Simulation Model of the Coupling between Nitrification and Denitrification in a Freshwater Sediment

A model was constructed to simulate the results of experiments which investigated nitrification and denitrification in the freshwater sediment of Lake Vilhelmsborg, Denmark (K. Jensen, N. P. Sloth, N. Risgaard-Petersen, S. Rysgaard, and N. P. Revsbech, Appl. Environ. Microbiol. 60:2094-2100, 1994). The model output faithfully represented the profiles of O₂ and NO₃^- and rates of nitrification, denitrification, and O₂ consumption as the O₂ concentration in the overlying water was increased from 10 to 600 μM. The model also accurately predicted the response, to increasing O₂ concentrations, of the integrated (micromoles per square meter per hour) rates of nitrification and denitrification. The simulated rates of denitrification of NO₃^- diffusing from the overlying water (D_w) and of NO₃^- generated by nitrification within the sediment (D_n) corresponded to the experimental rates as the O₂ concentration in the overlying water was altered. The predicted D_w and D_n rates, as NO₃^- concentration in the overlying water was changed, closely resembled those determined experimentally. The model was composed of 41 layers 0.1 mm thick, of which 3 represented the diffusive boundary layer in the water. Large first-order rate constants for nitrification and denitrification were required to completely oxidize all NH₄⁺ diffusing from the lower sediment layers and to remove much of the NO₃^- produced. In addition to the flux of NH₄⁺ from below, the model required a flux of an electron donor, possibly methane. Close coupling between nitrification and denitrification, achieved by allowing denitrification to tolerate some O₂ (~10 μM), was necessary to reproduce the real data. Spatial separation of the two processes (no toleration by denitrification of O₂) resulted in too high NO₃^- concentrations and too low rates of denitrification.     

Potential rates of nitrification and denitrification in an oligotrophic freshwater sediment system

Potential rates of nitrification and denitrification were measured in an oligotrophic sediment system. Nitrification potential was estimated using the CO oxidation technique, and potential denitrification was measured by the acetylene blockage technique. The sediments demonstrated both nitrifying and denitrifying activity. E_h, O₂, and organic C profiles showed two distinct types of sediment. One type was low in organic C, had high O₂ and E_h, and had rates of denitrification 1,000 times lower than the other which had high organic C, low O₂, and low E_h. Potential nitrification and denitrification rates were negatively correlated with E_h. This suggests that environmental heterogeneity in denitrifier and nitrifier populations in oligotrophic sediment systems may be assessed using E_h before sampling protocols for nitrification or denitrification rates are established. There was no correlation between denitrification and nitrification rates or between either of these processes and NH₄ ⁺ or NO₃ ^– concentrations. The maximum rate of denitrification was 0.969 nmole N cm^–3 hour^–1, and the maximum rate of nitrification was 23.6 nmole cm^–3 hour^–1, suggesting nitrification does not limit denitrification in these oligotrophic sediments. Some sediment cores had mean concentrations of 6.0 mg O₂/liter and still showed both nitrification and denitrification activity.     

Biosensor Determination of the Microscale Distribution of Nitrate, Nitrate Assimilation, Nitrification, and Denitrification in a Diatom-Inhabited Freshwater Sediment

High-resolution NO₃⁻ profiles in freshwater sediment covered with benthic diatoms were obtained with a new microscale NO₃⁻ biosensor characterized by absence of interference from chemical species other than NO₂⁻ and N₂O. Analysis of the microprofiles obtained indicated no nitrification during darkness, high rates of nitrification and a tight coupling between nitrification and denitrification during illumination, and substantial rates of NO₃⁻ assimilation during illumination. Nitrification during darkness could be induced by purging the bulk water with O₂ gas, indicating that the stimulatory effect on nitrification by illumination was caused by algal production of O₂. NH₄⁺ addition did not stimulate nitrification during darkness when O₂ was restricted to the upper 1-mm layer, and there was thus a low nitrification potential in the permanently oxic top 1 mm of the sediment.     

Stable isotopes indicate that zebra mussels (Dreissena polymorpha) increase dependence of lake food webs on littoral energy sources

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Nutrient cycling in shallow,oligotrophic Lake Kvie,Denmark

The importance of isoetids for the exchange of dissolved inorganic nitrogen (DIN) between sediment and water was studied in shallow Lake Kvie, Denmark. Vegetated sediments from the littoral zone (55% of lake area) were compared to unvegetated sediments from the littoral and profundal zone. Clear effects of the isoetids were found on DIN in the porewater. At the vegetated station, NH₄ ⁺ showed the highest concentrations just below the surface (< 40 µM) whereas NO₃ ^- was dominating below 5 cm depth with concentrations up to 100 µM during the spring. The unvegetated littoral sediment showed a distinct change between winter where NH₄ ⁺ dominated and summer where NO₃ ^- was most abundant. NH₄ ⁺ dominated in the profundal sediment and showed increasing concentration with depth. The E_h was high (> 400 mV) in the vegetated sediment, indicating isoetid release of O₂ in the rhizosphere. A low DIN uptake was observed at the vegetated station while, based on porewater data, a diffusive release from the sediment was expected. This difference was due to plant assimilation. In general a release of NH₄ ⁺ and an uptake of NO₃ ^- was seen in all sediments. The denitrification rate calculated from the mass balance for the entire lake was 0.4 mmol m - 2 d^-1 and accounted for removal of 77% of the annual N-input to Lake Kvie.     

Nitrogen transformations at the sediment–water interface across redox gradients in the Laurentian Great Lakes

The capacity of a lake to remove reactive nitrogen (N) through denitrification has important implications both for the lake and for downstream ecosystems. In large oligotropic lakes such as Lake Superior, where nitrate (NO₃ ^?) concentrations have increased steadily over the past century, deep oxygen penetration into sediments may limit the denitrification rates. We tested the hypothesis that the position of the redox gradient in lake sediments affects denitrification by measuring net N-fluxes across the sediment–water interface for intact sediment cores collected across a range of sediment oxycline values from nearshore and offshore sites in Lake Superior, as well as sites in Lake Huron and Lake Erie. Across this redox gradient, as the thickness of the oxygenated sediment layer increased from Lake Erie to Lake Superior, fluxes of NH₄ ⁺ and N₂ out of the sediment decreased, and sediments shifted from a net sink to a net source of NO₃ ^?. Denitrification of NO₃ ^? from overlying water decreased with thickness of the oxygenated sediment layer. Our results indicate that, unlike sediments from Lake Erie and Lake Huron, Lake Superior sediments do not remove significant amounts of water column NO₃ ^? through denitrification, likely as a result of the thick oxygenated sediment layer.     

Impacts of zebra mussels (Dreissena polymorpha) on isotopic niche size and niche overlap among fish species in a mesotrophic lake

Zebra mussels (Dreissena polymorpha) filter feed phytoplankton and reduce available pelagic energy, potentially driving fish to use littoral energy sources in lakes. However, changes in food webs and energy flow in complex fish communities after zebra mussel establishment are poorly known. We assessed impacts of zebra mussels on fish littoral carbon use, trophic position, isotopic niche size, and isotopic niche overlap among individual fish species using δ¹³C and δ¹⁵N data collected before (2014) and after (2019) zebra mussel establishment in Lake Ida, MN. Isotope data were collected from 11 fish species, and from zooplankton and littoral invertebrates to estimate baseline isotope values. Mixing models were used to convert fish δ¹³C and δ¹⁵N into estimates of littoral carbon and trophic position, respectively. We tested whether trophic position, littoral carbon use, isotopic niche size, and isotopic niche overlap changed from 2014 to 2019 for each fish species. We found few effects on fish trophic position, but 10 out of 11 fish species increased littoral carbon use after zebra mussel establishment, with mean littoral carbon increasing from 43% before to 67% after establishment. Average isotopic niche size of individual species increased significantly (2.1-fold) post zebra mussels, and pairwise-niche overlap between species increased significantly (1.2-fold). These results indicate zebra mussels increase littoral energy dependence in the fish community, resulting in larger individual isotopic niches and increased isotopic niche overlap. These effects may increase interspecific competition among fish species and could ultimately result in reduced abundance of species less able to utilize littoral energy sources.

     

In situ survival and growth of zebra mussels (Dreissena polymorpha) under chronic hypoxia in a stratified lake

Experiments and field surveys were conducted in Hargus Lake (Ohio, U.S.) to investigate the effect of lake stratification on the survival, growth and distribution of zebra mussels. During the lake stratification period, relatively stable temperature and dissolved oxygen (DO) gradients persisted across the water column, allowing us to examine the chronic effect of hypoxia on zebra mussels. Zebra mussels were incubated in cages and suspended at different depths in the water column at both pelagic (max. depth = 12 m) and littoral (max. depth = 3.5 m) sites from April 18 to September 28, 1994. No mussel survived to the end of the experiment in cages ≥ 5.5 m, whereas the highest survival rate (76%) occurred at 5 m depth where temperature and DO remained fairly stable for at least 3 months. The threshold oxygen level for survival was between 1.0–1.7 mg l^-1 when water temperature was at about 17–18 °C. While zebra mussels′ survival rate was not affected under the sublethal hypoxic conditions, their growth was greatly retarded by poor water quality. The field survey showed that the zebra mussels and macrophytes had about the same distribution and their biomasses were positively related. The percentage of mussels in aggregates increased towards their maximum distribution depth. The maximum distribution depth of the naturally occurring zebra mussels was only 2.8 m, whereas the adult mussels could survive the entire stratification period when being artificially placed on the 3.5 m bottom, and young mussels could colonize the 3.5 m bottom if solid substrates were provided. We conclude that lack of substrate, rather than hypoxia, was the limiting factor of zebra mussel distribution above 5 m depth in Hargus Lake. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nitrification and Denitrification in Lake and Estuarine Sediments Measured by the 15N Dilution Technique and Isotope Pairing

The transformation of nitrogen compounds in lake and estuarine sediments incubated in the dark was analyzed in a continuous-flowthrough system. The inflowing water contained ¹⁵NO₃^-, and by determination of the isotopic composition of the N₂, NO₃^-, and NH₄⁺ pools in the outflowing water, it was possible to quantify the following reactions: total NO₃^- uptake, denitrification based on NO₃^- from the overlying water, nitrification, coupled nitrification-denitrification, and N mineralization. In sediment cores from both lake and estuarine environments, benthic microphytes assimilated NO₃^- and NH₄⁺ for a period of 25 to 60 h after darkening. Under steady-state conditions in the dark, denitrification of NO₃^- originating from the overlying water accounted for 91 to 171 μmol m^-2 h^-1 in the lake sediments and for 131 to 182 μmol m^-2 h^-1 in the estuarine sediments, corresponding to approximately 100% of the total NO₃^- uptake for both sediments. It seems that high NO₃^- uptake by benthic microphytes in the initial dark period may have been misinterpreted in earlier investigations as dissimilatory reduction to ammonium. The rates of coupled nitrification-denitrification within the sediments contributed to 10% of the total denitrification at steady state in the dark, and total nitrification was only twice as high as the coupled process.     

Temporal Variation of Denitrification Activity in Plant-Covered, Littoral Sediment from Lake Hampen, Denmark

Diel and seasonal variations in denitrification were determined in a littoral lake sediment colonized by the perennial macrophyte Littorella uniflora (L.) Aschers. In the winter, the activity was low (5 μmol of N m⁻² h⁻¹) and was restricted to the uppermost debris layer at a depth of 0 to 1 cm. By midsummer, the activity increased to 50 μmol of N m⁻² h⁻¹ and was found throughout the root zone to a depth of 10 cm. The root zone accounted for as much as 50 to 70% of the annual denitrification. A significant release of organic substrates from the roots seemed to determine the high activities of root zone denitrification in the summer. The denitrification in the surface layer and in the root zone formed two distinct activity zones in the summer, when the root zone also contained nitrification activity, as indicated from the accumulations of NO₃⁻. Light conditions inhibited denitrification in both the surface layer and the upper part of the root zone, suggesting that a release of O₂ by benthic algae and from the roots of L. uniflora controlled a diel variation of denitrification. In midsummer, the rate of denitrification in both the surface layer and the upper part of the root zone was limited by NO₃⁻. In the growth season, there was evidence for a significant population of denitrifiers closely associated with the root surface.     

Invasive mussels modify the cycling,storage and distribution of nutrients and carbon in a large lake

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Northern map turtles (Graptemys geographica) derive energy from the pelagic pathway through predation on zebra mussels (Dreissena polymorpha)

1. Zebra mussels ( Dreissena polymorpha ) derive their energy from the pelagic energy pathway by filtering plankton. Because zebra mussels occur in high densities in littoral habitats, they potentially constitute an important trophic link between littoral consumers and pelagic energy sources. Northern map turtles ( Graptemys geographica ) are widespread in North America and consume zebra mussels.
2. We used stable isotopes analyses to quantify the flow of energy from the pelagic pathway to northern map turtles and to infer the contribution of zebra mussels to map turtle biomass. We then built a bioenergetic model to estimate the annual intake of zebra mussels by northern map turtles in Lake Opinicon, Ontario, Canada.
3. Stable isotopes analyses indicated that zebra mussels constitute between 0% and 14% of the diet of males and between 4% and 36% of the diet of females. Assuming that zebra mussels account for all of the pelagic contribution, we estimated that map turtles consume 3200 kg of zebra mussels annually. Because female map turtles are much larger than males and consume more zebra mussels, they are responsible for 95% of the zebra mussel biomass ingested annually.
4. The pelagic pathway supports an important part of the standing crop biomass of map turtles in Lake Opinicon. We highlight the importance of freshwater turtles in lake ecosystems. Unravelling the trophic interactions mediated by freshwater turtles will lead to a more integrated picture of lake ecosystems.     

Denitrification kinetics and denitrifier abundances in sediments of lakes receiving atmospheric nitrogen deposition (Colorado, USA)

The transport and deposition of anthropogenic nitrogen (N) to downwind ecosystems is significant and can be a dominant source of new N to many watersheds. Bacterially mediated denitrification in lake sediments may ameliorate the effects of N loading by permanently removing such inputs. We measured denitrification in sediments collected from lakes in the Colorado Rocky Mountains (USA) receiving elevated (5–8?kg?N?ha^?1?y^?1) or low (<2?kg?N?ha^?1?y^?1) inputs of atmospheric N deposition. The nitrate (NO₃ ^?) concentration was significantly greater in high-deposition lakes (11.3?μmol?l^?1) compared to low-deposition lakes (3.3?μmol?l^?1). Background denitrification was positively related to NO₃ ^? concentrations and we estimate that the sampled lakes are capable of removing a significant portion of N inputs via sediment denitrification. We also conducted a dose–response experiment to determine whether chronic N loading has altered sediment denitrification capacity. Under Michaelis–Menten kinetics, the maximum denitrification rate and half-saturation NO₃ ^? concentration did not differ between deposition regions and were 765?μmol?N?m^?2?h^?1 and 293?μmol?l^?1?NO₃ ^?, respectively, for all lakes. We enumerated the abundances of nitrate- and nitrite-reducing bacteria and found no difference between high- and low-deposition lakes. The abundance of these bacteria was related to available light and bulk sediment resources. Our findings support a growing body of evidence that lakes play an important role in N removal and, furthermore, suggest that current levels of N deposition have not altered the abundance of denitrifying bacteria or saturated the capacity for sediment denitrification.     

Efficient removal of NH4+ accumulated during anaerobic treatment of distillery wastewater from Shochu making

Propionate and NH₄⁺ were accumulated in the effluent during anaerobic treatment of five-fold diluted distillery wastewater from shochu making. Propionate could be removed efficiently during biological denitrification by the addition of NO₃⁻ (4.2 g/l) to the anaerobically treated wastewater. At a hydraulic retention time of more than 2 h, a TOC removal efficiency of 90% could be achieved. The wastewater was then treated aerobically by biological nitrification. With a hydraulic retention time of more than 14 h the efficiency of reduction of NH₄⁺ could be maintained above 97%. In order to reduce the amount of NO₃⁻ addition necessary for the removal of propionate, simultaneous removal of propionate and NH₄⁺ was studied by recirculating the effluent from a nitrification process to a denitrification process using denitrification and nitrification reactors connected in series. At a recirculation ratio of 2, the amount of NO₃⁻ that had to be added was reduced to 0.3 g/l of anaerobically treated wastewater, which corresponds to 6.9% of the theoretical value. Under the same conditions except for the addition of NO₃⁻ at 1.0 g/l, TOC and BOD in the effluent from the nitrification were 23 and 5 mg/l respectively, which are sufficiently low to allow discharge into river water. Moreover, the NO₃⁻ concentration in the effluent decreased with increases in the recirculation ratio.     

Microelectrode Measurements of Nitrate Gradients in the Littoral and Profundal Sediments of a Meso-Eutrophic Lake (Lake Vechten, The Netherlands)

NO₃⁻ concentration profiles were measured in the sediments of a meso-eutrophic lake with a newly developed microelectrode. The depth of penetration of NO₃⁻ varied from only 1.3 mm in organic-rich profundal silty sediments to 5 mm in organic-poor littoral sandy sediments. The thickness of the zone of denitrification in the organic-rich sediments was <500 μm. Oxygen profiles measured simultaneously revealed that the zone of denitrification was directly adjacent to the aerobic zone. The results demonstrate high denitrification rates (0.26 to 1.31 mmol m⁻² day⁻¹) at in situ nitrate concentrations in the overlying water (0.030 mmol liter⁻¹) and limitation of denitrification by nitrate availability.     

Altered trophic pathway and parasitism in a native predator (Lepomis gibbosus) feeding on introduced prey (Dreissena polymorpha)

Populations of invasive species tend to have fewer parasites in their introduced ranges than in their native ranges and are also thought to have fewer parasites than native prey. This ‘release’ from parasites has unstudied implications for native predators feeding on exotic prey. In particular, shifts from native to exotic prey should reduce levels of trophically transmitted parasites. We tested this hypothesis in native populations of pumpkinseed sunfish (Lepomis gibbosus) in Lake Opinicon, where fish stomach contents were studied intensively in the 1970s, prior to the appearance of exotic zebra mussels (Dreissena polymorpha) in the mid-1990s. Zebra mussels were common in stomachs of present-day pumpkinseeds, and stable isotopes of carbon and nitrogen confirmed their importance in long-term diets. Because historical parasite data were not available in Lake Opinicon, we also surveyed stomach contents and parasites in pumpkinseed in both Lake Opinicon and an ecologically similar, neighboring lake where zebra mussels were absent. Stomach contents of pumpkinseed in the companion lake did not differ from those of pre-invasion fish from Lake Opinicon. The companion lake, therefore, served as a surrogate “pre-invasion” reference to assess effects of zebra mussel consumption on parasites in pumpkinseed. Trophically transmitted parasites were less species-rich and abundant in Lake Opinicon, where fish fed on zebra mussels, although factors other than zebra mussel consumption may contribute to these differences. Predation on zebra mussels has clearly contributed to a novel trophic coupling between littoral and pelagic food webs in Lake Opinicon.     

Sediment Nitrification, Denitrification, and Nitrous Oxide Production in a Deep Arctic Lake

We used a combination of ¹⁵N tracer methods and a C₂H₂ blockage technique to determine the role of sediment nitrification and denitrification in a deep oligotrophic arctic lake. Inorganic nitrogen concentrations ranged between 40 and 600 nmol · cm⁻³, increasing with depth below the sediment-water interface. Nitrate concentrations were at least 10 times lower, and nitrate was only detectable within the top 0 to 6 cm of sediment. Eh and pH profiles showed an oxidized surface zone underlain by more reduced conditions. The lake water never became anoxic. Sediment Eh values ranged from −7 to 484 mV, decreasing with depth, whereas pH ranged from 6.0 to 7.3, usually increasing with depth. The average nitrification rate (49 ng of N · cm⁻³ · day⁻¹) was similar to the average denitrification rate (44 ng of N · cm⁻³ · day⁻¹). In situ N₂O production from nitrification and denitrification ranged from 0 to 25 ng of N · cm⁻³ · day⁻¹. Denitrification appears to depend on the supply of nitrate by nitrification, such that the two processes are coupled functionally in this sediment system. However, the low rates result in only a small nitrogen loss.     

Nitrate cycling in Lake Titicaca (Peru-Bolivia): the effects of high-altitude and tropicality

SUMMARY.

• 1 The vertical distribution of dissolved oxygen and inorganic nitrogen differed considerably between three stratification cycles in Lake Titicaca, a tropical lake (latitude 16°S) in the high Andes (3800 m altitude). In 1980/81 an anoxic layer of water extended from 200 to 275 m and contained high levels of NH₄ but zero NO₃. During the annual deep mixing period in 1981 this layer was substantially eroded, and was completely eliminated during deep-mixing in 1982.
• 2 Nitrapyrin assays of nitrification demonstrated highest activity in the surface mixed layer, lowest activity just beneath the thermocline, and then increasing nitrification rates with increasing depth towards the bottom anoxic zone. Denitrification rates were slow, but detectable, in the surficial sediments of Lake Titicaca during late 1982. Much faster rates were estimated for the periods of water column anoxia.
• 3 Lake Titicaca is a productive lake with low saturation levels of oxygen because of its high altitude. These features favour hypolimnetic anoxia, and thus denitrification which varies in magnitude from year to year.

     

Estimation of Nitrification and Denitrification from Microprofiles of Oxygen and Nitrate in Model Sediment Systems

The coupling between nitrification and denitrification and the regulation of these processes by oxygen were studied in freshwater sediment microcosms with O₂ and NO₃^- microsensors. Depth profiles of nitrification (indicated as NO₃^- production), denitrification (indicated as NO₃^- consumption), and O₂ consumption activities within the sediment were calculated from the measured concentration profiles. From the concentration profiles, it was furthermore possible to distinguish between the rate of denitrification based on the diffusional supply of NO₃^- from the overlying water and the rate based on NO₃^- supplied by benthic nitrification (D_w and D_n, respectively). An increase in O₂ concentration caused a deeper O₂ penetration while a decrease in D_w and an increase in D_n were observed. The relative importance for total denitrification of NO₃^- produced by nitrification thus increased compared with NO₃^- supplied from the water phase. The decrease in D_w at high oxygen was due to an increase in diffusion path for NO₃^- from the overlying water to the denitrifying layers in the anoxic sediment. At high O₂ concentrations, nitrifying activity was restricted to the lower part of the oxic zone where there was a continuous diffusional supply of NH₄⁺ from deeper mineralization processes, and the long diffusion path from the nitrification zone to the overlying water compared with the path to the denitrifying layers led to a stimulation in D_n.     

Nitrogen fluxes from marine sediments: quantification of the associated co-occurring bacterial processes

Different models for calculate on of di-nitrogen fluxes using ¹⁵NO₃ tracers were tested for their congruence with experimental data obtained with marine sediment samples. The co-occurence of nitrification as source of substrate and the simultaneous N₂ production from denitrification and/or Anammox were taken into account as well as nitrous oxide production in the total denitrification rate. The results highlighted that isotope technique provides a powerful tool to evaluate, in the same experimental set up, the rates of total N₂ fluxes: denitrification and/or Anammox if it is carefully applied and its limitations, mainly the range of ¹⁵NO₃ inputs are adapted to the studied samples and the linearity of the kinetics of the products checked.