Short-term influence of recolonisation by the polycheate worm [2pt] Nereis succinea on oxygen and nitrogen fluxes and [2pt] denitrification: a microcosm simulation

The short-term effects of sediment recolonisation by Nereis succinea on sediment-water column fluxes of oxygen and dissolved inorganic nitrogen, and rates of denitrification, were studied in microcosms of homogenised, sieved sediments. The added worms enhanced oxygen uptake by the sediments, due to the increased surface area provided by the burrow walls and the degree of stimulation was stable with time. Similarly, ammonium fluxes to the water column were stimulated by N. succinea, but declined over the 3 day incubation in all microcosms including the controls. Nitrate fluxes were generally greater in the faunated microcosms, but highly variable with time. Denitrification rates were positively stimulated by N. succinea populations, denitrification of water column nitrate was stimulated 10-fold in comparison to denitrification coupled to nitrification in the sediments. Rates of denitrification of water column nitrate were not significantly different from rates in undisturbed sediment cores with similar densities of N. succinea, whereas rates of coupled nitrification–denitrification were 3-fold lower in the experimental set-up. These results may reflect the relative growth rates of nitrifying and denitrifying bacteria, which allow more rapid colonisation of new burrow surfaces by denitrifier compared to nitrifier populations. The data indicate that recolonisation by burrowing macrofauna of the highly reduced sediments of the Sacca di Goro, Lagoon, Italy, following the annual dystrophic crisis, may play a significant role in the reoxidation and detoxification of the sediments. The increased rates of denitrification associated with the worm burrows, may promote nitrogen losses, but due to the low capacity of nitrifying bacteria to colonise the new burrow structures, these losses would be highly dependent upon water column nitrate concentrations.     

Seasonal distribution of nitrifying bacteria and rates of nitrification in coastal marine sediments

The distribution of nitrification potential (NP) with depth in sediment and season was investigated in a shallow sandy sediment (0.5 m water) and a deeper muddy sediment (17m water). In both sediments, nitrifying bacteria were present in the anoxic strata (oxygen penetration was 5 mm below the surface). The NP at 6–8 cm depth in the sediment was 50% and 10% of the surface NP at the sandy and muddy sediment, respectively. It is suggested that bioturbation and physical disturbance of the sediment were the most likely reasons for this distribution. The NP increased as sediment temperature decreased. This effect was less marked in the muddy sediment. It is concluded that during the summer, the numbers or specific activity of nitrifying bacteria diminished for the following reasons: There was decreased O₂ penetration into the sediment and increased competition for O₂ by heterotrophs; there was increased competition for NH₄ ⁺ and there was inhibition by H₂S. These effects counteracted the potentially higher growth rates and increased rates of NH₄ ⁺ production at the elevated summer temperatures. The potential nitrification rates in the upper 1 cm, which were measured at 22°C, were converted to calculated rates at the in situ temperature (Q₁₀=2.5) and in situ oxygen penetration. These calculated rates were shown to closely resemble the measured in situ rates of nitrification. The relationship between the in situ rates of nitrification and the nitrification potential is discussed.     

Nutrient biogeochemistry in the water column (N,P, Si) and porewater (N) of sandy sediment of the Scheldt estuary (SW-Netherlands)

The Scheldt river drains a densely populated and industrialized area in northern France, western Belgium and the south-west Netherlands. Mineralization of the high organic load carried by the river leads to oxygen depletion in the water column and high concentrations of dissolved nitrogen and phosphorus compounds. Upon estuarine mixing, dissolved oxygen concentrations are gradually restored due to reaeration and dilution with sea water. The longitudinal redox gradient present in the Scheldt estuary strongly affects the geochemistry of nutrients. Dissolved nutrients in the water column and dissolved nitrogen species in sediment porewaters were determined for a typical summer and winter situation. Water column concentration-salinity plots showed conservative behaviour of dissolved Si during winter. During summer (and spring) dissolved Si may be completely removed from solution due to uptake by diatoms. The geochemistry of phosphorus was governed by inorganic and biological processes. The behaviour of nitrogen was controlled by denitrification in the anoxic fluvial estuary, followed by nitrification in the upper estuary (prior to oxygen regeneration). In addition, nitrogen was taken up during phytoplankton blooms in the lower estuary. Dissolved inorganic nitrogen species in porewaters from the upper 20 cm of sediments were obtained from a subtidal site in the middle of the lower estuary. Dissolved nutrient concentrations were low in the upper 10–15 cm of the sandy and organic poor (<1% POC) sediments mainly as a result of strong sediment mixing. The porewater profiles of ammonium and nitrate were evaluated quantitatively, using a one-dimensional steady-state diagenetic model. This coupled ammonium-nitrate model showed ammonification of organic matter to be restricted to the upper 4 to 7 cm of the sediments. Total nitrification ranged from 3.7–18.1 mmol m^?2 d^?1, converting all ammonium produced by ammonification. The net balance between nitrification and denitrification depended on the season. Nitrate was released from the sediments during winter but is taken up from the water column during summer. These results are in good agreement with data obtained from the independently calibrated water column model for the Scheldt Estuary (VAN GILSet al., 1993).     

Oxygen-limited autotrophic nitrification/denitrification by ammonia oxidisers enables upward motion towards more favourable conditions

The hypothesis is formulated that in case of oxygen limitation in the sediment, nitrifiers switch from nitrification to oxygen-limited autotrophic nitrification-denitrification (OLAND) in order to survive and maintain activity. During OLAND, ammonium is oxidised using nitrite as e-acceptor to form dinitrogen gas. As an additional advantage they benefit from the gaseous N₂ formed as a means of transport. In this way, the nitrifiers can move out of the sediment and rise through the water column towards more favourable conditions. At the surface, the bacteria could take up oxygen, and recommence nitrification. In order to test this hypothesis, nitrifying sediment with an overlaying water column was simulated in lab-scale columns. Nitrogen transformations and material transport through the water column were followed after addition of different forms of nitrogen under oxygen-limited conditions. ¹⁵N-labelling experiments showed a large contribution of OLAND to the observed nitrogen deficits. Nitrifier enumerations, fluorescent in situ hybridisation and 16S rRNA gene analysis revealed increased populations of ammonia oxidising nitrifiers in the upper water layers. The results presented support the proposed hypothesis of transport using OLAND. Nitrifying activity in the sediment immediately recovered almost completely from prolonged oxygen-limited incubation when oxygen concentrations were increased. Electronic Publication     

Seasonal regulation of nitrification in a rooted macrophyte (Vallisneria spiralis L.) meadow under eutrophic conditions

Variable oxygen release from the root of macrophytes growing in ammonium-rich organic substrates can stimulate the process of nitrification. To verify this hypothesis, we performed seasonal measurements of potential nitrification activity in sediments with and without the perennial submersed plant Vallisneria spiralis L. (Hydrocharitaceae). Pore water and sediment features were simultaneously considered in order to provide insights into the regulation of the process. Results demonstrated a significant effect of season and plant presence on potential nitrification activity, with higher rates in winter and lower rates in summer. Vegetated sediment displayed lower pore water ammonium, but always higher potential nitrification activity compared to the unvegetated substrate, regardless the season. Nitrification activity was strongly correlated with pore water redox status, which were affected by both season and plant presence. Along its annual cycle V. spiralis promoted more oxidized conditions in the rhizosphere likely due to elevated radial oxygen loss and the consequent maintenance of a larger nitrifying community. These outcomes confirm the results of a limited number of studies that demonstrated how sediment biogeochemistry may be controlled by plant-released oxygen also in organic-rich systems.     

Rapid flow through the sediments of a headwater stream in the southern Appalachians

SUMMARY. 1. The flow of water through the sediment layer (underflow) of streams can influence nutrient uptake dynamics and the supply of materials to microbes, meiofauna and macroinvertebrates living within stream sediments. We examined the extent of underflow in Hugh White Creek, a headwater stream in the southern Appalachian Mountains and compared underflow at different depths and at different sites within the stream.
2. Initially rhodamine dye was used to trace the flow of water through the sediments; however, the dye was strongly absorbed by the sediments in Hugh White Creek. Thus rhodamine was not suitable as a tracer of water flow. Chloride reacted conservatively in laboratory experiments and was used as a tracer.
3. The tracer infiltrated the sediments within 5 min to depths of 10 cm at all six sites. Chloride infiltration tended to decrease with depth of sediments at all sites although there was no consistent statistical pattern in chloride concentration with depth for the different sites. Equilibrium between the water column and sediments was reached rapidly, within minutes for the sites with coarse sediments and within a few hours for sites with finer sediments. Minimum rates of chloride infiltration into the sediments ranged between 1.0 cm min⁻¹ for the sites with cobble substrate to 0.2 cm min⁻¹ for sites with fine sand sediments associated with debris dams. These data suggest that underflow may be a major component in the functioning of Appalachian mountain streams.     

The influence of water quality and sediment geochemistry on the horizontal and vertical distribution of phosphorus and nitrogen in sediments of a large,shallow lake

Distinct horizontal water column concentration gradients of nutrients and chlorophyll a (Chl a) occur within large, shallow, eutrophic Lake Taihu, China. Concentrations are high in the north, where some of the major polluted tributaries enter the lake, and relatively low in the south, where macrophytes generally are abundant. It is not clear, however, whether these water column concentration gradients are similarly reflected in spatial heterogeneity of nutrient concentrations within the bottom sediments. The main objective of this study was therefore to test if horizontal and vertical variations in the phosphorus and nitrogen content in bottom sediments of Lake Taihu are significantly related to (1) horizontal variations in overlying water column nutrient concentrations and (2) other sediment geochemical constituents. We measured the concentration of total phosphorus (TP) and total nitrogen (TN) in surficial sediments (0–2 cm) and TP, TN and Chl a concentrations in water column samples, collected from 32 sites in 2005. In 2006 sediment, TP, TN, carbon, iron and manganese concentrations were measured vertically at 2 cm intervals, extending to a depth of approximately 20 cm, at an additional eight sites. Linear correlation analysis revealed that surficial sediment TP concentrations across the 32 stations were related significantly, though weakly, to annual mean water column concentrations of TP, TN as well as Chl a. Correlations of surficial sediment TN with water column variables were, however, not significant (P > 0.05). Amongst the geochemical variables tested, the vertical variability of sediment TP concentrations was most strongly related to sediment manganese and carbon concentrations. A multiple stepwise linear regression revealed that the combination of sediment manganese and carbon concentrations explained 91% of the horizontal variability in sediment TP concentrations and 65% of the vertical variability. Handling editor: Luigi Naselli-Flores     

Linking Molecular Microbial Ecology to Geochemistry in a Coastal Hypoxic Zone

Multiple environmental mechanisms have been proposed to control bottom water hypoxia (<2 mg O₂ L^?1) in the northern Gulf of Mexico Louisiana shelf. Near-bottom hypoxia has been attributed to a direct consumption of oxygen through benthic microbial respiration and a secondary chemical reaction between oxygen and reduced metabolites (i.e. ferrous iron and total sulfide) from these populations. No studies to date have examined the metabolically active microbial community structure in conjunction with the geochemical profile in these sediments. Temporal and spatial differences in dissolved and solid phase geochemistry were investigated in the upper 20 cm of the sediment column. Pyrosequencing of reverse transcribed small subunit (SSU) ribosomal ribonucleic acid (rRNA) was used to determine population distribution. Results indicated that populations shallower than 10 cm below surface were temporally variable yet uniform between sites, while below this depth, populations were more site-specific. This suggests a potential interaction between the water column and the benthic microbial population limited to a shallow depth. The presence of dissolved reduced iron in the upper sediment column was indicative of low oxygen concentration, yet sulfide was at or below detection limits. Putative sulfate and iron reducing and oxidizing populations were metabolically active at similar depths suggesting potential recycling of products. Results from this study indicate low carbon concentrations in the shallow sediments limit general metabolic activity, reducing the potential for microbial respiration. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the supplemental file.     

Experimental measurement of sediment nitrification and denitrification in Hamilton Harbour,Canada

This research examines the role of sediment nitrification and denitrification in the nitrogen cycle of Hamilton Harbour. The Harbour is subject to large ammonia and carbon loadings from a waste-water treatment plant and from steel industries. Spring ammonia concentrations rapidly decrease from 4.5 to 0.5 mg 1⁻¹, while spring nitrate concentrations increase from 1 to 2 mg l⁻¹, by mid-summer. A three-layer sediment model was developed. The first layer is aerobic; in it, oxidation of organics and nitrification occurs. The second layer is for denitrification, and the third layer is for anaerobic processes. Ammonia sources for nitrification include diffusion from the water column, sources associated with the oxidation of organics, sources from denitrification and from anaerobic processes. Diffusion of oxygen, ammonia and nitrate across the sediment-water interface occurs. Temperature effects are modelled using the Arrhenius concept. A combination of zero-order kinetics for nitrate or ammonia consumption with diffusion results in a half-order reaction, with respect to the water column loss rate to sediments. From experimental measurement, the rate of nitrification is 200 mg N 1⁻¹ sediment per day, while that of denitrification is 85 mg N 1–1 sediment per day at 20 °C. The Arrhenius activation energy is estimated as 15 000 cal/ mole-K and 17 000 cal/ mole-K for nitrification and denitrification, respectively, between 10 °C and 20 °C. Calculations of the flux of ammonia with the sediments, using the biofilm model, compare favourably with experimental observations. The ammonia flux from the water column is estimated to account for 20% of the observed decrease in water column stocks of ammonia, while the nitrate flux from the water column is estimated to account for 25% of the total nitrogen produced by the sediments.     

Apparent and Measured Rates of Nitrification in the Hypolimnion of a Mesotrophic Lake

Three distinct phases were observed in the change of dissolved inorganic nitrogen concentrations in the hypolimnion of Grasmere. The second phase of decreasing ammonia and increasing nitrate concentrations was typical of the nitrification process. Observations on nitrate concentration gradients between surface sediments and the water column and experiments using the nitrification inhibitor N-Serve indicated the in situ activity of chemolithotrophic nitrifying organisms. Nitrification rates were estimated throughout the period of stratification by using the N-Serve and [¹⁴C]bicarbonate uptake method. Comparison of the field nitrate concentrations with the predicted nitrate concentrations (from estimates of the nitrification rate) indicated that the method underestimated the true rate of nitrification. Possible reasons for this are discussed.     

Effects of bioturbation by tube-dwelling chironomid larvae on oxygen uptake and denitrification in eutrophic lake sediments

1. Oxygen uptake and denitrification were determined in two bioturbated sediments from a eutrophic lake in southern Sweden. In laboratory mesocosms, an organic profundal sediment was incubated with Chironomus plumosus L. and a sandy littoral sediment with an organic-rich top layer was incubated with Polypedilum sp. Both species of chironomid are sediment tube-dwelling. 2. Oxygen consumption, expressed per gram of larval dry weight, was enhanced to the same extent by the larvae in both sediments. Measurements of the respiration rate of individual larvae revealed that the respiration per gram dry weight of the smaller Polypedilum sp. was more than three times higher than that of C. plumosus. 3. Denitrification was measured using the ‘nitrogen isotope pairing’ technique. In the organic sediment, denitrification of nitrate from the water phase (dw) and denitrification of nitrate from coupled nitrification (dn) were each correlated with the biomass of C. plumosus. In the sandy sediment, dw was correlated with the biomass of Polypedilum sp., while dn did not show any correlation with Polypedilum sp. 4. Oxygen uptake in the organic sediment was increased by a factor of 2.5, dw 5-fold and dn 2.5-fold at a biomass of 10 g m^–2 dry weight of C. plumosus. The same biomass of Polypedilum sp. in the sandy sediment resulted in a 2-fold stimulation of oxygen uptake and a 3-fold stimulation of dw, while dn was not affected. These differences in stimulation between oxygen uptake and denitrification by the larvae in the sediments suggest that the stimulation pattern cannot be explained by simple extension of the sediment surface. The burrows evidently reduce the distance between the nitrate source in the water column and the denitrifiers in the anoxic zones. 5. This study indicates that bioturbation by macrofauna elements can have a great impact on denitrification in lake sediments, and that different organisms can influence nitrogen turnover in specific ways.     

Nitrification in Freshwater Sediments as Influenced by Insect Larvae: Quantification by Microsensors and Fluorescence in Situ Hybridization

Sediment-reworking macrofauna can stimulate nitrification by increasing the O₂ penetration into sediments or it can reduce nitrification by grazing on nitrifying bacteria. We investigated the influence of Chironomus riparius larvae (Insecta: Diptera) on the in situ activity, abundance, and distribution of NH₄⁺-oxidizing (AOB) and NO₂^–-oxidizing bacteria (NOB) in two freshwater sediments with microsensors and fluorescence in situ hybridization. In organic-poor sediment, nitrification activity was reduced by the presence of C. riparius larvae, whereas no such effect was detected in organic-rich sediment. We explain this difference with the variable larval burrowing and grazing behavior in the two sediment types: In organic-poor sediment larval activities were intense and evenly distributed across the whole sediment surface, whereas in organic-rich sediment larval activities were locally restricted to the microenvironment of animal burrows. Surprisingly, the animals did not cause any significant change of the abundance of AOB and NOB. This implies that the observed reduction of nitrification activity was not density-regulated, but rather was due to the lowered metabolic activity of the nitrifiers. Partial digestion and redeposition of particle-associated bacteria by C. riparius larvae are believed to have caused this loss of metabolic activity.This revised version was published online in November 2004 with corrections to Volume 48.     

不同盐度对闽江河口沉积物硝化作用的影响

选择闽江河口鳝鱼滩湿地与道庆洲湿地为对象,采集湿地沉积物在室内进行沉积物硝化培养,分析不同盐度水平对湿地沉积物硝化作用的影响.结果表明:闽江河口湿地沉积物硝化速率普遍较低,鳝鱼滩湿地沉积物最高硝化速率只有0.193 mg·kg^-1·d^-1,而道庆洲湿地沉积物硝化速率最高不超过0.050 mg·kg^-1·d^-1.盐度的升高会抑制闽江河口湿地沉积物的硝化作用.低盐度时(5),硝化速率下降的主要原因是硝化细菌活性受到抑制;随着盐度的升高(10),硝化速率略有上升但仍低于初始值,这是由于随着盐度的升高,盐度对好氧氨化细菌活性的抑制程度有所加强,导致系统产生NH_4^+-N的速率下降,从而造成好氧氨化细菌对表观硝化速率下降的贡献减少.沉积物硝化活性对于盐度的响应存在地域差异.咸水湿地(鳝鱼滩湿地)沉积物中的微生物对盐度的变化适应性较强,从而使该湿地沉积物硝化活性在高盐度条件下仍然较高.淡水湿地(道庆洲湿地)沉积物对盐度变化的适应性较弱,导致其沉积物硝化活性在高盐度条件下低于中等盐度.闽江河口鳝鱼滩与道庆洲的短叶茳芏湿地沉积物硝化作用较低的主要原因是沉积物呈酸性及淹水状态下的缺氧条件弱化了沉积物的硝化作用.两处湿地的硝化速率与硝化活性随时间的变化趋势为先升高后下降,这是由初始NH_4^+-N浓度、氧气含量和反硝化共同作用造成的.     

Factors influencing the extent and development of the oxic zone in sediments

Dissolved oxygen concentrations in river-sediment porewaters are reported and modelled using a zero-order reaction rate and the Monod equation. After mixing the sediments and allowing settling, the dissolved oxygen profile in the bed-sediment was expected to reach a steady-state rapidly (< 1 h). However changes in the vertical profile of oxygen over a period of 38 days revealed that the penetration of oxygen increased and the dissolved oxygen flux at the interface decreased with time, probably as the oxidation kinetics of organic matter and redox reactions in the sediment changed. Experiments with three contrasting silt and sand dominated sediments (organic matter content between 0.9 and 18%) at two water velocities (ca 10 and 20 cm s^–1) showed that the dissolved oxygen profiles were independent of velocity for each of the sediments. The most important controls on the reaction rate were the organic matter content and specific surface area of the sediment. A viscous diffuse-boundary-layer above the sediment was only detected in the experiments with the silt sediment where the sediment oxygen demand was relatively high. In the coarser sediments, the absence of a diffuse layer indicated that slow oxidation processes in the sediment controlled the dissolved oxygen flux at the interface. The problem of determining a surface reference in coarse sediment is highlighted. The results are discussed with reference to other studies including those concerned with estuarine and marine sediments.     

The influence of an invasive plant on denitrification in an urban wetland

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Distribution of autotrophic nitrifying bacteria in a polluted river (the Passaic).

The abundance of nitrifying bacteria, determined by most-probable-number procedures, within habitats of the Passaic River was as follows: rooted aquatic plants greater than algae approximately equal to rocks greater than sediments greater than greater than water. On the average, NH4+ oxidizers were 540-fold more abundant in the topmost 1 cm of sediment than in the water, and NO2- oxidizers were 250-fold more abundant. The population densities in this surface sediment at two nearby stations, one with a predominantly mineral stream bed and the other an organic ooze, did not differ significantly. Large numbers of nitrifiers were present to a depth of about 5 cm in a mineral sediment core.     

Distribution of autotrophic nitrifying bacteria in a polluted river (the Passaic)

The abundance of nitrifying bacteria, determined by most-probable-number procedures, within habitats of the Passaic River was as follows: rooted aquatic plants greater than algae approximately equal to rocks greater than sediments greater than greater than water. On the average, NH4+ oxidizers were 540-fold more abundant in the topmost 1 cm of sediment than in the water, and NO2- oxidizers were 250-fold more abundant. The population densities in this surface sediment at two nearby stations, one with a predominantly mineral stream bed and the other an organic ooze, did not differ significantly. Large numbers of nitrifiers were present to a depth of about 5 cm in a mineral sediment core.     

Intensity of mineralization processes in mountain lakes in NW Slovenia

The potential and actual intensity of mineralization in sediments of fourteen mountain lakes and one subalpine lake in NW Slovenia have been measured. Potential mineralization was measured as the intensity of the electron transport system (ETS) activity of microzoobenthos and microbial communities and the actual mineralization as the oxygen consumption of respiration processes, both measured at a standard temperature of 20°C. The lakes are of different trophic levels and some exhibit seasonal anoxia. All but one are hardwater lakes. Two layers of sediment cores from the deepest point of the lakes were analysed: a surface layer and one below 15 cm. Significant differences among different lakes in their ETS activity and oxygen consumption in the surface and lower layers of sediment were observed. ETS activities and oxygen consumption rates were higher in the surface layers of all the lakes. From the three investigated deterministic factors (temperature, lake depth and total phosphorus in the water column) on sedimentary metabolism ETS activity in the surface layer correlated significantly with total phosphorus and lake depth, but oxygen consumption rate showed a significant correlation only with total phosphorus. The relationship between oxygen consumption and ETS activity was also investigated. ETS activities correlated with oxygen consumption rates according to the equation of logR = 0.421^* logETS + 0.898 (r=0.82; n=30; p<0.001). The R/ETS ratio was lower at the sediment surface than in the layers deeper than 15 cm. It is concluded that ETS activity and oxygen consumption are good indicators of the intensity of the metabolic activity and mineralization in lake sediments. As the characteristics of lakes and some environmental factors influence the ETS activity and the oxygen consumption differently, the same R/ETS ratio should not be used as conversion factor in calculations for different lakes.     

Nitrate and nitrite in interstitial waters of eelgrass beds in relation to the rhizosphere

The distribution of nitrate and nitrite in the interstitial water of the sediment of eelgrass (Zostera marina) bed of Izembek Lagoon, Alaska, were investigated. Their concentrations were relatively high (0 to 9.8 μg-at.N·1^?1, average 4.8 for nitrate; 0 to 4.0 μ-at.N·1^?1, average 1.9 for nitrite) although the sediments were anoxic and contained hydrogen sulphide. The rates of bacterial denitrification measured by ¹⁵N tracer technique ranged from 0.49×10^?10 to 1.2 × 10^?9 g-atN·g^?1·h^?1. When a steady state is maintained, the loss of nitrate and nitrite must be balanced by their production by bacterial nitrification. Experimentally determined rate of nitrification in the sediment was of the same order. A model experiment demonstrated that oxygen is transported from leaves to rhizomes and roots of eelgrass and released into the sediment. The oxygen is used for nitrification in the rhizosphere in anoxic sediments.     

Measurement of in situ rates of nitrification in sediment

A method has been developed for the measurement of nitrification rates in intact sediment cores without disturbing the concentration gradients of oxygen and ammonium. N-serve (2-chloro-6-trichloromethyl-pyridine), a specific inhibitor of the autotrophic ammonium oxidation, was injected into a 0–2 cm surface layer of the sediment (20 ppm) and added to the water column of sediment cores (5 ppm). N-serve in these concentrations was sufficient to inhibit nitrification, but did not change the rate of ammonium production or incorporation in sediment suspensions, which were incubated aerobically and anaerobically. The ammonium accumulation in cores injected with N-serve was thus equal to the amount of ammonium which was oxidized to nitrate in the control cores. Nitrification rates were in the range of 0–3 mmol N m^{–2 –1}