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1.
A floodplain aquifer within an agricultural watershed near Madison, Wisconsin (USA), was studied to determine whether denitrification was occurring below the surface organic layer. Groundwater levels and concentrations of O2, Cl?, NO 3 ? , SO 4 2? , dissolved organic carbon (DOC), and major cations were monitored over a 1-year period along a 230-m transect between an agricultural field and a stream discharge point. Seventeen groundwater samples were analyzed for δ15NNO3 and δ18ONO3 composition. Samples in which NO 3 ? was too low for stable isotope analysis were analyzed for excess dissolved N2. Groundwater NO 3 ? concentrations declined between the agricultural field and the discharge point. Chloride and δ15NNO3/δ18ONO3 data indicated that the drop in NO 3 ? was caused primarily by dilution of shallow NO 3 ? -rich water with deeper, NO 3 ? -depleted groundwater. Two localized zones of denitrification were identified in the upland-wetland transition by their δ15NNO3 and δ18ONO3 signatures, and two in the stream hyporheic zone by the presence of excess dissolved N2. The combined stratigraphic, hydrologic, and geochemical data in these locations correspond to groundwater mixing zones where NO 3 ? is delivered to subsurface layers that support denitrification fueled by dissolved (e.g. DOC or dissolved Fe(II)) and/or solid-phase (e.g. particulate organic carbon, solid-associated Fe(II), or pyrite) electron donors.  相似文献   

2.
Sediment denitrification was monthly evaluated in two tropical coastal lagoons with different trophic states using the 15N isotope pairing technique. Denitrification rates were very low in both environments, always <5.0 μmol N2 m?2 h?1 and were not significantly different between them. Oxygen consumption varied from 426 to 4248 μmol O2 m?2 h?1 and was generally three times higher in the meso-eutrophic than the oligotrophic lagoon. The low denitrification activity was ascribed to both low water NO3 ? concentrations (<2.0 μM) and little nitrate supply from nitrification. There was no correlation of denitrification with nitrate or ammonium fluxes. Sediments in temperate environments with similar oxygen consumption rates usually presented a higher proportion of nitrification–denitrification rates. Sediment oxygen consumption was a good predictor of sediment denitrification in both studied lagoons.  相似文献   

3.
In this study rates of oxygen, ammonium (NH4 +), nitrate (NO3 −), nitrite (NO2 −), and nitrous oxide (N2O) fluxes, nitrogen (N) fixation, nitrification, and denitrification were compared between two intertidal sites for which there is an abundant global literature, muddy and sandy sediments, and two sites representing the rocky intertidal zone where biogeochemical processes have scarcely been investigated. In almost all sites oxygen production rates greatly exceeded oxygen consumption rates. During daylight, NH4 + and NO3 − uptake rates together with ammonification could supply the different N requirements of the primary producer communities at all four sites; N assimilation by benthic or epilithic primary producers was the major process of dissolved inorganic nitrogen (DIN) removal; N fixation, nitrification, and denitrification were minor processes in the overall light DIN cycle. At night, distinct DIN cycling processes took place in the four environments, denitrification rates ranged from 9 ± 2 to 360 ± 30 μmol N2 m−2 h−1, accounting for 10–48% of the water column NO3 − uptake; nitrification rates varied from 0 to 1712 ± 666 μmol NH4 + m−2 h−1. A conceptual model of N cycle dynamics showed major differences between intertidal sediment and rocky sites in terms of the mean rates of DIN net fluxes and the processes involved, with rocky biofilm showing generally higher fluxes. Of particular significance, the intertidal rocky biofilms released 10 times the amount of N2O produced in intertidal sediments (up to 17 ± 6 μmol N2O m−2 h−1), representing the highest N2O release rates ever recorded for marine systems. The biogeochemical contributions of intertidal rocky substrata to estuarine and coastal processes warrant future detailed investigation.  相似文献   

4.
Groundwater samples from boreholes and springs in the unconfined Chalk aquifer of Cambridgeshire were analysed for N2O and other N species on a monthly basis between March 1995 and February 1996. Land use in the study area is devoted to intensive arable farming supported by the application of N-based fertilisers. All groundwater samples were strongly oversaturated with N2O, with concentrations ranging from 13 to 320 times the saturation concentration with respect to air-equilibrated water.A very good positive correlation between N2O and NO3 concentrations was obtained (r2=0.80), but no relationship was established between N2O and NO2 or NH4 concentrations. Concentrations of N2O and NO3 increased continuously in the direction of groundwater flow, with molar net gain ratios of NO3 to N2O varying between 204 and 410. These ratios are within the range reported in previous studies of nitrification. Corresponding dissolved O2 levels in groundwater samples were moderately undersaturated, further indicating that the main source of N2O in Chalk groundwater in Cambridgeshire is probably nitrification. No consumption of N2O seems to take place within the unconfined aquifer with degassing to the atmosphere apparently being the sole mechanism for N2O removal from groundwater.An estimated N2O flux of around 0.05 kg N2O ha−1 a−1 from the sampled groundwater discharge points to the atmosphere was calculated for the study area. This figure is likely to be much higher, since it does not account for diffuse N2O emissions from groundwater seepage areas or any degassing from the unconfined aquifer through the unsaturated zone. Both these processes will contribute substantially to the total aerial flux, thus suggesting that groundwater may be a significant contributor to the global N2O budget.  相似文献   

5.
Constructed wetlands (CWs) are considered important sources of nitrous oxide (N2O). Various reports in the literature indicate that CWs have high N2O emission rates. The release of N2O from CWs treating wastewater emissions range from ?16.7 to 188 mg N2O m?2day?1. N2O in CWs is produced mainly by nitrification, denitrification, nitrifier denitrification, and nitrate-ammonification. Denitrification is considered the major source of N2O under most conditions. In recent years, two main methods of sampling N2O gas in CWs have been employed, including the headspace equilibration technique and the closed static chambers technique. N2O emission may be affected by various operating parameters and environmental conditions. One of the main environmental factors affecting the removal of nitrogen in CWs is dissolved oxygen, which affects nitrification and denitrification processes, thus greatly influencing N2O emission. CW gas dynamics is affected mainly by season and weather conditions, especially temperature and moisture. Aquatic plants, flow regime, oxidation–reduction potential, nitrate concentration, C/N ratio and other factors can affect N2O emission in CWs.  相似文献   

6.
The biological and physical controls on microbial processes that produce and consume N2O in soils are highly complex. Isotopomer ratios of N2O, with abundance of 14N15N16O, 15N14N16O, and 14N14N18O relative to 14N14N16O, are promising for elucidation of N2O biogeochemistry in an intact ecosystem. Site preference, the nitrogen isotope ratio of the central nitrogen atom minus that of the terminal nitrogen atom, is useful to distinguish between N2O via hydroxylamine oxidation and N2O via nitrite reduction.We applied this isotopomer analysis to a groundwater system in a temperate coniferous-forested ecosystem. Results of a previous study at this location showed that the N2O concentration in groundwater varied greatly according to groundwater chemistry, i.e. NO3−, DOC, and DO, although apportionment of N2O production to nitrification or denitrification was ambiguous. Our isotopic analysis (δ15N and δ18O) of NO3− and N2O implies that denitrification is the dominant production process of N2O, but definitive information is not derived from δ15N and δ18O analysis because of large variations in isotopic fractionations during production and consumption of N2O. However, the N2O site preference and the difference in δ15N between NO3− and N2O indicate that nitrification contributes to total N2O production and that most measured N2O has been subjected to further N2O reduction to N2. The implications of N2O biogeochemistry derived from isotope and isotopomer data differ entirely from those derived from conventional concentration data of DO, NO3−, and N2O. That difference underscores the need to reconsider our understanding of the N cycle in the oxic-anoxic interface.  相似文献   

7.
Fjords and estuaries exchange large amounts of solutes, gases, and particulates between fluvial and marine systems. These exchanges and their relative distributions of compounds/particles are partially controlled by stratification and water circulation. The spatial and vertical distributions of N2O, an important greenhouse gas, along with other oceanographic variables, are analyzed from the Reloncaví estuary (RE) (~41° 30′ S) to the gulf of Corcovado in the interior sea of Chiloé (43° 45′ S) during the austral winter. Freshwater runoff into the estuary regulated salinity and stratification of the water column, clearly demarking the surface (<5 m depth) and subsurface layer (>5 m depth) and also separating estuarine and marine influenced areas. N2O levels varied between 8.3 and 21 nM (corresponding to 80 and 170 % saturation, respectively), being significantly lower (11.8 ± 1.70) at the surface than in subsurface waters in the Reloncaví estuary (14.5 ± 1.73). Low salinity and NO3 ?, NO2 ?, and PO4 3? levels, as well as high Si(OH)4 values were associated with low surface N2O levels. Remarkably, an accumulation of N2O was observed in the subsurface waters of the Reloncaví sound, associated with a relatively high consumption of O2. The sound is exposed to increasing anthropogenic impacts from aquaculture and urban discharge, occurring simultaneously with an internal recirculation, which leads to potential signals of early eutrophication. In contrast, within the interior sea of Chiloé (ISC), most of water column was quasi homohaline and occupied by modified subantarctic water (MSAAW), which was relatively rich in N2O (12.6 ± 2.36 nM) and NO3 ? (18.3 ± 1.63 μM). The relationship between salinity, nutrients, and N2O revealed that water from the open ocean, entering into ISC (the Gulf of Corcovado) through the Guafo mouth, was the main source of N2O (up to 21 nM), as it gradually mixed with estuarine water. In addition, significant relationships between N2O excess vs. AOU and N2O excess vs. NO3 ? suggest that part of N2O is also produced by nitrification. Our results show that the estuarine and marine waters can act as light source or sink of N2O to the atmosphere (air–sea N2O fluxes ranged from ?1.57 to 5.75 μmol m?2 day?1), respectively; influxes seem to be associated to brackish water depleted in N2O that also caused a strong stratification, creating a barrier to gas exchange.  相似文献   

8.
With rapidly industrial and agricultural development,more and more fertilizers,chemicals and heavy ions will be discharged into lakes and rivers,which would cause lake eutrophication and quality deterioration in drinking water sources.Therefore,denitrification is essential for controlling the amounts of nitrogen,During the transformation process from nitrate to the end products-nitrogen and several intermediated[e.g.nitrite(NO2^-),nitrous oxide(N2O) and nitric oxide(NO)]may be accumulated,which have more toxic influences on the environment.in This study,the denitrification effect of Paracoccus Denitrificans was examined on the changes between oxic and anoxic conditions at varying pH.At pH=7.5,denitrification proceeded well after 3 switches from oxic to anoxic conditions and vice versa,Production of N2 was constant and the amounts of NO2-,N2O and NO were extremely low.How ever,at pH=6.8,denitrification activity was inhitied and there large amounts of the intermaediates.The denitrifying bacteria decreased violently in dry weight and were washed out.  相似文献   

9.
Fate of riverine nitrate entering a well defined turbid estuary receiving discharges from the Atchafalaya River, a distributary of the Mississippi River, was determined. Seasonal distribution of NO3 and its transformations were measured in Four League Bay (9,300 ha). Denitrification was estimated by incubating wet samples in the presence of acetylene and monitoring N2O production. The annual sediment accumulation of N was also determined within the bay and within the adjacent marshes. Nitrogen accumulation ranged from 6.0 to 23 gN per m2 per yr on the marsh and 6.1 to 11.2 gN per m2 per yr in the bay. Denitrification in this system was controlled by the availability of NO3 ? with fluxes ranging from 2 to 70 ngN per g per hr. The annual (N2O +N2)-N emission was equivalent to 142 and 120 μg per g or 2.1 and 1.7 gN per m2 from the 5 bay and 5 marsh stations, respectively. Approximately 1.95×105 kgN, predominantly as N2, is being returned to the atmosphere via denitrification. We estimate this to be equivalent to 50% of the riverine NO3 ? entering this estuary. A significant amount was also assimilated within the estuary.  相似文献   

10.
Rates of nitrification along an estuarine gradient in Narragansett Bay   总被引：1，自引：0，他引：1  
Rates of pelagic nitrification, measured using N-Serve-sensitive [14C]bicarbonate uptake, varied by as much as an order-of-magnitude among three sites along the salinity gradient of Narragansett Bay (Rhode Island, United States). Rates were always higher at the Providence River estuary site (0.04–11.2 μmol N I?1 d?1) than at either the lower Narragansett Bay site (0.02–0.98 μmol N I?1d?1) or the freshwater Blackstone River site (0.04–1.7 μmol N I?1d?1). Although temperature was the most important variable regulating the annual cycle of nitrification, ammonium concentrations were most likely responsible for the large differences in rates among the three sites in summer. At the levels found in this estuarine system, salinity and concentrations of oxygen or total suspended matter did not appear to have a direct measurable effect on nitrification and pH did only occasionally. Nitrification played an important role in the nitrogen cycle at all three sites. In Narragansett Bay, nitrification contributed 55% of the NO2 ? and NO3 ? entering annually, and was the major source during spring and summer. Water from offshore was the only other large source of NO2 ? and NO3 ?, contributing 34%. High summer rates of nitrification could support much of the phytoplankton uptake of NO2 ? and NO3 ?. In the Providence River estuary, the largest annual input of NO2 ? and NO3 ? was from rivers (54%), although nitrification (28%) and water from lower portions of the bay (11%) also made large contributions. Again, nitrification was most important in the summer. The high rates of nitrification in the Providence River estuary during summer were also likely to be important in terms of oxygen demand, and the production of nitric and nitrous oxides. In the Blackstone River, NO2 ? and NO3 ? concentrations increased as the river flowed through Rhode Island, and nitrification was a possible source.  相似文献   

11.
A “snap shot” survey of the Mississippi estuary was made during a period of low river discharge, when the estuarine mixing zone was within the deltaic channels. Concentrations of H+, Ca2+, inorganic phosphorus and inorganic carbon suggest that the waters of the river and the low salinity (<5‰) portion of the estuary are near saturation with respect to calcite and sedimentary calcium phosphate. An input of oxidized nitrogen species and N2O was observed in the estuary between 0 and 4‰ salinity. The concentrations of dissolved NH4 + and O2, over most of the estuary, appeared to be influenced by decomposition of terrestrial organic matter in bottom sediments. The estuarine bottom also appears to be a source of CH4 which has been suggested to originate from petroleum shipping and refining operations. Estuarine mixing with offshore Gulf waters was the dominant influence on distributions of dissolved species over most of the estuary (i.e., from salinities >5‰). The phytoplankton abundance (measured as chlorophylla) increased as the depth of the mixed layer decreased in a manner consistent with that expected for a light-limited ecosystem. Fluxes of NO3 ?+NO2 ? and soluble inorganic phosphorus to the Gulf of Mexico were estimated to be 3.4±0.2×103 g N s?1 and 1.9±0.2 g P s?1 respectively, at the time of this study.  相似文献   

12.
The stable isotope nitrogen-15 (15N) is a robust indicator of nitrogen (N) source, and the joint use of δ15N and δ18O–NO3 ? values can provide more useful information about nitrate source discrimination and N cycle process. The δ15N and δ18O–NO3 ? values, as well as major ion tracers, from Taihu Lake in east China were investigated to identify the primary nitrate sources and assess nitrate biogeochemical process in the present study. The results show that the nitrate concentration in West Taihu Lake (WTL) was generally higher than those in East Taihu Lake (ETL) and its upstream inflow rivers. The NO3 ?/Cl? value combined with mapping of δ15N–NO3 ? and NO3 ? concentration suggest that the mixing process should play a major effect in WTL, and denitrification was the dominant N transformation process in WTL. A linear relationship of close to ~1: 2 was observed between δ15N–NO3 ? and δ18O–NO3 ? values in WTL, confirming the occurrence of denitrification in WTL. The δ15N–NO3 ? data imply that sewage and manure were the principal nitrate sources in WTL and its feeder rivers, while the nitrate in ETL might derive from soil organic nitrogen and atmospheric deposition. The δ18O–NO3 ? data indicate most of nitrate from microbial nitrification of organic nitrogen matter possibly make a significant contribution to the lake.  相似文献   

13.
The δ15N-value has often tentatively been used for the assignment of nitrate to its origin. However, the very complex correlations between the different nitrogen pools, mostly accompanied by isotope discriminations, oppose a very limited application of this method. On the other hand, the oxygen isotope abundance should be more indicative, because industrially produced NO−3 must nearly exclusively contain oxygen from O2 (δ18O = +23.5%.), while NO−3 originating from a nitrification process must have water (δ18O ≅ -10%.) as the main oxygen source.For the proof of this reflection a method for the precise oxygen isotope analysis of NO−3 was developed. Its application to the δ-value determination of commercial fertilizers and NO−3 formed by nitrification absolutely confirmed the above predictions. Similarly, the isotope abundance of NO−3-samples from ground and drinking water of known origin corresponded to the expected values. On the basis of these results and taking into account the known isotope abundance shifts due to isotope effects of nitrification and denitrification, a diagram between δ15N- and δ18O-values for NO−3 was developed, which permitted the assignment of NO−3 in unknown water samples to its probable source and origin.  相似文献   

14.
Nitrous oxide evolution may contribute to partial destruction of the ozone layer in the stratosphere. A two year study of the release of N2O from adjoining salt, brackish, and fresh marsh sediment indicates that the annual emission was 31, 48, and 55 mg N m?2 respectively. Emission from open water area was less than the corresponding emission from the marsh sediment. In vitro experiments indicate that the N2O emission was increased when the sediment was drained for extended periods of time. The addition of NO3? significantly increased the rate of N2O evolution, indicating that a large potential for denitrification exists in the anoxic sediment. Appreciable losses of N2O would only be expected when the marshes receive an extraneous source of nitrate such as sewage and/or wastewater.The contribution of the Gulf Coast wetlands to the atmospheric N2O balance is estimated to be 3.3 × 109 g N2O. The maximum average daily emission was equivalent to 1.5 g N2O-N ha?1, which is less than the measured emission from uncultivated soils (Mosieret al., 1981) but greater than the estimates from noncropped land (CAST, 1976).  相似文献   

15.
Measurements of15N/14N in dissolved molecular nitrogen (N2), nitrate (NO 3 − ) and nitrous oxide (N2O) and18O/16O in N2O [expressed as δ15N and δ18O, relative to atmospheric N2 and oxygen (O2), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest and most intense O2 minima observed in the open ocean. Microbially-mediated reduction of NO 3 − to N2 (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ15N of NO 3 − increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250–350 m); the δ15N of N2 concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (ε) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ15N of NO 3 − is observed from ∼ 200m (> 11‰) to 80m (∼ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen fixation. Isotopic analysis of N2O reveals extremely large enrichments of both15N and18O within the OMZ, presumably due to the preferential reduction of lighter N2O to N2. However, isotopically light N2O is observed to accumulate in high concentrations above the OMZ indicating that the N2O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentrations of N2O ever found at the sea surface are observed, show moderate depletion of15N, but slight enrichment of18O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly18O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or sinks of N2O in the atmosphere. Also, the N2O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling between the two processes as an important mechanism of N2O production.  相似文献   

16.
The change in porewater nitrate (NO2 ? + NO3 ?) concentrations during exposure of intertidal sediment was studied at a fixed location in the Sado estuary, southwest Portugal, in November 1994. In order to follow nitrate concentration and dynamics from pre-ebb to post flood, during the day, high vertical resolution profiles (0.2 cm) were studied. As a complement, in February 1995, potential nitrification rates in the sediment were measured by laboratory incubations, with high vertical resolution (0.2 cm) up to 3 cm depth. Oxygen penetration was measured with polarographic mini-electrodes. The sediment’s texture as well as the organic matter composition in carbon and nitrogen were studied in deeper (30 cm) cores. In February 1993,210Pb activity depth profiles were measured in a core sampled at the beginning of exposure, in order to evaluate the possibility of nonlocal particle exchange. C:N ratios and210Pb activity profiles show evidence of nonlocal exchange of solid phase particles between the surface and deeper sediment, most likely due to macrofaunal activity. As a consequence, fresh organic matter is brought from the surface to 7–9 cm depth, causing enhancement of nutrient concentrations. Results of this study suggest nitrate dynamics in intertidal sediments of the Sado estuary are strongly influenced by tidal action. Periodic submersion and exposure allow for the diversification of pathways of oxygen supply to the sediment. Tidal stress at the sediment-water interface during the arrival (flooding) and departure (exposure) of the tidal front at the site has an important bearing on the effective depth of the nitrification zone. A denitrification rate of 2.16 μmol N dm?5 h?1 was measured directly from the nitrate inventory in the 1.5–6 cm depth layer. The schematic model of N cycling in these sediments suggests that 20% of the N pool is denitrified during exposure, and that this process is limited by O2 availability for nitrification.  相似文献   

17.
Aquatic ecosystems have been identified as a globally significant source of nitrous oxide (N2O) due to continuous active nitrogen involvement, but the processes and influencing factors that control N2O production are still poorly understood, especially in reservoirs. For that, monthly N2O variations were monitored in Dongfeng reservoir (DFR) with a mesotrophic condition. The dissolved N2O concentration in DFR displayed a distinct spatial–temporal pattern but lower than that in the eutrophic reservoirs. During the whole sampling year, N2O saturation ranging from 144% to 640%, indicating that reservoir acted as source of atmospheric N2O. N2O production is induced by the introduction of nitrogen (NO3 ?, NH4 +) in mesotrophic reservoirs, and is also affected by oxygen level and water temperature. Nitrification was the predominate process for N2O production in DFR due to well-oxygenated longitudinal water layers. Mean values of estimated N2O flux from the air–water interface averaged 0.19 µmol m?2 h?1 with a range of 0.01–0.61 µmol m?2 h?1. DFR exhibited less N2O emission flux than that reported in a nearby eutrophic reservoir, but still acted as a moderate N2O source compared with other reservoirs and lakes worldwide. Annual emissions from the water–air interface of DFR were estimated to be 0.32 × 105 mol N–N2O, while N2O degassing from releasing water behind the dam during power generation was nearly five times greater. Hence, N2O degassing behind the dam should be taken into account for estimation of N2O emissions from artificial reservoirs, an omission that historically has probably resulted in underestimates. IPCC methodology should consider more specifically N2O emission estimation in aquatic ecosystems, especially in reservoirs, the default EF5 model will lead to an overestimation.  相似文献   

18.
《Applied Geochemistry》1995,10(4):391-405
Extensive NO3− contamination of groundwater in the Abbotsford aquifer to levels above drinking water limits is a major problem in the Fraser Lowlands of southwestern British Columbia, Canada. Nitrate concentrations in the aquifer ranged from 0 to 151 mg/l NO3−, with a median concentration of 46 mg/l NO3−. Of 117 wells sampled, 54% had NO3− concentrations exceeding the drinking water limit of 45 mg/1. Approximately 80% of the study area had groundwater NO3− concentrations exceeding 40 mg/1 NO3−. Potential NO3− source materials were poultry manure N and synthetic NH4 based fertilizers. Theδ15N of solid poultry manure samples ranged between + 7.9 and + 8.6‰ (AIR). Four brands of synthetic fertilizers commonly used hadδ15N values between −1.5 and −0.6‰. Ammonia volatilization caused theδ15N of groundwater NO3− produced from poultry manure N to range between +8 and +16‰. Theδ18O values of groundwater NO3−, by contrast, mostly ranged between +2 and +5‰ (SMOW). This narrow range ofδ18O values fell within the expected range of NO3− produced by nitrification of reduced N forms such as poultry manure N and NH4 fertilizers, and had a similar range ofδ18O values as NO3− in the upper part of the unsaturated zone below raspberry fields and beneath former manure piles. Theδ15N-NO3− andδ18O-NO3− data confirmed that NO3− in the aquifer was predominantly derived from poultry manure and to a lesser extent from synthetic fertilizers. Theδ18O-NO3− data further suggested the nitrification process occurred mainly in the summer months, with the soil NO3− produced subsequently flushed into the aquifer during fall recharge. Theδ15N-NO3−andδ18O-NO3− data conclusively indicated that no significant bacterial denitrification is taking place in the Abbotsford aquifer.  相似文献   

19.
Filter-feeding bivalves, like oysters, couple pelagic primary production with benthic microbial processes by consuming plankton from the water column and depositing unassimilated material on sediment. Conceptual models suggest that at low to moderate oyster densities, this deposition can stimulate benthic denitrification by providing denitrifying bacteria with organic carbon and nitrogen (N). While enhanced denitrification has been found at oyster reefs, data from oyster aquaculture are limited and equivocal. This study measured seasonal rates of denitrification, as well as dissimilatory nitrate reduction to ammonium (DNRA), and dissolved inorganic N fluxes at a rack and bag eastern oyster (Crassostrea virginica) aquaculture farm. Consistent with models, denitrification was enhanced within the farm, with an average annual increase of 350% compared to a reference site. However, absolute denitrification rates were low relative to other coastal systems, reaching a maximum of 19.2 μmol m?2 h?1. Denitrification appeared to be nitrate (NO3 ?) limited, likely due to inhibited nitrification caused by sediment anoxia. Denitrification may also have been limited by competition for NO3 ? with DNRA, which accounted for an average of 76% of NO3 ? reduction. Consequently, direct release of ammonium (NH4 +) from mineralization to the water column was the most significant benthic N pathway, with seasonal rates exceeding 900 μmol m?2 h?1 within the farm. The enhanced N processes were spatially limited however, with significantly higher rates directly under oysters, compared to in between oyster racks. For commercial aquaculture farms like this, with moderate oyster densities (100–200 oysters m?2), denitrification may be enhanced, but nonetheless limited by biodeposition-induced sediment anoxia. The resulting shift in the sediment N balance toward processes that regenerate reactive N to the water column rather than remove N is an important consideration for water quality.  相似文献   

20.
Sediment-water exchanges of ammonium (NH4 +), nitrate + nitrite (NOx ?), filterable reactive phosphorus (FRP, primarily ortho-phosphate), and oxygen (O2) under aphotic (heterotrophic) conditions were determined at 2–5 stations in the Neuse River Estuary, from 1987 to 1989. Shallow (1 m), sandy stations were sampled along the salinity gradient. Fluxes from deep (>2 m) sites were compared to the shallow sites in two salinity zones. Grain size became finer and organic content increased with depth in the oligohaline zone but not in the mesohaline zone. Net release of NH4 + and FRP occurred at all sites. Fluxes varied from slight uptake to releases of 200–500 μmol m?2 h?1 (NH4 +) and 150–900 μmol m?2 h?1 (FRP). Net NOx ? exchange was near zero, but were ±100 μmol m?2 h?1 over the year. Release of NH4 + and FRP from the shallow sandy stations decreased with distance down the estuary, but O2 uptake did not change. The deeper oligohaline site had twofold higher rates of NH4 + and FRP release and O2 uptake than the shallow site, but no differences occurred between depths in the mesohaline zone. Temperature and organic content were important controls for all fluxes, but water column NOx ? concentration was also important in regulating NOx ? exchanges. Ratios of oxygen consumption to NH4 + release were near the predicted ratio (Redfield model) at oligohaline sites but increased down estuary at mesohaline sites. This may be due to greater nitrification rates promoted by autotrophy in the sediments.  相似文献   

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