Impact of trace metals on denitrification in estuarine sediments of the Douro River estuary,Portugal

Denitrification influences the nitrogen budget in estuaries by removing fixed nitrogen from the inorganic pool; rates are dependent on both geological and geographic conditions as well as increasing anthropogenic impacts. In this study the effects of copper (Cu), chromium (Cr), zinc (Zn), cadmium (Cd) and lead (Pb), on the denitrification pathway were evaluated in subtidal and intertidal sediments of the Douro River estuary. Dinitrogen, N₂O and NO₂⁻ production rates were measured in triplicate slurries of field samples under different treatments of metal concentrations. Results demonstrated that similar metal amendments led to different site responses for denitrification, suggesting that variations in sediment properties (metal concentrations, grain size, organic matter content, etc.) and/or differences in denitrifying community tolerance modulate the level of metal toxicity. Denitrifying communities in subtidal muddy sediments were not affected by increasing concentrations of metals. In contrast, intertidal sandy sites revealed high sensitivity to almost all trace metals tested; almost complete inhibition by Cr (95%) and Cu (85%) was observed for 98 and 79 μg per gram of wet sediment respectively, and by Zn (92%) at the highest concentration added (490 μg per gram of wet sediment). Moreover, the addition of trace metals stimulated N₂O and NO₂⁻ accumulation in intertidal sandy (Zn, Cu, Cr and Cd) and muddy sediments (Cu and Zn), demonstrating a pronounced inhibitory effect on specific steps within the denitrification enzymatic system. In summary, the results obtained suggest that, according to the type of estuarine sediment, trace metals cannot only reduce total N removal from an estuary via denitrification but also can enhance the release of N₂O, a powerful greenhouse gas.     

Denitrification prevails over anammox in tropical mangrove sediments (Goa,India)

Denitrification, anammox (Anx) and di-nitrogen fixation were examined in two mangrove ecosystems- the anthropogenically influenced Divar and the relatively pristine Tuvem. Stratified sampling at 2 cm increments from 0 to 10 cm depth revealed denitrification as the main process of N₂ production in mangrove sediments. At Divar, denitrification was ∼3 times higher than at Tuvem with maximum activity of 224.51 ± 6.63 nmol N₂ g⁻¹ h⁻¹ at 0–2 cm. Denitrifying genes (nosZ) numbered up to 2 × 10⁷ copies g⁻¹ sediment and belonged to uncultured microorganisms clustering within Proteobacteria. Anammox was more prominent at deeper depths (8–10 cm) mainly in Divar with highest activity of 101.15 ± 87.73 nmol N₂ g⁻¹ h⁻¹ which was 5 times higher than at Tuvem. Di-nitrogen fixation was detected only at Tuvem with a maximum of 12.47 ± 8.36 nmol N₂ g⁻¹ h⁻¹. Thus, in these estuarine habitats prone to high nutrient input, N₂-fixation is minimal and denitrification rather than Anx serves as an important mechanism for counteracting N loading.     

Seasonal and spatial patterns of sedimentary denitrification rates in the Chukchi sea

Recent constructions of the global nitrogen budget estimate that at least half of the ocean's fixed nitrogen is lost by sedimentary denitrification, the majority of which occurs in continental shelves. The Arctic contains approximately 20% of the world's continental shelf, suggesting it is a substantial contributor to the global sedimentary denitrification rate. During two cruises in the summer and spring of 2002 and 2004, respectively, denitrification rates were calculated from the downward diffusive flux of nitrate in the shelf and slope sediments of the Chukchi Sea in the western Arctic. Additionally, in the spring of 2004, denitrification rates were determined by whole-core incubations in which the flux of nitrogen gas out of the sediments was measured. Measurements were made along three transects crossing the shelf and slope (50–3000 m), each transect having different overlying water characteristics. Denitrification rates generally decreased with increasing water depth: rates varied from about 1.6 mmol N m⁻² d⁻¹ for the shallow-water sediments to undetectable in deep-water sediments. Rates showed little variation between the two seasons. However, rates were found to correspond with differences in annual overlying primary productivities and particulate organic carbon export fluxes. An extrapolation to the whole Arctic yielded an average Arctic sedimentary denitrification rate of 13 Tg N yr⁻¹. Taken in the context of the global nitrogen budget, it is about 4–13% of the total sink of fixed nitrogen in the ocean.     

The effects of semi-lunar spring and neap tidal change on nitrification, denitrification and N2O vertical distribution in the intertidal sediments of the Yangtze estuary, China

To explore the influences of semi-lunar spring and neap tidal changes on nitrogen cycling in intertidal sediments, a comparative study among waterlogged, desiccated and reflooded systems was carried out in August 2005 and February 2006 by analyzing nitrification, denitrification and N₂O depth profiles in the intertidal flats of the Yangtze estuary. Laboratory experiments showed that alternating emersion and inundation resulted in the significant changes in nitrification and denitrification rates in the intertidal sediment systems. Due to the desiccation-related effects, lowest nitrification and denitrification rates were observed in the desiccated sediment cores. Highest nitrification and denitrification rates were however detected in the waterlogged and reflooded systems, respectively. It is hypothesized that the highest nitrification rates in the waterlogged sediments were mainly attributed to higher nitrifier numbers and NH₄⁺ being more available, whereas the availability of NO₃⁻ might dominate denitrification in the reflooded sediments. In addition, the highest N₂O concentrations were detected in the reflooded sediment cores, and the lowest found in the dried sediment cores. It was also shown that N₂O in the intertidal sediments was mainly from nitrification under the desiccated condition. In contrast, N₂O in the intertidal sediments was produced mainly via denitrification under the waterlogged and reflooded conditions. It is therefore concluded that the semi-lunar tidal cycle has a significant influence on nitrification, denitrification and N₂O production in the intertidal sediment systems.     

Nitrous oxide production and denitrification rates in estuarine intertidal saltmarsh and managed realignment zones

Increasing concerns over habitat loss and rising costs of sea defence maintenance due to rising sea levels, has seen increases in the practice of managed realignment and reflooding of former reclaimed areas of intertidal saltmarsh and mudflat around the world. These practices are taking place with little knowledge of their impact on soil biogeochemical processes. Rates of denitrification (using the acetylene inhibition technique) and nitrous oxide (N₂O) production were measured from a long-established saltmarsh (SM) and an adjacent, recently re-flooded managed realignment (MR) site comprising former arable land in the estuary of the River Torridge, Devon, UK. Incubations were carried out in closed chambers in which patterns of tidal flooding were simulated automatically. Measurements were made during periods of flood and non-flood over a total of four tidal inundations with estuarine water. During the latter two flooding episodes floodwater was amended with nitrate (NO₃⁻). Nitrous oxide production in the SM soil generally was lower than in the MR soil, with mean values and standard errors over the whole incubation of 0.27 ± 0.16 mg N₂O-N m⁻² h⁻¹ and 0.65 ± 0.15 mg N₂O-N m⁻² h⁻¹ respectively. Denitrification rates demonstrated a similar trend although generally were an order of magnitude higher than N₂O production, with mean rates and standard errors of 2.88 ± 1.12 mg N₂O-N m⁻² h⁻¹ in the SM soil and 3.39 ± 1.16 mg N₂O-N m⁻² h⁻¹ in the MR soil. The data suggest that both soils are net sinks for NO₃⁻ and net sources for N₂O. Both patterns of tidal inundation and floodwater chemistry affect the process rates in each soil differently. The impact of flooding with NO₃⁻ – amended water was greater on the SM soil than the MR soil, and it is likely that decomposing vegetation buried in the accreting sediments following reflooding at the MR site were supplying a source of N in the soil, and so process rates were less dependent upon external supplies. The act of managed realignment in intertidal zones could therefore result in an increase in mean production of N₂O in intertidal zones, at least in the short term.     

N2O Production, Nitrification and Denitrification in an Estuarine Sediment

The mechanisms regulating N₂O production in an estuarine sediment (Tama Estuary, Japan) were studied by comparing the change in N₂O production with those in nitrification and denitrification using an experimental continuous-flow sediment–water system with¹⁵N tracer (¹⁵N-NO−3 addition). From Feburary to May, both nitrification and denitrification in the sediment increased (246 to 716 μmol N m⁻² h⁻¹and 214 to 1260 μmol N m⁻² h⁻¹, respectively), while benthic N₂O evolution decreased slightly (1560 to 1250 nmol N m⁻² h⁻¹). Apparent diffusion coefficients of inorganic nitrogen compounds and O₂at the sediment–water interface, calculated from the respective concentration gradients and benthic fluxes, were close to the molecular diffusion coefficients (0·68–2·0 times) in February. However, they increased to 8·8–52 times in May except for that of NO−2, suggesting that the enhanced NO−3 and O₂supply from the overlying water by benthic irrigation likely stimulated nitrification and denitrification. Since the progress of anoxic condition by the rise of temperature from February to May (9 to 16 °C) presumably accelerated N₂O production through nitrification, the observed decrease in sedimentary N₂O production seems to be attributed to the decrease in N₂O production/occurrence of its consumption by denitrification. In addition to the activities of both nitrification and denitrification, the change in N₂O metabolism during denitrification by the balance between total demand of the electron acceptor and supply of NO−3+NO−2 can be an important factor regulating N₂O production in nearshore sediments.     

Benthic denitrification and nitrogen cycling at the slope and rise of the N.W. European Continental Margin (Goban Spur)

The rate of benthic denitrification in slope and rise sediments of a transect across the N.W. European Continental Margin (Goban Spur) was evaluated from 31 pore water nitrate profiles obtained during six cruises between May and October. All profiles had well separated zones of nitrification and denitrification. High near-surface nitrate concentrations prevented the influx of nitrate from the bottom water. The denitrification rates obtained from steady-state-modelling ranged from 0.13 to 2.56 μmol N cm⁻² y⁻¹ and showed an exponential increase both with decreasing water depth and with increasing rate of organic carbon degradation. Denitrification rates in a nearby canyon, which did not follow these relationships, were estimated to be much higher as a result of erosion and redistribution of organic matter. Denitrification at the Goban Spur slope and rise is much lower than previously reported for similar environments in the Pacific resulting predominantly from the different oxygen and nitrate concentrations in the bottom water. A weighted average for the whole slope and rise sediment system shows that 17% of the particulate organic nitrogen input (8.93 μmol N cm⁻² y⁻¹) is denitrified and only 1% is buried, the rest being released as nitrate. Although being ten times higher compared with basin sediments, denitrification on the slope and rise is several times lower than on the adjacent shelf.     

Nitrogen transformations as inferred from the activities of key enzymes in the Arabian Sea oxygen minimum zone

Vertical distributions of the potential activities of some key enzymes mediating nitrification and denitrification were investigated within the oxygen (O₂) minimum zone of the Arabian Sea at a number of locations between latitudes 17°N and 21°N and longitudes 63°E and 68°E so as to get an insight into the predominant biochemical mode(s) of production and consumption of nitrous oxide (N₂O). Results revealed that the dissimilatory nitrate (NO⁻₃) reduction activity was generally very low or absent within the σ_θ range 26.6–26.8, which corresponds to the Persian Gulf Watermass (PGW). Depth profiles of nitrate reductase (NaR), nitrite reductase (NiR) and ammonia monooxygenase (AMO) activities were compared with those of O₂, NO⁻₃, nitrite (NO⁻₂) and N₂O, and it is concluded that nitrifier denitrification rather than heterotrophic denitrification is active within the core of PGW. The presence of multiple peaks of AMO activity coinciding with distinct maxima in the O₂ profile and with a trend opposite to that of NaR activity indicates that the two processes, viz., classical and nitrifier denitrification, occur in discrete layers, probably determined by the variations in the ambient O₂ concentrations at various depths surrounding the PGW core. Further, it appears that at the depths where nitrifier denitrification is active in the absence of heterotrophic denitrification, N₂O builds up as its consumption may be inhibited by O₂. Possible reasons for the occurrence of appreciable nitrate deficit within the core of PGW, where dissimilatory NO⁻₃ reduction is lacking, are discussed.     

N2, N2O and O2Profiles in a Tagus Estuary Salt Marsh

Vertical gas profiles of N₂, N₂O and O₂were obtained in intact sediment cores from a Tagus estuary salt marsh using membrane inlet mass spectrometry. This technique allows direct measurements of dissolved gas concentrations with minimal disturbance. O₂concentrations decreased sharply with depth, becoming undetectable below 14mm. Denitrification products (N₂and N₂O) occurred in the surface layer of the sediment where O₂was present. Diffusion of N₂and N₂O from the anaerobic zone, denitrification in anaerobic microsites and aerobic denitrification are possible explanations for this observation. N₂was the sole product of denitrification in control sediment cores probably because of the great demand for electron acceptors in this sediment. The addition of NO₃ ⁻ and CH₃CO₂ ⁻ increased the concentrations of N₂and N₂O in the sediment. Significantly higher concentrations in treated cores occurred between 1·5 and 2·0cm for N₂and between 0·5 and 1·5cm for N₂O. The peak in N₂concentration occurred in the anaerobic zone of the sediment, close to the aerobic–anaerobic interface while the peak in N₂O concentration occurred above this interface where concentrations of O₂were approximately 10μM. This is indicative that, in this sediment, production of N₂O is less sensitive to the presence of O₂than reduction of N₂O to N₂.     

Microbial nitrogen transformations in sediments and inorganic nitrogen fluxes across the sediment/water interface on the South Island West Coast,New Zealand

In January 1982, sediment microbial N transformations and inorganic N fluxes across the sediment/water interface were studied at nine sites off the South Island West Coast, New Zealand. The sediments showed a great variety in physical, chemical and biological properties. The sediment organic matter had a molar $\text{C}\text{N}$ ratio of 5.9–10.9, and the total $\text{N}\text{P}$ ratio was 1.2–4.0. The denitrification capacity in the top 7.5 cm of sediment was 0.1–77.2 mmol N m^?2 day^?1 and generally declined with increasing sediment depth. The in situ denitrification rate was 0.02–1.84 mmol N m^?2 day^?1 and highest activities were generally found in surface sediments and at 6–7.5 cm depth. Denitrification accounted for 82–100% of total nitrate reduction. Net N mineralization was indirectly estimated at 0.6–2.4 mmol N m^?2 day^?1, and the experimental determination of this N transformation gave 0.6–3.2 mmol N m^?2 day^?1. Denitrification accounted for 3–75% of net N mineralization. The diffusive flux of ammonium and nitrate across the sediment/water interface was 0.1–0.7 and 0.1–0.6 mmol N m^?2 day^?1, respectively.     

A role for nitrite in the production of nitrous oxide in the lower euphotic zone of the oligotrophic North Pacific Ocean

Understanding the role of the oceans in the Earth's changing climate requires comprehension of the relevant metabolic pathways which produce climatically important trace gases. The global ocean represents one of the largest natural sources of nitrous oxide (N₂O) that is produced by selected archaea and/or bacteria during nitrogen (N) metabolism. In this study, the role of nitrite (NO₂⁻) in the production of N₂O in the upper water column of the oligotrophic North Pacific Subtropical Gyre was investigated, focusing primarily on the lower euphotic zone where NO₂⁻ concentrations at the primary NO₂⁻ maximum reached 195 nmol L⁻¹. Free-drifting sediment trap arrays were deployed to measure N cycle processes in sinking particulate material and the addition of selected N substrates to unpreserved sediment traps provided an experimental framework to test hypotheses regarding N₂O production pathways and controls. Sinking particles collected using NO₂⁻-amended, unpreserved sediment traps exhibited significant production of N₂O at depths between 100 and 200 m. Subsequent stable isotope tracer measurements conducted on sediment trap material amended with ¹⁵NO₂⁻ yielded elevated δ¹⁵N values of N₂O, supporting N₂O production via a NO₂⁻ metabolism pathway. Experiments on seawater collected from 150 m showed N₂O production via NO₂⁻ metabolism also occurs in the water-column and indicated that the concentration of NO₂⁻ relative to NH₄⁺ availability may be an important control. These findings provide evidence for the production of N₂O via nitrifer-denitrification in the lower euphotic zone of the open ocean, whereby NO₂⁻ is reduced to N₂O by ammonia-oxidizing microorganisms.     

Nitrous oxide and N-nutrient cycling in the oxygen minimum zone off northern Chile

Measurements of dissolved gases (O₂, N₂O), nutrients (NO₃⁻, NO₂⁻, PO₄³⁻), and oceanographic variables were performed off northern Chile (∼21°S) between March 2000 and July 2004, in order to characterize the existing oxygen minimum zone (OMZ) and identify processes involved in N₂O cycling. Both N₂O and NO₃⁻ displayed sharp, shallow peaks with concentrations of up to 124 nM (1370% saturation) and 26 μM, respectively, in association with a strong oxycline that impinges on the euphotic zone. NO₂⁻ accumulation below the oxycline's base reached up to 9 μM. The vertical distribution of physical and chemical parameters and the existing relationships between apparent oxygen utilization (AOU), apparent N₂O production (ΔN₂O), and NO₃⁻ revealed three main layers within the upper OMZ. The first layer, or the upper part of the oxycline, is located between the base of the mixed layer and the mid-point of the oxycline (around σ_t=25.5 kg m⁻³). There the O₂ declines from ∼250 to ∼50 μM, and strong (but opposing) O₂ and NO₃⁻ gradients and their associated AOU–ΔN₂O and AOU–NO₃⁻ relationships indicate that nitrification produces N₂O and NO₃⁻ in the presence of light. The second layer, or lower part of the oxycline, represents the upper OMZ boundary and is located between the middle and the base of the oxycline (25.9<σ_t<26.1 kg m⁻³). In this layer NO₃⁻ reduction begins at O₂ levels ranging from ∼50 to ∼11 μM and accumulation of 41–68% of the ΔN₂O pool occurs. The accumulation of N₂O (but not of NO₂⁻ or NH₄⁺) and the observed AOU–ΔN₂O and AOU–NO₃⁻ relationships (which are opposite to those of the overlying first layer) suggest that a coupling between nitrification and NO₃⁻ reduction is involved in N₂O cycling in this second layer. The third layer is the OMZ core, where the O₂ concentration remains constant (O₂<11 μM). It coincides with σ_t>26.2 kg m⁻³, which is typical of Equatorial Subsurface Water (ESSW). In this layer, N₂O and NO₃⁻ continue to decrease, but a large NO₂⁻ accumulation is observed. Considering all the data, a biogeochemical model for the upper OMZ off northern of Chile is proposed, in which nitrification and denitrification differentially mediate N₂O cycling in each layer.     

Benthic fluxes in a tidal salt marsh creek affected by fish farm activities: Río San Pedro (Bay of Cádiz,SW Spain)

Benthic fluxes of dissolved inorganic carbon, total alkalinity, oxygen, nutrients, nitrous oxide and methane were measured in situ at three sites of Río San Pedro salt marsh tidal creek (Bay of Cádiz, SW Spain) during three seasons. This system is affected by the discharges of organic carbon and nutrients from the surrounding aquaculture installations. Sediment oxygen uptake rates and inorganic carbon fluxes ranged respectively from 16 to 79 mmol O₂ m^? 2 d^? 1 and from 48 to 146 mmol C m^? 2 d^? 1. Benthic alkalinity fluxes were corrected for the influence of NH₄⁺ and NO₃^? + NO₂^? fluxes, and the upper and lower limits for carbon oxidation rates were inferred by considering two possible scenarios: maximum and minimum contribution of CaCO₃ dissolution to corrected alkalinity fluxes. Average C_ox rates were in all cases within ± 25% of the upper and lower limits and ranged from 40 to 122 mmol C m^? 2 d^? 1. Whereas carbon mineralization did not show significant differences among the sites, C_ox rates varied seasonally and were correlated with temperature (r² = 0.72). During winter and spring denitrification was estimated to account for an average loss of 46% and 75%, respectively, of the potentially recyclable N, whereas during the summer no net removal was observed. A possible shift from denitrification to dissimilatory nitrate reduction to ammonium (DNRA) during this period is argued. Dissolved CH₄ and N₂O fluxes ranged from 5.7 to 47 μmol CH₄ m^? 2 d^? 1 and 4.3 to 49 μmol N–N₂O m^? 2 d^? 1, respectively, and represented in all cases a small fraction of total inorganic C and N flux. Overall, about 60% of the total particulate organic matter that is discharged into the creek by the main fish farm facility is estimated to degrade in the sediments, resulting in a significant input of nutrients to the system.     

Denitrification and the nitrogen gas excess in the eastern tropical South Pacific oxygen deficient zone

Recent studies of the nitrogen gas excess produced during water column denitrification have indicated that water column denitrification rates calculated using nitrate deficit-type methods could be a substantial underestimate. Since there are no other significant processes that produce (or consume) N₂ in the oxygen deficient zone (ODZ), its excess above background can be used to estimate the amount of denitrification, avoiding assumptions made in nitrate deficit calculations of the composition of the respired organic matter and also uncertainties in the nitrogen removal pathways. Dissolved N₂, Ar, and nutrient concentrations were measured at 2 stations in the ODZ of the eastern tropical South Pacific (ETSP) in order to compare the nitrogen gas excess with the dissolved inorganic nitrogen (DIN) deficit due to denitrification. In contrast with previous findings in the Arabian Sea ODZ, the shapes of the N₂ excess and DIN deficit profiles were similar in the ETSP ODZ, with maxima at the top of the ODZ. Maximum DIN deficits at each station were 19 and 18 μM N compared to the maximum N₂ excesses of 15 and 20 μM N, respectively. Given the same considerations of the volume and residence time for the oxygen deficient zone waters, denitrification rates for the ETSP estimated from the N₂ excess would be comparable or no greater than 30% larger than the one determined using the DIN deficit. This implies that the source of the DIN removed from the ODZ is either deep sea nitrate or organic matter with an N:P ratio close to Redfield.     

Nitrogen and oxygen isotopic composition of N2O from suboxic waters of the eastern tropical North Pacific and the Arabian Sea—measurement by continuous-flow isotope-ratio monitoring

Nitrous oxide (N₂O) is a trace gas that is increasing in the atmosphere. It contributes to the greenhouse effect and influences the global ozone distribution. Recent reports suggest that regions such as the Arabian Sea may be significant sources of atmospheric N₂O.In the ocean, N₂O is formed as a by-product of nitrification and as an intermediary of denitrification. In the latter process, N₂O can be further reduced to N₂. These processes, which operate on different source pools and have different magnitudes of isotopic fractionation, make separate contributions to the ¹⁵N and¹⁸O isotopic composition of N₂O. In the case of nitrification in oxic waters, the isotopic composition of N₂O appears to depend mainly on the ¹⁵N/¹⁴N ratio of NH⁺₄ and the ¹⁸O/¹⁶O ratio of O₂ and H₂O. In suboxic waters, denitrification causes progressive ¹⁵N and ¹⁸O enrichment of N₂O as a function of degree of depletion of nitrate and dissolved oxygen. Thus the isotopic signature of N₂O should be a useful tool for studying the sources and sinks for N₂O in the ocean and its impact on the atmosphere.We have made observations of N₂O concentrations and of the dual stable isotopic composition of N₂O in the eastern tropical North Pacific (ETNP) and the Arabian Sea. The stable isotopic composition of N₂O was determined by a new method that required only 80–100 nmol of N₂O per sample analysis. Our observations include determinations across the oxic/suboxic boundaries that occur in the water columns of the ETNP and Arabian Sea. In these suboxic waters, the values of δ¹⁵N and δ¹⁸O increased linearly with one another and with decreasing N₂O concentrations, presumably reflecting the effects of denitrification. Our results suggest that the ocean could be an important source of isotopically enriched N₂O to the atmosphere.     

Response of Nereis diversicolor (Polychaeta, Nereidae) populations to reduced wastewater discharge in the polluted estuary of Oued Souss, Bay of Agadir, Morocco

Field investigations on the population dynamics of Nereis diversicolor were carried out from January 2002 to December 2003 in the estuary of Oued Souss (southwestern Morocco) to determine the changes caused by setting up of a domestic and industrial wastewater purification plant (M'zar) before and after by the end of wastewater discharges in November 2002 on the structure of the ecosystem. Samples of N. diversicolor were collected monthly in the intertidal zone at low tide before (during 2002) and after (during 2003) the end of wastewater discharges.Separation of cohorts using the Algorithm EM method (McLachlan, G.J., Krishnan, T., 1997. The EM algorithm and extensions. Wiley Series in Probability and Statistics. Wiley, New York, 274 pp.) allowed determination of the growth rate (mm day⁻¹) by cohort and the annual production. The data showed significant differences between populations of Nereis diversicolor before and after the end of wastewater discharges. During the wastewater discharge period (2002), the population had a mean annual density of 1992 ind m⁻², a mean annual biomass of 75.52 g DW m⁻² and an annual secondary production of 141.3 g DW m⁻² with a P/B ratio of 1.87. After the end of discharges (2003), density, biomass and secondary production decreased significantly. The annual averages for these parameters were 740 ind m⁻², 14.16 g DW m⁻² and 23.83 g DW m⁻², respectively, with a P/B ratio of 1.68.The important decrease observed in density, biomass and secondary production of Nereis diversicolor may be attributed (a) to the environmental changes observed after the end of wastewater discharges in the estuary of Oued Souss, namely the increase of salinity and the decrease of organic matter content, and (b) to the migration of this species towards other areas.     

Upwelling and river runoff as sources of dissolved nitrous oxide to the Alsea estuary,Oregon

Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N₂O to the atmosphere. Large and rapid increases in the concentrations of N₂O, NO₃^?, and NO₂^? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N₂O in rivers.     

源自生物絮团产絮凝剂的异养硝化-好氧反硝化菌xt1的鉴定及其脱氮特性

为了高效进行水体脱氮,本实验从形成于凡纳滨对虾(Litopenaeus vannamei)养殖水体的生物絮团中分离到一株具产絮能力的脱氮菌xt1,经16S r RNA基因测序与生理生化分析确定菌株xt1为短小芽孢杆菌(Bacillus pumilus)。在此基础上,本文研究了该菌的脱氮特性。结果表明:菌株xt1最佳碳源为葡萄糖,以其为底物对氨氮、硝态氮去除率分别达95.56%和57.40%。以蔗糖为碳源亦具较高脱氮率,对氨氮、硝态氮去除率分别达69.95%和49.50%;该菌能利用有机氮加速生长,添加0.25%、0.5%、1%和2%的蛋白胨能促进OD600,分别达到0.925、1.034、1.103和1.314,均高于未加蛋白胨下的生长,且氨氮去除率均超过90%,硝态氮去除率均超过88%;该菌能适应20—200mg/L无机氮浓度;该菌能以NH4+-N、NO2–-N或NO3–-N为唯一氮源进行异养硝化-好氧反硝化,反应84h去除率分别达到94.16%、47.60%和91.17%。其中,该菌的硝化形式是将氨氮转化为气态氮脱除,其硝态氮反硝化形式是先将硝态氮转化为亚硝氮,再以气态氮脱除。在进行异养硝化-好氧反硝化同时,菌株xt1体现絮凝特性,絮凝率最高分别达到82.28%、73.15%和75.60%;此外,添加该菌于养殖水体中能加速生物絮团形成,同时提高脱氮率。各项结果表明,菌株xt1可作为水产养殖水体脱氮的备选菌株。     

Dry atmospheric deposition and diazotrophy as sources of new nitrogen to northwestern Mediterranean oligotrophic surface waters

Atmospheric dry deposition of nitrogen (N) and dinitrogen (N₂) fixation rates were assessed in 2004 at the time-series DYFAMED station (northwestern Mediterranean, 43°25′N, 7°52′E). The atmospheric input was monitored over the whole year. Dinitrogen fixation was measured during different seasonal trophic states (from mesotrophy to oligotrophy) sampled during nine cruises. The bioavailability of atmospherically deposited nutrients was estimated by apparent solubility after 96 h. The solubility of dry atmospheric N deposition was highly variable (from ∼18% to more than 96% of total N). New N supplied to surface waters by the dry atmospheric deposition was mainly nitrate (NO₃⁻) (∼57% of total N, compared to ∼6% released as ammonium (NH₄⁺)). The mean bioavailable dry flux of total N was estimated to be ∼112 μmol m⁻² d⁻¹ over the whole year. The NO₃⁻ contribution (70 μmol NO₃⁻ m⁻² d⁻¹) was much higher than the NH₄⁺ contribution (1.2 μmol NH₄⁺ m⁻² d⁻¹). The N:P ratios in the bioavailable fraction of atmospheric inputs (122.5–1340) were always much higher than the Redfield N:P ratio (16). Insoluble N in atmospheric dry deposition (referred to as “organic” and believed to be strongly related to anthropogenic emissions) was ∼40 μmol m⁻² d⁻¹. N₂ fixation rates ranged from 2 to 7.5 nmol L⁻¹ d⁻¹. The highest values were found in August, during the oligotrophic period (7.5 nmol L⁻¹ at 10 m depth), and in April, during the productive period (4 nmol L⁻¹ d⁻¹ at 10 m depth). Daily integrated values of N₂ fixation ranged from 22 to 100 μmol N m⁻² d⁻¹, with a maximum of 245 μmol N m⁻² d⁻¹ in August. No relationship was found between the availability of phosphorus or iron and the observed temporal variability of N₂ fixation rates. The atmospheric dry deposition and N₂ fixation represented 0.5–6% and 1–20% of the total biological nitrogen demand, respectively. Their contribution to new production was more significant: 1–28% and 2–55% for atmospheric dry deposition and N₂ fixation, respectively. The dry atmospheric input was particularly significant in conditions of water column stratification (16–28% of new production), while N₂ fixation reached its highest values in June (46% of new production) and in August (55%).     

Stable isotopes of carbon and nitrogen in suspended matter and sediments from the Godavari estuary

Spatial distribution of the carbon and nitrogen content and their isotopic enrichment in suspended matter and sediments were measured in the Godavari estuary to identify the sources and transformation mechanism of organic matter. Significant variability in isotopic distribution was found over the entire length of the Godavari estuary, suggesting multiple sources of organic matter. The mean isotopic ratios (δ¹³C_sed −25.1 ± 0.9, δ¹³C_sus −24.9 ± 1, δ¹⁵N_sed 8.0 ± 2 and δ¹⁵N_sus 6.5 ± 0.9‰) and elemental concentrations (C_sed 0.45 ± 0.2%, C_sus 0.9 ± 0.7%, N_sed 0.07 ± 0.05% and N_sus 0.16 ± 0.1%) support a predominantly terrigenous source. Significant enrichment in the isotopic ratios of δ¹³C from the upper to lower estuary in both suspended (−27.5 and −24.3‰, respectively) and sedimentary (−26.2 and −24.9‰, respectively) phases indicates a decrease in the influence of terrigeneous material toward the mouth of the estuary. A significant positive relationship exists between the δ¹³C of suspended and sediment, which indicates that these two organic carbon pools are likely coupled in the form of a significant exchange between the two phases. A positive relationship exists between chlorophyll a and suspended organic matter, which may mean that a significant source of organic carbon is the in situ produced phytoplankton. But, applying a simple mixing model to our isotopes, data yielded about 46% as the contribution of the terrestrial source to suspended matter, which may support the excessive heterotrophic activity in the Godavari estuary reported earlier.