-subtropical section, though few Fe sources were apparent in the Southern Ocean. Combining our size-fractionated Fe data with numerical modeling study suggests that the Fe physical–chemical form in seawater differs between the sources and is a key factor for controlling residence time and explaining the large scale distributed hydrothermal Fe.

Following the N<sub>2</sub>O consumption in the oxygen minimum zone of the eastern South Pacific

Abstract. Oxygen minimum zones (OMZs), such as those found in the eastern South Pacific (ESP), are the most important N2O sources in the global ocean relative to their volume. N2O production is related to low O2 concentrations and high primary productivity. However, when O2 is sufficiently low, canonical denitrification takes place and N2O consumption can be expected. N2O distribution in the ESP was analyzed over a wide latitudinal and longitudinal range (from 5° to 30° S and from 71–76° to ~ 84° W) based on ~ 890 N2O measurements. Intense N2O consumption, driving undersaturations as low as 40%, was always associated with secondary NO2– accumulation (SNM), a good indicator of suboxic/anoxic O2 levels. First, we explore relationships between ΔN2O and O2 based on existing data of denitrifying bacteria cultures and field observations. Given the uncertainties in the O2 measurements, a second relationship between ΔN2O and NO2– (> 0.75 μM) was established for suboxic waters (O2 < 8 μM). We reproduced the apparent N2O production (ΔN2O) along the OMZ in ESP with high reliability (r2 = 0.73 p = 0.01). Our results will contribute to the quantification of the N2O that is recycled in O2 deficient waters, and improve the prediction of N2O behavior under future scenarios of OMZ expansion and intensification.

Understanding why the volume of suboxic waters does not increase over centuries of global warming in an Earth System Model

Abstract. Global warming is expected to reduce oxygen solubility and vertical exchange in the ocean, changes which would be expected to result in an increase in the volume of hypoxic waters. A simulation made with a full Earth System model with dynamical atmosphere, ocean, sea ice and biogeochemical cycling (the Geophysical Fluid Dynamics Laboratory's Earth System Model 2.1) shows that this holds true if the condition for hypoxia is set relatively high. However, the volume of the most hypoxic (i.e., suboxic) waters does not increase under global warming, as these waters actually become more oxygenated. We show that the rise in dissolved oxygen in the tropical Pacific is associated with a drop in ventilation time. A term-by-term analysis within the least oxygenated waters shows an increased supply of dissolved oxygen due to lateral diffusion compensating an increase in remineralization within these highly hypoxic waters. This lateral diffusive flux is the result of an increase of ventilation along the Chilean coast, as a drying of the region under global warming opens up a region of wintertime convection in our model. The results highlight the potential sensitivity of suboxic waters to changes in subtropical ventilation as well as the importance of constraining lateral eddy transport of dissolved oxygen in such waters.

A Vigorous Specialized Microbial Food Web in the Suboxic Waters of a Shallow Subtropical Coastal Lagoon

To examine the extent of the microbial food web in suboxic waters of a shallow subtropical coastal lagoon, the density and biomass of bacteria and protozooplankton were quantified under different dissolved oxygen (DO) levels. In addition, bottom waters of a stratified site were compared with bottom waters of a homogeneous site under periods of high and low biological oxygen production/consumption in the lagoon. At the stratified site, microbial biomass decreased with oxygen decline, from oxia to suboxia, with a recovery of the initial total biomass after a 20-day period of persistent suboxia. A peak in density and biomass of purple sulfur bacteria (PSB) (90 μg C L(-1)) occurred in the suboxic waters 20 days prior to the peak in biomass of ciliates >50 μm (Loxophyllum sp. of 150 μm) (160 μg C L(-1)), demonstrating a top down biomass control. Ciliates >50 μm were positively correlated with PSB and bacteriochlorophyll a (photosynthetic pigment of PSB). Total protozoan biomass reached 430 μg C L(-1) in the suboxic waters of the stratified site, with ciliates >50 μm accounting for 90% of the total ciliate biomass and of 55 % of biomass of protozoa. At the homogeneous site, total protozoan biomass was only 66 μg C L(-1), where flagellates and ciliates <25 μm were the dominant microorganisms. Therefore, as light is available for primary producers in the bottom waters of shallow stratified coastal lagoons or estuaries, one can expect that high primary production of PSB may favor a specialized microbial food web composed by larger microorganisms, accessible to zooplankton that tolerate low DO levels.

Evidence for the direct oxidation of organic nitrogen to N2 gas in the Arabian Sea

We performed a suite of (15)N incubations ((15)NO(2)(-), (15)NO(3)(-) and (15)NH(4)(+)) with and without the organic-nitrogen (N) compound allylthiourea (ATU), in the suboxic waters of the Arabian Sea. Production of (29)N(2) in control (-ATU) incubations with either (15)NH(4)(+)+(14)NO(2)(-), or their analogues, (15)NO(2)(-)+(14)NH(4)(+), though small, confirmed the presence of anammox. In contrast, when we added ATU, along with (15)NO(2)(-) and (14)NH(4)(+), there was a much greater production of (29)N(2), with 92% of the (15)N-label being recovered as (29)N(2) on average. Such stimulated production of (29)N(2) could not be due to anammox, as the addition of ATU, along with (15)NH(4)(+)+(14)NO(2)(-), only produced (29)N(2) equivalent to that in the controls. The ratios of (29)N(2) to (30)N(2) produced also precluded stimulation of denitrification. We present this as evidence for a hitherto uncharacterised metabolism potentially capable of oxidising organic-N (e.g. NH(2) groups) directly to N(2) gas at the expense of NO(2)(-).

Controls on Mo isotope fractionations in a Mn-rich anoxic marine sediment, Gullmar Fjord, Sweden

Molybdenum isotopes have emerged as an important tool for determining the spatial extent of past ocean redox conditions. However, our understanding of Mo cycling and associated isotopic fractionations in modern and ancient continental margin sediments is limited, restricting attempts to fully utilize the potential of Mo isotopes. This is particularly the case with regard to the relative significance of Mo isotope fractionations associated with biogeochemical Mn and Fe oxide cycling under conditions of low dissolved sulfide. To rectify this, we have investigated sediments from Gullmar Fjord (Sweden), which contain high concentrations of Mn oxides and exhibit a strong redox zonation between Mn and Fe reduction. A combination of dissolved and particulate Mo isotope measurements, with porewater and solid phase Mn and Fe characterization, reveals that the main influence on porewater δ98Mo compositions is exerted by adsorption and desorption processes during reductive dissolution of both Mn and Fe (oxyhydr)oxide minerals. In areas enriched in Mn oxides distinctively low solid phase δ98Mo values are observed (~−1.0 and 0.4‰), which occur due to the large Mo isotope fractionation associated with Mo adsorption to Mn oxides. However, where Mn oxide concentrations are lower, authigenic δ98Mo is largely controlled by Fe (oxyhydr)oxide minerals, resulting in more positive δ98Mo isotopic compositions (~−0.5 and 2‰). Our results suggest that for modern anoxic marine sediments where deposition occurs beneath an oxic to suboxic water column, δ98Mo is dominantly controlled by the relative availability of Mn and Fe oxides, and the production of dissolved sulfide.

Dissolved redox sensitive elements, Re, U and Mo in intense denitrification zone of the Arabian Sea

The concentrations of redox sensitive elements rhenium, uranium and molybdenum have been measured in the suboxic water column of the Arabian Sea to determine their response (behavior) to intense oxygen minima and denitrifying conditions in the water column. North of 12°N, within the core of the intense oxygen minimum zone (OMZ) of the Arabian Sea, the dissolved oxygen levels drop to values as low as < 5 μM accompanied by the presence of secondary nitrite maxima (SNM) indicating the occurrence of denitrification in the water column. The distributions of Re, U and Mo in the Arabian Sea show that their dissolved concentrations in the suboxic layer are indistinguishable from those in the overlying and underlying oxic waters suggesting that there is no discernible removal of these elements from the suboxic denitrifying layers. In contrast, the lateral and vertical distribution of dissolved Re, U and Mo concentrations vary as a function of salinity suggesting their conservative behavior. The salinity normalized (35) Re, U and Mo concentrations in the Arabian Sea are 40 pmol/kg, 13.8 nmol/kg and 114 nmol/kg respectively. These concentrations are nearly identical to their abundances in the Bay of Bengal, a basin adjacent to the Arabian Sea characterized by high freshwater influx and fluvial sediments. The similarity in concentration suggests that the large variability in the biogenic and detrital particulate fluxes and the suboxic/denitrifying conditions between these two oceanic regions do not affect the concentrations of Re, U and Mo and that the prevailing biogeochemical conditions in these regions are inadequate for their removal from the water column.

Metabolic strategies of free-living and aggregate-associated bacterial communities inferred from biologic and chemical profiles in the Black Sea suboxic zone

The Black Sea is a permanently anoxic basin with a well-defined redox gradient. We combine environmental 16S rRNA gene data from clone libraries, terminal restriction fragment length polymorphisms, and V6 hypervariable region pyrosequences to provide the most detailed bacterial survey to date. Furthermore, this data set is informed by comprehensive geochemical data; using this combination of information, we put forward testable hypotheses regarding possible metabolisms of uncultured bacteria from the Black Sea's suboxic zone (microaerophily, nitrate reduction, manganese cycling, and oxidation of methane, ammonium, and sulfide). Dominant bacteria in the upper suboxic zone included members of the SAR11, SAR324, and Microthrix groups and in the deep suboxic zone included members of BS-GSO-2, Marine Group A, and SUP05. A particulate fraction (30μm filter) was used to distinguish between free-living and aggregate-attached communities in the suboxic zone. The particulate fraction contained greater diversity of V6 tag sequences than the bulk water samples. Lentisphaera, Epsilonproteobacteria, WS3, Planctomycetes, and Deltaproteobacteria were enriched in the particulate fraction, whereas SAR11 relatives dominated the free-living fraction. On the basis of the bacterial assemblages and simple modeling, we find that in suboxic waters, the interior of sinking aggregates potentially support manganese reduction, sulfate reduction, and sulfur oxidation.

Nitrification from the lower euphotic zone to the sub-oxic waters of a highly productive British Columbia fjord

Nitrification rates were measured monthly from April to October 2008, at depths ranging from the lower euphotic zone to the sub-oxic waters of Saanich Inlet. Ammonium (NH 4 +) and nitrite (NO 2 −) oxidation rates ranged from undetectable to 0.319 and 0.478 μmol L − 1 d − 1 , respectively. NH 4 + oxidation rates and concentrations were positively correlated at substrate concentrations less than 0.8 μmol NH 4 + L − 1 . Positive correlations between NH 4 + oxidation rates, NO 2 − concentrations, and NO 2 − oxidation rates were also observed, highlighting the important role that NH 4 + oxidation plays in supporting NO 2 − oxidation in Saanich Inlet. Despite the apparent dependence of NO 2 − oxidation rates on NH 4 + oxidation rates, the former was still 44% higher than the latter and we concluded that Saanich Inlet NO 2 − oxidation rates were augmented by fortnightly spring-tide nutrient renewal. From May to October, sub-oxic zone waters were isolated from any significant mixing events, and we estimated that nitrification was responsible for approximately 25% of dissolved oxygen consumption. This estimate is in close agreement with that calculated using Redfield stoichiometry, and as such highlights the accuracy with which nitrification rates can be quantified using incubation techniques. Finally, nitrification rates in the euphotic zone were at times substantial, and we suggest that earlier estimates of new production in Saanich Inlet may have been overestimated by approximately 15%.

Novel lineages of Prochlorococcus thrive within the oxygen minimum zone of the eastern tropical South Pacific

The eastern tropical Pacific Ocean holds two of the main oceanic oxygen minimum zones of the global ocean. The presence of an oxygen-depleted layer at intermediate depths, which also impinges on the seafloor and in some cases the euphotic zone, plays a significant role in structuring both pelagic and benthic communities, and also in the vertical partitioning of microbial assemblages. Here, we assessed the genetic diversity and distribution of natural populations of the cyanobacteria Prochlorococcus and Synechococcus within oxic and suboxic waters of the eastern tropical Pacific using cloning and sequencing, and terminal restriction fragment length polymorphism (T-RFLP) analyses applied to the 16S-23S rRNA internal transcribed spacer region. With the T-RFLP approach we could discriminate 19 cyanobacterial clades, of which 18 were present in the study region. Synechococcus was more abundant in the surface oxic waters of the eastern South Pacific, while Prochlorococcus dominated the subsurface low-oxygen waters. Two of the dominant clades in the oxygen-deficient waters belong to novel and yet uncultivated lineages of low-light adapted Prochlorococcus.

Primary Production in a Subtropical Stratified Coastal Lagoon—Contribution of Anoxygenic Phototrophic Bacteria

Anaerobic anoxygenic phototrophic bacteria can be found in the suboxic waters of shallow stratified coastal systems, and may play important roles in the total primary production of subtropical stratified coastal lagoons. We investigated the spatiotemporal variability of light CO(2) fixation and net oxygen production in the stratified Conceição Lagoon (Brazil) in summer and fall of 2007, as well as the contribution of bacteriochlorophyll a (BChl a)-containing bacteria to photosynthetically driven electron transfer. Both chlorophyll a (Chl a) and BChl a varied in space, while only BChl a varied in time (three-fold increase from summer to fall). In summer, net oxygen production and light CO(2) fixation were correlated, with both having higher rates with higher Chl a concentrations in the enclosed region of the lagoon. In fall, CO(2) fixation was decoupled from oxygen production. Denaturing gradient gel electrophoresis revealed that bacterial communities of oxic site 12 and suboxic site 33 formed one cluster, different from other oxic samples within the lagoon. In addition, BChl a/Chl a ratios at these sites were high, 40% and 45%, respectively. Light acted as the main factor controlling the BChl a concentration and CO(2) fixation rates. High turbidity within the enclosed area of the lagoon explained high BChl a and decoupling between CO(2) fixation and oxygen production in oxygenated waters. Contribution of purple sulfur bacteria to total bacterial density in suboxic waters was 1.2%, and their biomass contributed to a much higher percentage (12.2%) due to their large biovolume. Our results indicate a significant contribution of anaerobic anoxygenic bacteria to the primary production of the "dead zone" of Conceição Lagoon.

Heterotrophic denitrification vs. autotrophic anammox – quantifying collateral effects on the oceanic carbon cycle

Abstract. The conversion of fixed nitrogen to N2 in suboxic waters is estimated to contribute roughly a third to total oceanic losses of fixed nitrogen and is hence understood to be of major importance to global oceanic production and, therefore, to the role of the ocean as a sink of atmospheric CO2. At present heterotrophic denitrification and autotrophic anammox are considered the dominant sinks of fixed nitrogen. Recently, it has been suggested that the trophic nature of pelagic N2-production may have additional, "collateral" effects on the carbon cycle, where heterotrophic denitrification provides a shallow source of CO2 and autotrophic anammox a shallow sink. Here, we analyse the stoichiometries of nitrogen and associated carbon conversions in marine oxygen minimum zones (OMZ) focusing on heterotrophic denitrification, autotrophic anammox, and dissimilatory nitrate reduction to nitrite and ammonium in order to test this hypothesis quantitatively. For open ocean OMZs the combined effects of these processes turn out to be clearly heterotrophic, even with high shares of the autotrophic anammox reaction in total N2-production and including various combinations of dissimilatory processes which provide the substrates to anammox. In such systems, the degree of heterotrophy (ΔCO2:ΔN2), varying between 1.7 and 6.5, is a function of the efficiency of nitrogen conversion. On the contrary, in systems like the Black Sea, where suboxic N-conversions are supported by diffusive fluxes of NH4+ originating from neighbouring waters with sulphate reduction, much lower values of ΔCO2:ΔN2 can be found. However, accounting for concomitant diffusive fluxes of CO2, the ratio approaches higher values similar to those computed for open ocean OMZs. Based on this analysis, we question the significance of collateral effects concerning the trophic nature of suboxic N-conversions on the marine carbon cycle.

Biogeochemistry and dissolved oxygen dynamics at a subglacial upwelling, Midtre Lovénbreen, Svalbard

AbstractThere is a growing awareness that biological processes affect solute acquisition in glacial meltwaters. An unprecedented, high-resolution record of dissolved oxygen (DO) in emergent subglacial meltwaters at polythermal Midtre Lovénbreen, Svalbard, is discussed in conjunction with the major-ion chemistry of periodic water samples within the catchment. The subglacial outburst increased solutes passing through the proglacial area and was coupled to a seasonal transition in upwelling character from suboxic waters to those with large diurnal fluctuations in the levels of DO saturation, latterly returning to sustained suboxic runoff. During the period of daily variation, DO correlated positively with discharge and inversely to total dissolved solute concentration. Consideration of SO42− concentrations showed they exceed those achievable with complete consumption of DO in saturated supraglacial meltwater, and dissolution experiments illustrated protracted abiotic sulphide oxidation remains an unlikely cause. Similarly, relatively elevated ratios of NO3−/Cl− preclude denitrification aiding the catalysis of sulphide oxidation. Results here tentatively suggest sulphide oxidation is mediated by both aerobic and anoxic biochemical processes and that transition metals are the most likely oxidants. Bacteria are shown to impart a major control on the ionic composition of subglacial upwelling with variability in the ratio of oxygenated surface meltwaters and suboxic subglacial waters.