The influence of long emersion on biota, ammonium fluxes and nitrification in intertidal sediments of Marennes-Oléron Bay, France

A comparative study between waterlogged and reflooded intertidal sediments was undertaken in March and June 1999 through statistical analysis of selected sediment parameters (biota, salinity, O2, Eh), pool sizes and benthic fluxes of nutrients (NH4+, NO2-, NO3-) and nitrification rates. In March samples, absence of polychaetes and oligochaetes from upper sediment horizons were due to erosional events sweeping away surface sediments. Presence of richer annelid assemblages in June samples indicated more stable hydrodynamic conditions that favoured the development of benthic microalgae biofilms. Dewatering of sediments during a 3-day emersion period promoted a salinity rise on top layers, migration of pore water ions towards the sediment surface, and created sediment fissures that accelerated water exchange on reflooding. Reflooded and waterlogged sediment systems were comparable with respect to the release of NH4+ to overlying water but were different with respect to nitrification rates. Sediment-water NH4+ fluxes were higher (P = 0.011) in March (3.3 mmol m(-2) day(-1) compared to June (1.4 mmol m(-2) day(-1) due to higher macrofauna biomasses and lower benthic microalgae concentrations in March samples. Potential nitrification rates (range from 19 to 60 mmol NO3- (-2) day(-1)) were not statistically different between March and June. A thinner oxic layer in reflooded compared with waterlogged systems reflects a decrease of O2 diffusion into sediment at high salinities which resulted in the fall of the actual nitrification rates (P < 0.05). Our data suggest that long term dessication of intertidal sediments may depress the nitrification process at the ecosystem level.     

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.     

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 fluxes of nitrogen in the tidal reaches of a turbid, high-nitrate sub-tropical river

Benthic fluxes of dissolved inorganic nitrogen (NO₃⁻ and NH₄⁺), dissolved organic nitrogen (DON), N₂ (denitrification), O₂ and TCO₂ were measured in the tidal reaches of the Bremer River, south east Queensland, Australia. Measurements were made at three sites during summer and winter. Fluxes of NO₃⁻ were generally directed into the sediments at rates of up to −225 μmol N m⁻² h⁻¹. NH₄⁺ was mostly taken up by the sediments at rates of up to −52 μmol N m⁻² h⁻¹, its ultimate fate probably being denitrification. DON fluxes were not significant during winter. During summer, fluxes of DON were observed both into (−105 μmol m⁻² h⁻¹) and out of (39 μmol m⁻² h⁻¹) the sediments. Average N₂ fluxes at all sampling sites were similar during summer (162 μmol N m⁻² h⁻¹) and winter (153 μmol N m⁻² h⁻¹). Denitrification was fed both by nitrification within the sediment and NO₃⁻ from the water column. Sediment respiration rates played an important role in the dynamics of nitrification and denitrification. NO₃⁻ fluxes were significantly related to TCO₂ fluxes (p<0.01), with a release of NO₃⁻ from the sediment only occurring at respiration rates below 1000 μmol C m⁻² h⁻¹. Rates of denitrification increased with respiration up to TCO₂ fluxes of 1000 μmol C m⁻² h⁻¹. At sediment respiration rates above 1000 μmol C m⁻² h⁻¹, denitrification rates increased less rapidly with respiration in winter and declined during summer. On a monthly basis denitrification removed about 9% of the total nitrogen and 16% of NO₃⁻ entering the Bremer River system from known point sources. This is a similar magnitude to that estimated in other tidal river systems and estuaries receiving similar nitrogen loads. During flood events the amount of NO₃⁻ denitrified dropped to about 6% of the total river NO₃⁻ load.     

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.     

Temporal changes and spatial variation of soil oxygen consumption, nitrification and denitrification rates in a tidal salt marsh of the Lagoon of Venice, Italy

The aim of the present study was to investigate seasonal and spatial patterns of soil oxygen consumption, nitrification, denitrification and fluxes of dissolved inorganic nitrogen (DIN) in a tidal salt marsh of the Lagoon of Venice, Italy. In the salt marsh, intact soil cores including overlying water were collected monthly at high tide from April to October in salt marsh creeks and in areas covered by the dominant vegetation, Limonium serotinum. In May, cores were also collected in areas with vegetation dominated by Juncus maritimus and Halimione portulacoides. In laboratory incubations at in situ temperature in the dark, flux rates of oxygen and DIN were monitored in the overlying water of the intact cores. ¹⁵N-nitrate was added to the overlying water and nitrification and denitrification were measured using isotope-dilution and -pairing techniques. The results show that highest soil oxygen consumption coincided with the highest water temperature in June and July. The highest denitrification rates were recorded in spring and autumn coinciding with the highest nitrate concentrations. Soil oxygen consumption and nitrification rates differed between sampling sites, but denitrification rates were similar among the different vegetation types. The highest rates were recorded in areas covered with L. serotinum. Burrowing soil macrofauna enhanced oxygen consumption, nitrification and denitrification in April and May. The data presented in this study indicate high temporal as well as spatial variations in the flux of oxygen and DIN, and nitrogen transformations in the tidal salt marshes of the Venice lagoon during the growth season. The results identify the salt marshes of the Venice lagoon as being metabolically very active ecosystems with a high capacity to process nitrogen.     

Evaluation of the N2 flux approach for measuring sediment denitrification

Direct gas chromatographic measurement of denitrification rates via N₂ fluxes from aquatic sediments can avoid some of the artifacts and complexities associated with indirect approaches and tracer techniques. However, measurement protocols have typically been determined based upon initial results or previous studies. We present a process-level study and simulation model for evaluating and optimizing N₂ gas flux approaches in closed chamber incubations. Experimental manipulations and simulations of both artificial and natural sediments were used to conduct sensitivity analyses of key design parameters in N₂ flux measurements. Experimental results indicated that depletion of labile organic matter during the long incubations required by common protocols (for diffusive off-gassing of porewater N₂) may result in underestimates of denitrification rates in some systems. Simulations showed that the required incubation time was primarily a function of sediment thickness. The best approach found to minimize incubation time and reduce errors was to select the minimum sediment thickness necessary to include the entire depth distribution of nitrification–denitrification for a particular sediment system. Attempts to increase measurement sensitivity and shorten incubation times by reducing the headspace thickness to 1–2 cm generally cause denitrification to be underestimated by 3–13% for gas headspaces, and up to 80% for water headspaces. However, errors were negligible with gas and water headspace thicknesses of 10 cm and 15 cm, respectively. Anaerobic cores to control for non-denitrification N₂ fluxes shortened incubation time, but introduced artifacts in sediments with extensive macrofaunal irrigation.     

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.     

Potential rates and environmental controls of denitrification and nitrous oxide production in a temperate urbanized estuary

Denitrification may play a major role in inorganic nitrogen removal from estuarine ecosystems, particularly in those subjected to increased nitrate and organic matter loads. The Douro estuary (NW Portugal) suffers from both problems: freshwater input of nitrate and organic load from untreated wastewater discharges. To assess how these factors might control sediment denitrification, a 12-month survey was designed. Denitrification potential and nitrous oxide (N₂O) production were measured at different locations using the slurry acetylene blockage technique. Denitrification rate ranged from 0.4 to 38 nmol N g⁻¹ h⁻¹, increasing towards the river mouth following an urban pollution gradient. N₂O production, a powerful greenhouse gas implicated on the destruction of the ozone layer, was significantly related with sediment organic matter and accounted for 0.5–47% of the N gases produced. Additional enrichment experiments were consistent with the results found in the environment, showing that sediments from the upper less urban stretch of the estuary, mostly sandy, respond positively to carbon and, inversely, in organic rich sediments from the lower estuary, the denitrification potential was limited by nitrate availability. The obtained results confirmed denitrification as an important process for the removal of nitrate in estuaries. The presence of wastewater discharges appears to stimulate nitrogen removal but also the production of N₂O, a powerful greenhouse gas, exacerbating the N₂O:N₂ ratio and thus should be controlled.     

Seasonal and spatial variation in the denitrifying activity in estuarine and lagoonal sediments

We have elucidated the seasonal and spatial variation in the potential denitrifying activity in estuarine and coastal lagoonal sediments in Lakes Shinji and Nakaumi, Japan. The denitrifying activity increased from summer through autumn and was positively correlated with the temperature of the overlying water at all sites except one, where the bottom was always more reductive than at the other sites and there was no NO₃ ⁻ as a substrate for denitrification from spring to autumn. Moreover, the relationship between the denitrifying activity and the distance from the sea showed different trends in estuarine and lagoonal sediments. These spatial differences indicate that different factors regulate the denitrification in estuarine and lagoonal sediments. Denitrifying activity in estuarine sediment was regulated by the discharge of freshwater containing NO₃ ⁻ or organic matter, while in the lagoonal sediments the occurrence of nitrification via the intrusion of oxic seawater into the reductive sediment appears to be a key requirement for the process of denitrification. Therefore, the denitrifying activity in the lagoonal sediment appears to be greater near the sea. Water intrusion is one of the key factors controlling denitrification in coastal marine ecosystems by affecting the supply of substrate available for denitrification.     

Tidal effect on dynamics of pore water nitrate in intertidal sediment of a eutrophic Estuary

To evaluate the effect of the tidal cycle on the pore water nitrate dynamics in intertidal sediment, concentrations of inorganic nitrogen in water and sediment were monitored during tidal cycles in the mud flat of Tama Estuary, Japan. During submergence, nitrate concentration was highest in the overlying water and decreased monotonically with increasing depth in the sediment, suggesting that the primary source of nitrate in the sediment was nitrate transported from the overlying water. Pore water nitrate decreased remarkably during the initial 3–4 hours after the onset of exposure. Thereafter, it was constant or slightly increased until tidal flooding.In situ accumulation of nitrate at the end of exposure, however, did not exceed the nitrate concentrations in the overlying water. The inhibition of nitrate reduction and the stimulation of nitrification would explain the change of nitrate concentration, both consistent with the input of oxygen into the sediment following a 10 mm drop of the water table. In Tama Estuary sediments, the effect of the tidal cycle on the removal of combined nitrogen is rather negative, because high nitrate concentrations in the overlying water canceled the positive effect of nitrate accumulation by nitrification during exposure, while tidal oxygen intrusion have an inhibitory effection sedimentary denitrification.     

Denitrification in coastal Louisiana: A spatial assessment and research needs

By transforming fixed nitrogen (N) into nitrogen gas, the biochemical processes that support denitrification provide a function critical to maintaining the integrity of ecosystems subjected to increased loading of N from anthropogenic sources. The Louisiana coastal region receives high nitrate (NO₃^?) concentrations (> 100 µM) from the Mississippi–Ohio–Missouri River Basin and is also an area undergoing high rates of wetland loss. Ongoing and anticipated changes in the Louisiana coastal region promise to alter biogeochemical cycles including the net rate of denitrification by ecosystems. Projecting what these changes could mean for coastal water quality and natural resources requires an understanding of the magnitude and patterns of variation in denitrification rates and their connection to estuarine water quality at large temporal and spatial scales under current conditions. We compile and review denitrification rates reported in 32 studies conducted in a variety of habitats across coastal Louisiana during the period 1981– 2008. The acetylene inhibition and ¹⁵N flux were the preferred techniques (95%); most of the studies used sediment slurries rather than intact sediment cores. There are no estimates of denitrification rates using the N₂/Ar ratio and isotope pairing techniques, which address some of the problems and limitations of the acetylene inhibition and ¹⁵N flux techniques. These studies have shown that sediments from estuaries, lakes, marshes, forested wetlands, and the coastal shelf region are capable of high potential denitrification rates when exposed to high NO₃^? concentrations (> 100 µM). Maximum potential denitrification rates in experimental and natural settings can reach values > 2500 µmol m² h^? 1. The lack of contemporary studies to understand the interactions among critical nitrogen transformations (e.g., organic matter mineralization, immobilization, aquatic plant assimilation, nitrification, nitrogen fixation, dissimilatory nitrate reduction to ammonium (DNRA) and anaerobic ammonium oxidation (annamox) limits our understanding of nitrogen cycling in coastal Louisiana, particularly the role of respiratory and chemolithoautotrophic denitrification in areas undergoing wetland restoration.     

Rapid sediment accumulation and microbial mineralization in forests of the mangrove Kandelia candel in the Jiulongjiang Estuary, China

Rates of sediment accumulation and microbial mineralization were examined at three Kandelia candel forests spanning the intertidal zone along the south coastline of the heavily urbanized Jiulongljiang Estuary, Fujian Province, China. Mass sediment accumulation rates were rapid (range: 10–62 kg m⁻² y⁻¹) but decreased from the low- to the high-intertidal zone. High levels of radionuclides suggest that these sediments originate from erosion of agricultural soils within the catchment. Mineralization of sediment carbon and nitrogen was correspondingly rapid, with total rate of mineralization ranging from 135 to 191 mol C m⁻² y⁻¹ and 9 to 11 mol N m⁻² y⁻¹; rates were faster in summer than in autumn/winter. Rates of mineralization efficiency (70–93% for C; 69–92% for N) increased, as burial efficiency (7–30% for C; 8–31% for N) decreased, from the low-to the high-intertidal mangroves. Sulphate reduction was the dominant metabolic pathway to a depth of 1 m, with rates (19–281 mmol S m⁻² d⁻¹) exceeding those measured in other intertidal deposits. There is some evidence that Fe and Mn reduction-oxidation cycles are coupled to the activities of live roots within the 0–40 cm depth horizon. Oxic respiration accounted for 5–12% of total carbon mineralization. Methane flux was slow and highly variable when detectable (range: 5–66 μmol CH₄ m⁻² d⁻¹). Nitrous oxide flux was also highly variable, but within the range (1.6–106.5 μmol N₂O m⁻² d⁻¹) measured in other intertidal sediments. Rates of denitrification were rapid, ranging from 1106 to 3780 μmol N₂ m⁻² d⁻¹, and equating to 11–20% of total sediment nitrogen inputs. Denitrification was supported by rapid NH₄ release within surface deposits (range: 3.6–6.1 mmol m⁻² d⁻¹). Our results support the notion that mangrove forests are net accumulation sites for sediment and associated elements within estuaries, especially Kandelia candel forests receiving significant inputs as a direct result of intense human activity along the south China coast.     

Distribution of nitrous oxide dissolved in water masses in the eastern subtropical North Pacific and its origin inferred from isotopomer analysis

N₂O concentration and its isotopomer ratios were measured over a wide area from San Diego to Honolulu in the eastern subtropical North Pacific (ESNP). Waters in the study area had an N₂O maximum (38.2–50.5 nmol kg^?1) at 600–1000 m depth, which is similar to the profiles obtained previously in other areas in the North Pacific. We separated the seawater into five water masses (two for the surface layer, two for the middle layer, and one for the deep layer) and deduced N₂O production–consumption mechanisms in each water body by use of N₂O isotopomer ratios. The results showed that the mechanisms differ slightly among water masses. In the “coastal” surface layer, N₂O is produced by nitrification (NH₂OH oxidation). In the “open ocean” surface layer, it is produced mainly by nitrifier denitrification and to a lesser extent by nitrification under substrate-limited conditions. In both “upwelling” and “open ocean” middle layers it is produced mainly by denitrification and to a lesser extent by nitrifier denitrification. It is also partly reduced. In the deep layer, it is produced predominantly by denitrification with partial reduction. In this way, isotopomers aid elucidation of production–consumption mechanisms of N₂O in the sea even though the mechanisms cannot always be ascertained.     

Vertical profiles of nitrous oxide and dissolved oxygen in marine sediments

The whole core squeezing method was used to simultaneously obtain profiles of nitrous oxide (N₂O), nitrogenous nutrients, and dissolved oxygen in sediments of Koaziro Bay, Japan (coastal water), the East China Sea (marginal sea), and the central Pacific Ocean (open ocean). In the spring of Koaziro Bay, subsurface peaks of interstitial N₂O (0.5–3.5 cm depth) were observed, at which concentrations were higher than in the overlying water. This was also true for nitrate (NO₃⁻) and nitrite (NO₂⁻) profiles, suggesting that the transport of oxic overlying water to the depth through faunal burrows induced in situ N₂O production depending on nitrification. In the summer of Koaziro Bay, sediment concentrations of N₂O, NO₃⁻ and NO₂⁻ were lower than in the overlying water. In most East China Sea sediments, both N₂O and NO₃⁻ decreased sharply in the top 0.5–2 cm oxic layer (oxygen: 15–130 μM), which may have indicated N₂O and NO₃⁻ consumption by denitrification at anoxic microsites. N₂O peaks at subsurface depth (0.5–6.5 cm) implied in situ production of N₂O and/or its supply from the overlying water through faunal burrows. However, the occurrence of the latter process was not confirmed by the profiles of other constituents. In the central Pacific Ocean, the accumulation of N₂O and NO₃⁻ in the sediments likely resulted from nitrification. Nitrous oxide fluxes from the sediments, calculated using its gradient at the sediment–water interface and the molecular diffusion coefficient, were −45 to 6.9 nmolN m⁻² h⁻¹ in Koaziro Bay in the spring, −29 to −21 nmolN m⁻² h⁻¹ in the summer, −46 to 37 nmolN m⁻² h⁻¹ in the East China Sea, 0.17 to 0.23 nmolN m⁻² h⁻¹ in the equatorial Pacific, and <±0.2 nmolN m⁻² h⁻¹ in the subtropical North Pacific, respectively.     

The effect of tidal range on the flushing of ammonium from intertidal sediments of the Tagus estuary,Portugal

The time-course evolution of ammonium concentration has been examined in the flood water during the first 25 min of tidal inundation. The way this transport fluctuates with the tidal ranges and wind conditions was investigated. Flood water was collected at three sites, located along a transect from the lower to the upper intertidal area of the Tagus estuary. At spring and intermediate tides, the periods of air exposure vary slightly along the transect due to the high tidal amplitude and the flatness of the area, but the upper site remains uncovered at neap tide over the entire tidal cycle. At each site, sampling was performed at different tidal ranges covering the neap-spring tidal cycle and wind conditions. Ammonium was determined in the flood water at short time intervals: 1, 2, 3, 4, 5, 10, 15, 20 and 25 min. A clear pattern was observed along the transect: considerable quantities of ammonium were exported from the sediment to the water column at the beginning of the inundation, ranging from 0.2 to 4.8 mmol m⁻² d⁻¹. The highest transport was recorded at the lower intertidal site under spring tide conditions, which corresponds to the higher energetic situation and shorter emersion period. The lowest transport was observed at the upper intertidal site during the first inundation that followed three days of neap tide and continuous exposure of the sediment to the air. The value rates (0.2–4.8 mmol m⁻² d⁻¹) were one order of magnitude higher than those calculated from molecular diffusion (0.07 – 0.16 mmol m⁻² d⁻¹). This study points to the importance of the tidal flushing of ammonium from the intertidal sediments, and its spatial and tidal fluctuation.     

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.     

Growth dynamics of eelgrass, Zostera marina, in the intertidal zone of Seomjin Estuary, Korea

To examine the growth dynamics of eelgrass, Zostera marina, in the intertidal zone of Seomjin Estuary, Korea, we surveyed environmental factors such as water temperature, underwater irradiance, tidal exposure, and nutrient concentrations in the water column and sediment pore water in relation to the shoot density, biomass, morphological characteristics, and growth of Z. marina inhabiting the upper and lower intertidal zones. The survey was conducted monthly from January 2003 to December 2004. The water temperature of the two areas displayed seasonal fluctuations. Underwater irradiance was significantly higher in the upper intertidal zone than in the lower intertidal zone. Tidal exposure was also markedly longer in the upper intertidal zone than in the lower intertidal zone, whereas tidal exposure was highest in the spring and lowest in the summer in both areas. Water column NH₄ ⁺ and sediment pore water NO₃ ^?+NO₂ ^? concentrations were significantly higher in the upper intertidal zone than the lower intertidal zone. The eelgrass shoot density, biomass, morphology, and leaf productivity were significantly higher in the lower intertidal zone than in the upper intertidal zone. Both areas displayed a clear seasonal variation depending on changes in water temperature. However, leaf turnover time was significantly shorter in the upper intertidal zone than in the lower intertidal zone, with a higher turnover rate in the upper intertidal zone. Compared to the seagrasses in the lower intertidal zone, those in the upper intertidal zone showed more effective adaptations to the stress of long tidal exposure through downsizing and increased turnover time. These results suggest that tidal exposure, coupled with desiccation stress, can be a limiting factor for seagrass growth in the intertidal zone, along with underwater irradiance, water temperature, and nutrient availability.     

Nitrogen isotopic discrimination by water column nitrification in a shallow coastal environment

Temporal changes in nitrogen isotopic composition (δ¹⁵N) of the NO₃ ⁻ pool in the water column below the pycnocline in Ise Bay, Japan were investigated to evaluate the effect of nitrification on the change in the δ¹⁵N in the water column. The δ¹⁵N of NO₃ ⁻ in the lower layers varied from −8.5‰ in May to +8.4‰ in July in response to the development of seasonal hypoxia and conversion from NH₄ ⁺ to NO₃ ⁻. The significantly ¹⁵N-depleted NO₃ ⁻ in May most likely arose from nitrification in the water column. The calculated apparent isotopic discrimination for water column nitrification (ɛ_nit = δ¹⁵N_substrate − δ¹⁵N_product) was 24.5‰, which lies within the range of previous laboratory-based estimates. Though prominent deficits of NO₃ ⁻ from hypoxic bottom waters due to denitrification were revealed in July, the isotopic discrimination of denitrification in the sediments was low (ɛ_denit = ∼1‰). δ¹⁵N_NO3 in the hypoxic lower layer mainly reflects the isotopic effect of water column nitrification, given that water column nitrification is not directly linked with sedimentary denitrification and the effect of sedimentary denitrification on the change in δ¹⁵N_NO3 is relatively small.     

Tidal,diel and semi-lunar changes in the faunal assemblage of an intertidal salt marsh creek

The utilisation of a brackish estuarine marsh by nekton was investigated over a semi-lunar cycle in August 1994. Nekton migrating in and out of the intertidal creeks of the marsh ‘Het Verdronken Land van Saeftinghe’ in the Westerschelde estuary, SW Netherlands, was sampled passively during seven complete tidal cycles. Sampling one tidal cycle yielded three consecutive flood samples and four consecutive ebb samples. Sampling occurred every 2–3 days, covering diel, tidal and semi-lunar situations, thus allowing comparison of tidal, diel and semi-lunar influences on the composition of the intertidal fauna.Two different tidal-migration modes were observed. The mysid shrimp, Mesopodopsis slabberi, showed maximum abundance around high tide. For the remaining common species, the mysid (Neomysis integer), the shrimp (Palaemonetes varians), the crab (Carcinus maenas) and the goby (Pomatoschistus microps) and the amphipod (Corophium volutator), highest densities were recorded during lower water heights. The faunal assemblage shifts between the different tidal stages.On two occasions, consecutive day and night samples were taken. Total densities were higher during the night samples. During spring tide, difference in community composition was noticed between the night and the day samples. During neap tide, day–night differences were less clear. Recorded total densities were highest during spring tide and lowest during neap tide. At maximum water levels, a drop in total density was observed. A shift in community composition occurred between spring and neap tides.