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1.
Subterranean estuaries are highly dynamic in processing dissolved inorganic nitrogen (DIN). Here we investigate DIN turnover in surface sediments (0–20 cm depth) at the higher, medium and lower intertidal of a seepage face, i.e., the outer “mouth” of the subterranean estuary, during four consecutive seasons in Sanggou Bay, China. Throughout the studied period, ammonium (NH4+) and nitrite (NO2?) concentrations in the sampled porewaters did not vary significantly with depth or season. In contrast, peaks in porewater nitrate (NO3?) concentration and decreases in δ15N-NO3? and δ18O-NO3? were observed in the 15–20 cm depth (bottom) sediment, particularly during summer and autumn. Coupled with NO3? production, the sediment total nitrogen was also markedly peaking in the bottom layer of the studied seepage face. Together with abundant heterotrophic microbes in the sediment, this NO3? accumulation was linked to a reaction chain including organic matter decomposition, ammonification and nitrification. During winter, porewater enrichment in total nitrogen occurred closer to the surface of the seepage face but triggered also active NO3? production. This pattern reinforced the importance of pelagic organic matter supply on NO3? production. In the shallower depths of the seepage face (<12 cm), active net NO3? removal occurred except in winter. The isotopic fractionation (δ15N-NO3? and δ18O-NO3?) and metagenomic results revealed denitrification as the main pathway for NO3? reduction. Biological assimilation from benthic primary producers may also consume a fraction of NO3? at the sediment water interface. Both NO3? production and removal significantly varied in magnitude with season (?13.6 to 6.2 nmol cm?3 h?1). Substrate supply was the key driver for nitrate cycling, as evidenced by the high NO3? production rate in spring by comparison to autumn. The highest NO3? turnover rates were found in summer, suggesting the combined influence of advection rates and sediment microbiota composition. In spite of active removal (peak NO3? removal capability: 61%), a significant amount of NO3? was still transported from the seepage face into the bay waters. The magnitude of NO3? fluxes ranged from 312 to 476 kg N d?1, accounting for approximately 15% of the total exogenous NO3? loading into the bay. NO3? isotopic fingerprint revealed chemical fertilizer as the main source of terrestrial NO3? in SGD, highlighting the importance of land use to coastal system nitrogen budgets.  相似文献   

2.
During a semi-lunar tidal cycle from full moon till new moon, tide is characterized by the periodic change in spring and neap tide. Under the in situ conditions of light and temperature, the influence of a semi-lunar spring and neap tidal cycle on nutrient cycling in intertidal flat of the Yangtze estuary was simulated in the laboratory in July 2002. Lab experiments showed that NH4+ and PO43– were always released into overlying waters, while NO3– was directed into sediments in the permanently waterlogged systems, suggesting that the long-term waterlogged sediment acts as a significant source for NH4+ and PO43–, and a sink for NO3– in water columns. In contrast, reflooding of intertidal sediments after long-term desiccation promoted the considerable effluxes of NH4+, NO3– and PO43– into overlying waters, reflecting that the long-term exposed sediment is an important source for nutrients in overlying waters. In addition, the semi-lunar tidal cycle led to the intricate depth distribution patterns of nutrients in intertidal sediments. During long-term exposure, NH4+ in sediments was quickly transformed into NO2– and NO3–, while organic P pool might be converted to slightly adsorbed and iron-bound P. Therefore, it is considered that the semi-lunar tidal circulation has the significant influence on the biogeochemical cycle of nutrients in intertidal systems.  相似文献   

3.
Tidal currents and the spatial variability of tidally-induced shear stress were studied during a tidal cycle on four intertidal mudflats from the fluvial to the marine part of the Seine estuary. Measurements were carried out during low water discharge (<400 m3 s−1) in neap and spring tide conditions. Turbulent kinetic energy, covariance, and logarithmic profile methods were used and compared for the determination of shear stress. The cTKE coefficient value of 0.19 cited in the literature was confirmed. Shear stress values were shown to decrease above mudflats from the mouth to the fluvial part of the estuary due to dissipation of the tidal energy, from 1 to 0.2 N m−2 for spring tides and 0.8 to 0.05 N m−2 for neap tides. Flood currents dominate tidally-induced shear stress in the marine and lower fluvial estuary during neap and spring tides and in the upper fluvial part during spring tides. Ebb currents control tidally-induced shear stress in the upper fluvial part of the estuary during neap tides. These results revealed a linear relationship between friction velocities and current velocities. Bed roughness length values were calculated from the empirical relationship given by Mitchener and Torfs (1996) for each site; these values are in agreement with the modes of the sediment particle-size distribution. The influence of tidal currents on the mudflat dynamics of the Seine estuary was examined by comparing the tidally-induced bed shear stress and the critical erosion shear stress estimated from bed sediment properties. Bed sediment resuspension induced by tidal currents was shown to occur only in the lower part of the estuary.  相似文献   

4.
Estuaries act as an organic matter and nutrient filter in the transition between the land, rivers and the ocean. In the past, high nutrient and organic carbon load and low oxygen concentration made the Elbe River estuary (NW Europe) a sink for dissolved inorganic nitrogen. A recent reduction in loads and subsequent recovery of the estuary changed its biogeochemical function, so that nitrate is no longer removed on its transition towards the coastal North Sea. Nowadays in the estuary, nitrification appears to be a significant nitrate source. To quantify nitrification and determine actively nitrifying regions in the estuary, we measured the concentrations of ammonium, nitrite and nitrate, the dual stable isotopes of nitrate and net nitrification rates in the estuary on five cruises from August 2012 to August 2013. The nitrate concentration increased markedly downstream of the port of Hamburg in summer and spring, accompanied by a decrease of nitrate isotope values that was clearest in summer exactly at the location where nitrate concentration started to increase. Ammonium and nitrite peaked in the Hamburg port region (up to 18 and 8 μmol L?1, respectively), and nitrification rates in this region were up to 7 μmol L?1 day?1. Our data show that coupled re-mineralization and nitrification are significant internal nitrate sources that almost double the estuary’s summer nitrate concentration. Furthermore, we find that the port of Hamburg is a hot spot of nitrification, whereas the maximum turbidity zone (MTZ) only plays a subordinate role in turnover of nitrate.  相似文献   

5.
In an attempt to characterize localized rates of sediment accretion, 10 sediment cores were collected from the lower reach of the Passaic River, a major tributary of Newark Bay, New Jersey. Sediments were assayed for 210Pb activity at predetermined depths and the rate of sediment accretion (cm yr?1) was estimated from the least squares regression of the log of unsupported activity versus depth. Sediment accretion rates, derived from 210Pb measurements (RPb) were used to predict the depth interval within the core containing sediments deposited around 1954; subsequent 137Cs analyses were focused on this depth interval. Sediment accretion rates derived from 137Cs measurements (RCs) were extrapolated from the depth of the 1954 horizon. Lead-210 derived sediment accretion rates in cores collected from a sediment bench extending along the inside bend on the southern shore of a meander in the river, ranged from 4.1 cm yr?1 to 10.2 cm yr?1 and averaged 6.8 cm yr?1. The RCs estimates for cores from this area ranged from 3.8 cm yr?1 to 8.9 cm yr?1 and averaged 6.6 cm yr?1. The RCs for cores collected in a more hydrologically dynamic reach of the river upstream of the sediment bench, were only 0.41 cm yr?1 and 0.66 cm yr?1. The results of this investigation indicate that this reach of the lower Passaic River is an area of high sediment accumulation, retaining much of the sediment load deposited from upstream and downstream sources. The rates of sediment accretion in the lower Passaic River are among the highest reported anywhere in the Newark Bay estuary.  相似文献   

6.
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   

7.
The potential mineralization and immobilization of soil nitrogen (N), phosphorus (P) and sulfur (S) are relatively high in natural ecosystems. This study was conducted to investigate the changes in essential plant macronutrients; N, P, and S status in response to different soil depth in rangeland ecosystems in vitro. The net nutrient mineralization was measured during 90 days at different depths (0–15, 15–30, 30–45 and 45–60 cm), using kinetic models to estimate the release rate. The net ammonification and mineralization of P and S were described using parabolic diffusion equation, while the power function equation was used to describe the net nitrification. The results indicated that the amount of released ammonium (NH4 +) decreased with time and depth and the rates of net ammonification were negative in all samples. Conversely, nitrification increased with time and depth and the rates were all positive. The net mineralization for both P and S reduced with time. The concentration of mineralized SO4 2? increased with depth like nitrate (NO3 ?). Accumulation of SO4 2? and NO3 ? in subsurface soils and NH4 + and P at surface horizons can increase the potential of their loss by leaching or volatilization.  相似文献   

8.
Cores were collected from the length of Pakuranga estuary, a small urban estuary in Auckland, New Zealand, to determine sedimentation and contaminant history, and in particular the impact of urbanization. Catchment sediment loads for the most recent history (1953–1995), including urbanization since 1960, were reconstructed using the landcover history and soil erosion modeling. Pollen and14C dating and pre-urban landcover history were used to reconstruct early estuary sedimentation (i.e., post-3000 yr BP to 1960). Heavy metal concentrations, particle size,137Cs, pollen, and catchment sediment loads were all needed to disentangle the complex estuarine response to urbanization.137Cs profiles did not reflect the historical fallout pattern, but deposition of137Cs-labelled eroded catchment soil, coinciding with peaks in urban construction. Temporal variations in stormwater137Cs concentrations are likely due to varying contributions from137Cs-rich topsoil and137Cs-poor subsoils. A similar pattern was observed in heavy-metal concentrations and attributed to street runoff rather than topsoil being diluted by metal-poor subsoils. Dating of the sediment profiles showed that during urbanization sedimentation rates in the tidal creek and estuary were higher than sedimentation rates associated with past agricultural landuse and the original forest landcover. Urbanization has brought about substantial environmental changes in the upper estuary through continued infilling of shallow, intertidal areas, contamination by heavy metals to levels of ecological concern, sediment textural changes, and rapid mangrove colonization of formerly bare intertidal sediments.  相似文献   

9.
Quarterly field sampling was conducted to characterize variations in water column and sediment nutrients in a eutrophic southern California estuary with a history of frequent macroalgal blooms. Water column and sediment nutrient measures demonstrated that Upper Newport Bay (UNB) is a highly enriched estuary. High nitrate (NO3 −) loads from the river entered the estuary at all sampling times with a rainy season (winter) maximum estimated at 2,419 mol h−1. This resulted in water NO3 − concentration in the estuary near the river mouth at least one order of magnitude above all other sampling locations during every seasons; maximum mean water NO3 − concentration was 800 μM during springer 1997. Phosphorus (P)-loading was high year round (5.7–90.4 mol h−1) with no seasonal pattern. Sediment nitrogen (N)-content showed a seasonal pattern with a spring maximum declining through fall. sediment and water nutrients, as well as percent cover of three dominant macroalgae, varied between the main channel and tidal creeks. During all seasons, water column NO3 − concentrations were higher in the main channel than in tidal creeks while tidal creeks had higher levels of sediment total Kjeldhal nitrogen (TKN) and P. During each of the four sampling periods, percent cover ofEntermorpha intestinalis andCeramium spp. was higher in tidal creeks than in the main channel, while percent cover ofUlva expansa was always higher in the main channel. Decreases in sediment N in both creek and channel habitats were concurrent with increases in macroalgal cover, possibly reflecting use of stored sediment TKN by macroalgae. Our data suggest a shift in primary nutrient sources for macroalgae in UNB from riverine input during winter and spring to recycling from sediments duirng summer and fall.  相似文献   

10.
Streams draining agricultural catchments in loward England commonly have high nitrate concentrations. Laboratory microcosms were used to study nitrogen transformations in the sediment from such a stream. Solutions of either 0.5 mmol dm−3 calcium nitrate or calcium chloride (control) were passed once only over the sediment surface. After 5 days incubation, Eh in the nitrate treatment stabilized at approximately 500 mV at 5 mm below the water/sediment interface, and 285 mV at 10 mmn, whereas in the chloride treatment Eh was −200 mV to −250 mV at all measured depths (5 mm and below). Inorganic N mesurements of the seidment profiles showed that much more ammonium-N was being lost from the nitrate-treated sediment that the control. It was hypothesized that, when nitrate was present in the streamwater, the combined processed of nitrate diffusion and denitrification progressively consumed organic matter, thereby allowing dissolved oxygen to diffuse further into the sediment. A consequence of these processes would be a greater depth of nitrification and hence more ammounium loss. The plausibility of this hypothesis was evaluated by constructing a deterministic model which simulated the nitrate and ammonium profiles from the processes of diffusion, denitrification, nitrification, and ammonification. Kinetic parameters and diffusion impedance factor were determined from independent experiments. This calibrated model provided reasonable simulations of experimental data obtained from an additional experiment, which also showed that peaks in denitrification and nitrification activity were offset from each other by 2–3 mm, and that they migrated down the profile with increasing incubation time.  相似文献   

11.
Net ecosystem metabolism (NEM) was measured in the Piauí River estuary, NE Brazil. A mass balance of C, N, and P was used to infer its sources and sinks. Dissolved inorganic carbon (DIC) concentrations and fluxes were measured over a year along this mangrove dominated estuary. DIC concentrations were high in all estuarine sections, particularly at the fluvial end member at the beginning of the rainy season. Carbon dioxide concentrations in the entire estuary were supersaturated throughout the year and highest in the upper estuarine compartment and freshwater, particularly at the rainy season, due to washout effects of carbonaceous soils and different organic anthropogenic effluents. The estuary served as a source of DIC to the atmosphere with an estimated flux of 13 mol CO2 m?2 year?1. Input from the river was 46 mol CO2 m?2 year?1. The metabolism of the system was heterotrophic, but short periods of autotrophy occurred in the lower more marine portions of the estuary. The pelagic system was more or less balanced between auto- and heterotrophy, whereas the benthic and intertidal mangrove region was heterotrophic. Estimated annual NEM yielded a total DIC production in the order of 18 mol CO2 m?2 year?1. The anthropogenic inputs of particulate C, N, and P, dissolved inorganic P (DIP), and DIC were significant. The fluvial loading of particulate organic carbon and dissolved inorganic nitrogen (DIN) was largely retained in two flow regulation and hydroelectric reservoirs, promoting a reduction of C:N and C:P particulate ratios in the estuary. The net nonconservative fluxes obtained by a mass balance approach revealed that the estuary acts as a source of DIP, DIN, and DIC, the latter one being almost equivalent to the losses to the atmosphere. Mangrove forests and tidal mudflats were responsible for most of NEM rates and are the main sites of organic decomposition to sustain net heterotrophy. The main sources for this organic matter are the fluvial and anthropogenic inputs. The mangrove areas are the highest estuarine sources of DIP, DIC, and DIN.  相似文献   

12.
The Avon-Heathcote is a small, microtidal, predominantly intertidal, weather-dominated estuary. It has experienced large alterations to its physical environment as a result of the establishment and growth the adjacent Christchurch City, on what was previously a swampy, dune-bordered coastal plain. During the first 25 years of settlement the volume of the tidal compartment of the estuary decreased from 7.7 × 106 m3 to about 5 × 106 m3. A combination of rapid drain laying in the 1880s and 1890s and the conversion of much of the catchment of the estuary to urban uses altered the sediment yields and runoff characteristics of the area and resulted in a reversal of the early trend. By 1925 the tidal compartment had returned to its original volume, and by 1975 it had reached a volume of 10.93 × 106 m3. The early decrease was accompanied by widespread deposition of a 30–60 cm thick layer of muddy sediment, some of which remains. The estuary inlet underwent major adjustments to accomodate the increasing tidal flow and now appears to be approaching a new, posturban equilibrium.  相似文献   

13.
The influences of exposure to the atmosphere on ammonium cycle in the intertidal surface sediments were in situ studied with a geochemical approach at a typical station in the Yangtze Estuary during three tidal cycles in September 2003. During an about 8-h emersion period of each diurnal tide, six high-resolution vertical profiles of adsorbed and dissolved ammonium were measured. It was observed that both adsorbed and dissolved ammonium generally had an increasing trend in sediment cores during the exposure. The rate of ammonium regeneration in sediments was estimated using the accumulation amount of ammonium including adsorbed and soluble fractions during the daytime emersion. The calculation result showed that there was relatively high ammonification rate (˜500 nmol N cm−3 day−1), which reflected that organic nitrogen in sediments was quickly decomposed with a residence time of ˜52.7 days. Due to the dramatic temperature difference observed in sediment profiles, free convection was considered an important mechanism of regulating the efflux of produced ammonium into overlying waters. The total estimated amount of regenerated ammonium was ˜1.35×105 t N year−1 in the intertidal flat of the Yangtze Estuary, which occupied 7.6% of the total inorganic nitrogen annually transported to the estuarine ecosystem.  相似文献   

14.
Optical in situ chemical sensors enable sampling intervals and durations that rival acoustic techniques used for measuring currents. Coupling these high-frequency biogeochemical and physical measurements in estuaries to address ecosystem-scale questions, however, is still comparatively novel. This study investigated how tides affect ecosystem metabolism in a mesotidal estuary in central California (Elkhorn Slough). Dissolved oxygen measurements were used to estimate the terms in a control volume budget for a tidal creek/marsh complex at tidal timescales over several weeks. Respiration rates were 1.6 to 7.3 g O2 m?2 day?1; net community production approached 20 g O2 m?2 day?1. We found that aquatic NCP integrated throughout the creek complex varied significantly over the spring-neap cycle. The intertidal contribution to aquatic metabolism was net heterotrophic during spring tides and generally in balance during neap tides because spring-tide marsh inundation was limited to nighttime, and therefore the marsh could not contribute any primary production to the water column. At the estuary scale, the fortnightly export of oxygen from the main channel to the intertidal was largely balanced by an advective flux up-estuary.  相似文献   

15.
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   

16.
Bertioga Channel is a partially mixed (type 2) tidal estuary on the coastal plain of São Paulo, Brazil. Hourly current and salinity measurements during neap and spring tides in July 1991 yielded information about the physical structure of the system. Peak along-channel velocities varied from 40 cm s?1 to 60 cm s?1 during flood tides and from 70 cm s?1 to 100 cm s?1 during ebb tides. Net vertical velocity profiles indicate that the net current reverses directions at a depth of 2.5–3.0 m in the halocline. Due to appreciable fortnightly tidal modulation, the estuary alternates from being highly stratified (type 2b) during neap tides, with advection and diffusion contributing equally to the net upstream salt flux, to being moderately stratified (type 2a) during spring tides, when 90% of the net upstream salt transport is the result of effective tidal diffusion. Decomposition of the salt flux indicates that the relative contribution to the upstream salt transport by gravitational circulation shear is greater than the oscillatory tidal flux by a factor of 2.6 during neap tides. The oscillatory tidal flux is generated by the correlation of the tidal components of the u-velocity and salinity and is responsible for approximately the same amount of upstream salt transport, during neap and spring tides. However, during spring tides, this oscillatory term is greater than the other salt flux terms by a factor of 1.4. The total salt transport, through a unit width of the section perpendicular to the flow, was within 2% of the sum of the seven major decomposed, advective and dispersive terms. On the assumption that the Bertioga Channel is laterally homogeneous, the results also indicate that the estuary is not in steady state with respect to salt flux.  相似文献   

17.
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   

18.
Tidal freshwater sections of the Cooper River Estuary (South Carolina) include extensive wetlands, which were formerly impounded for rice culture during the 1,700s and 1,800s. Most of these former rice fields are now open to tidal exchange and have developed into productive wetlands that vary in bottom topography, tidal hydrography and vegetation dominants. The purpose of this project was to quantify nitrogen (N) transport via tidal exchange between the main estuarine channel and representative wetland types and to relate exchange patterns to the succession of vegetation dominants. We examined N concentration and mass exchange at the main tidal inlets for the three representative wetland types (submerged aquatic vegetation [SAV], floating leaf vegetation, and intertidal emergent marsh) over 18-21 tidal cycles (July 1998–August 2000). Nitrate + nitrite concentrations were significantly lower during ebb flow at all study sites, suggesting potential patterns of uptake by all wetland types. The magnitude of nitrate decline during ebb flow was negatively correlated with oxygen concentration, reflecting the potential importance of denitrification and nitrate reduction within hypoxic wetland waters and sediments. The net tidal exchange of nitrate + nitrite was particularly consistent for the intertidal emergent marsh, where flow-weighted ebb concentrations were usually 18–40% lower than during flood tides. Seasonal patterns for the emergent marsh indicated higher rates of nitrate + nitrite uptake during the spring and summer (> 400 μmol N m-2 tide-1) with an annual mean uptake of 248 ± 162 μmol m–2 tide–1. The emergent marsh also removed ammonium through most of the year (207 ± 109 μmol m–2 tide–1), and exported dissolved organic nitrogen (DON) in the fall (1,690 ± 793 μmol m–2 tide–1), suggesting an approximate annual balance between the dissolved inorganic N uptake and DON export. The other wetland types (SAV and floating leaf vegetation) were less consistent in magnitude and direction of N exchange. Since the emergent marsh site had the highest bottom elevation and the highest relative cover of intertidal habitat, these results suggest that the nature of N exchange between the estuarine waters and bordering wetlands is affected by wetland morphometry, tidal hydrography, and corresponding vegetation dominants. With the recent diversion of river discharge, water levels in the upper Cooper estuary have dropped more than 10 cm, leading to a succession of wetland communities from subtidal habitats toward more intertidal habitats. Results of this study suggest that current trends of wetland succession in the upper Cooper River may result in higher rates of system-wide inorganic N removal and DON inputs by the growing distributions of intertidal emergent marshes.  相似文献   

19.
We measured fluxes of NH4+ and NO3− and δ15N of NH4+, sediment, and porewater NH4+ from incubated sediment cores along a nitrate gradient and in different seasons from Childs River, MA. NH4+ flux was low at the downstream site with the lowest concentration of organic matter (high salinity) but otherwise did not differ along the estuary. The δ15N of regenerated NH4+ ranged from +6.1‰ to +15.3‰ but did not vary significantly with season or salinity; the mean for the entire estuary was +10.4 ± 0.5‰. Based on differences between the δ15N of regenerated NH4+ and sediment, and expected isotopic fractionation due to remineralization, we concluded that nitrification occurred after remineralization of NH4+. Differences between the δ15N of regenerated NH4+ and the δ15N of porewater NH4+ provided further evidence of nitrification. We estimated that 11% to 48% of remineralized NH4+ underwent coupled nitrification–denitrification before release into the water column. In spite of losses to denitrification, NH4+ flux released 1.4 mol N m−2 year−1 to the water column and could provide 42% of phytoplankton nitrogen requirements.  相似文献   

20.
The Nauset Marsh estuary is the most extensive (9.45 km2) and least disturbed salt marsh/estuarine system within the Cape Cod National Seashore, even though much of the 19 km2 watershed area of the estuary is developed for residential or commercial purposes. Because all of the Nauset watershed is serviced by on-site individual sewage disposal systems, there is concern over the potential impact of groundwater-derived nutrients passing from these systems to the shallow receiving waters of the estuary. The purpose of this study was to determine whether denitrification (the bacterial conversion of nitrate to gaseous nitrogen) in estuarine sediments could effectively remove the nitrate from contaminated groundwater before it passed from the watershed to the estuary. Rates of denitrification were measured both in situ and in sediment cores, in areas of active groundwater discharge, in relatively pristine locations, and in areas situated down-gradient of moderate to heavily developed regions of the watershed. Denitrification rates for 47 sediment cores taken over an annual cycle at 5 stations ranged from non-detectable to 47 μmol N2 m−2 h−. Mean denitrification rates were positively correlated with sediment organic content, and varied seasonally due to changes in sediment organic content and to the effect of water temperatures on sediment oxygen penetration depths. There was no correlation between observed denitrification rates and corresponding nitrate concentrations in groundwater. A comparison of in situ denitrification rates (supported by groundwater nitrate) with denitrification rates observed in sediment cores (supported by remineralized nitrate) showed that groundwater-driven denitrification rates were small, and not in excess of denitrification rates supported by remineralized nitrate. Most of the denitrification in Nauset sediments was apparently fueled by remineralized nitrate through coupled nitrification/denitrification. Denitrification did not contribute significantly to the direct loss of nitrate from incoming groundwater at Nauset Marsh estuary. Groundwater flow was rapid, and much of it occurred in freshwater springs and seeps through very coarse, sandy, well-oxygenated sediments of limited organic content. There was little opportunity for denitrification to occur during groundwater passage through these sediments. These results have important management implications because they suggest that the majority of nitrogen from contaminated groundwater crosses the sediment/water interface and arrives at Nauset Estuary, where it is available to primary producers. Preliminary budget calculations suggest that while denitrification was not an effective mechanism for the direct removal of nitrate in contaminated groundwater flowing to Nauset Marsh estuary, it may contribute to significant nitrogen losses from the estuary itself.  相似文献   

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