Denitrification Rates in a Marine Sediment as Measured by the Acetylene Inhibition Technique

A method has been developed for measurement of denitrification activity in sediments by application of the acetylene inhibition technique. Acetylene-saturated water was injected, at close intervals, into sediment cores which were then incubated for a few hours at the in situ temperature. Frozen segments of the cores were assayed for accumulation of N₂O by a combined gas extraction and detection system. The segments were thawed under a stream of helium from which N₂O (and other gases) was collected in a liquid N₂ trap, and the quantity of N₂O was measured by gas chromatography. The maximum rate of denitrification in a coastal marine sediment was 35 nmol of N per cm³ of sediment per day at 2.5°C, and the rate of denitrification for the total sediment was 0.99 nmol of N per m² per day.     

Flux of reduced chemical constituents (Fe2+, Mn2+, NH inf4 sup+ and CH4) and sediment oxygen demand in Lake Erie

Sediment pore water concentrations of Fe²⁺, Mn²⁺, NH _inf4 ^sup+ and CH₄ were analyzed from both diver-collected cores and anin situ equilibration device (peeper) in Lake Erie's central basin. Sediment oxygen demand (SOD) was measured at the same station with a hemispheric chamber (including DO probe and recorder) subtending a known area of sediments. The average SOD was 9.4 mM m⁻² day⁻¹ (0.3 g m⁻² day⁻¹). From pore water gradients within the near-surface zone, the chemical flux across the interface was calculated indirectly using Fick's first law modified for sediments. These calculations, using core and peeper gradients, always showed sediment loss to overlying waters, and variations between the two techniques differed by less than an order of magnitude for Fe²⁺ and CH₄. The transport of these reduced constituents can represent a sizeable oxygen demand, ranging from less than 1% for Fe²⁺ and Mn²⁺ to as high as 26% for NH _inf4 ^sup+ , and 30% for CH₄. The average flux of these constituents could account for about a third of the SOD at the sediment-water interface of this station.     

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.     

Contribution of Sediment Respiration to Summer CO2 Emission from Low Productive Boreal and Subarctic Lakes

We measured sediment production of carbon dioxide (CO₂) and methane (CH₄) and the net flux of CO₂ across the surfaces of 15 boreal and subarctic lakes of different humic contents. Sediment respiration measurements were made in situ under ambient light conditions. The flux of CO₂ between sediment and water varied between an uptake of 53 and an efflux of 182 mg C m⁻² day⁻¹ from the sediments. The mean respiration rate for sediments in contact with the upper mixed layer (SedR) was positively correlated to dissolved organic carbon (DOC) concentration in the water (r² = 0.61). The net flux of CO₂ across the lake surface [net ecosystem exchange (NEE)] was also closely correlated to DOC concentration in the upper mixed layer (r² = 0.73). The respiration in the water column was generally 10-fold higher per unit lake area compared to sediment respiration. Lakes with DOC concentrations <5.6 mg L⁻¹ had net consumption of CO₂ in the sediments, which we ascribe to benthic primary production. Only lakes with very low DOC concentrations were net autotrophic (<2.6 mg L⁻¹) due to the dominance of dissolved allochthonous organic carbon in the water as an energy source for aquatic organisms. In addition to previous findings of allochthonous organic matter as an important driver of heterotrophic metabolism in the water column of lakes, this study suggests that sediment metabolism is also highly dependent on allochthonous carbon sources.     

Gasstoffwechselphysiologische Schädigungskriterien bei submersen Makrophyten vom Typ Fontinalis antipyretica L. unter Einwirkung von Schwermetallen oder Phenol

The water moss Fontinalis antipyretica L. is well suited for measuring CO₂ exchange by infrared gas analysis (IRGA), as only CO₂ is used for assimilation. Both in a state of full activity and of reduced activity in the course of a toxical charge the ratio of net primary productivity to respiration related to intermittent illumination is used as a bioassay. Three types regarding the proportion of net assimilation (A_N) to respiration (R_D) are refered to the toxical charge (phenol, HgCl₂, CuSO₄, CdCl₂). In the case of displacing the balance of A_N/R_D in the diurnal cycle to the side of respiration, photosynthetic oxygenation in the water ecosystem decreases. The combination of measuring CO₂ exchange by IRGA with a cyto-physiological investigation by determining the time of deplasmolysis of leaves is used for the prediction of vitality long before damage including lethal effects are to be recognizend morphologically.     

Vertical stratification in sediments from a young oligotrophic South African impoundment: implications in phosphorus cycling

Changes at the mud surface in Midmar Dam, following impoundment, were studied by examining vertical profiles of selected parameters in sediment cores. Distinct stratification in organic carbon, pH and exchangeable Al³⁺ was evident. Phosphate adsorption characteristics in the stratified sediments was quantified using Langmuir adsorption isotherms. The adsorption maxima and bonding energy constants in the surface sediments (0–3 cm) were markedly lower than those below 3 cm, indicating that the surface layers are less efficient at binding phosphate than the deeper layers. Radiotracer experiments indicate that the layers comprising the top 3 cm of sediment predominate in PO₄-P exchange with the overlying water.     

A method to estimate net community metabolism from profiles of dissolved O2 and N2

A method to estimate net community metabolism (NCM) in natural waters using vertical profiles of water temperature, salinity, dissolved O₂, gas tension, and calculated dissolved N₂ is presented. The method utilizes the disparate biological activity of dissolved O₂ and N₂ to estimate metabolism at different depths in the water column. For well-mixed surface waters, N₂ saturation levels are assumed to be the result of a quasi steady state balance of net warming or cooling and air–water gas exchange. Dissolved O₂ levels are assumed to maintain a similar balance, subject to net biological activity, and NCM is then calculated based on the difference between N₂ and O₂ saturation levels and the estimated timescale required to equilibrate the layer with the atmosphere. For deeper stratified layers of water that warmed after layer formation in isolation from the atmosphere, the temperature at formation is calculated using the measured N₂ concentration and an assumed N₂ saturation level of 100% at formation. By assuming that initial N₂ and O₂ saturation levels were equal, the initial O₂ concentration is calculated based on solubility relationships. NCM of the deeper waters is then estimated based on this information and knowledge of the general seasonal heating cycle of the waters. Daily mean water temperature and dissolved gas levels are used in the calculations. The method was assessed using profile measurements collected at Long Pond, Plymouth, Massachusetts, USA, on 23 August 2002. Oxygen was supersaturated relative to N₂ by approximately 4% in the 0–6 m deep epilimnion, and undersaturated relative to N₂ by approximately 7% in the stratified water at 9 m depth. The estimated 4-day average NCM for the epilimnion was 140 ± 70 mgC m⁻²day⁻¹. For waters at 9 m depth, the temperature at formation was calculated to be 6.58 °C, and the estimated 100-day average NCM was −2.5 ± 0.6 mgC m⁻³ day⁻¹. An independent estimate of −4.6 ± 0.9 mgC m⁻³ day⁻¹ was derived from the measured O₂ decline at 9 m depth over the same period of 2003.     

Plumbing the Global Carbon Cycle: Integrating Inland Waters into the Terrestrial Carbon Budget

ABSTRACT Because freshwater covers such a small fraction of the Earth’s surface area, inland freshwater ecosystems (particularly lakes, rivers, and reservoirs) have rarely been considered as potentially important quantitative components of the carbon cycle at either global or regional scales. By taking published estimates of gas exchange, sediment accumulation, and carbon transport for a variety of aquatic systems, we have constructed a budget for the role of inland water ecosystems in the global carbon cycle. Our analysis conservatively estimates that inland waters annually receive, from a combination of background and anthropogenically altered sources, on the order of 1.9 Pg C y⁻¹ from the terrestrial landscape, of which about 0.2 is buried in aquatic sediments, at least 0.8 (possibly much more) is returned to the atmosphere as gas exchange while the remaining 0.9 Pg y⁻¹ is delivered to the oceans, roughly equally as inorganic and organic carbon. Thus, roughly twice as much C enters inland aquatic systems from land as is exported from land to the sea. Over prolonged time net carbon fluxes in aquatic systems tend to be greater per unit area than in much of the surrounding land. Although their area is small, these freshwater aquatic systems can affect regional C balances. Further, the inclusion of inland, freshwater ecosystems provides useful insight about the storage, oxidation and transport of terrestrial C, and may warrant a revision of how the modern net C sink on land is described.     

The influence of lime and biological activity on sediment pH,redox and phosphorous dynamics

The addition of powdered limestone to intact sediment cores from oligotrophic, acid Lake Hovvatn caused pH to increase, redox potential (E₇) to drop, and permitted net precipitation of phosphorous (P) from the water column. Significant pH increase was found to a sediment depth of 6 cm and a maximum increase in pH from 4.9 to 6.5 was found at a depth of 0.5 cm when dosed with 36 g m^–2 of lime. Such pH increase creates important changes in sediment equilibrium chemistry and enhances habitat suitability. In the case of Hovvatn, however, sediments would consume only 5 kg of the 91 tons of applied limestone. Superficial sediments remained oxidized, but below 0.5 cm, E₇ in limed sediment declined significantly more than in unlimed sediments, with a maximum difference of 102 mV versus –66 mV at a depth of 6 cm in unlimed and limed cores, respectively. Abiotic reactions account for 82 ± 54% of this reduction and the remainder is due to the oxidation of organic matter by bacteria. Precipitation of CaSO₄, reduction of the sediments by organic compounds at elevated pH and inhibition of the downward diffusion of O₂ by the limestone powder are potential abiotic mechanisms which could drive E₇ down. Enhanced P release was not found at lowered E₇, and supernatent TP concentrations dropped from 11.7 to 4.4 µg P l^–1. More P was swept from solution in cores which recieved larger lime doses. The presence of chironomids caused sediment pH to increase by as much as 1.2 pH units, presumably due to NH₄ release, reduced sediment E₇ by as much as 171 mV and facilitated TP release during the first 17 d of core incubation. Field measurements of vertical distributions of sediment pH and E₇ before and after the liming of Hovvtn corroborated laboratory findings.     

Epilimnetic sulfate reduction and its relationship to lake acidification

Sulfate reduction occurred from 0–3 cm below the surface of the epilimnetic sediments of three northwestern Ontario lakes, including L.223, which has been experimentally acidified by additions of sulfuric acid. Shallow water sites were conducive to SO₄ ^2– reduction because decomposition in these predominantly sandy sediments caused oxygen concentrations to decrease rapidly within mm below the interface. The occurrence of methanogenesis just below the depth of minimum SO₄ ^2- concentration demonstrated that availability of organic carbon was not a limiting factor for sulphate reduction.Laboratory studies showed that SO₄ ^2- reduction rates in mixed sediments were lower at pH 4 than at pH 6. However, sulfate gradients in sediments indicated that there was no effect of acidification on sulfate reduction in situ. This was probably because microbial H⁺ consumption in the epilimnetic sediments maintained steep pH gradients below the sediment-water interface. The pH increased from = 5.0 to 6.5 or higher by a depth of 3.0 cm into the sediments.     

Nitrous oxide in brackish Lakes Shinji and Nakaumi, Japan

Nitrous oxide (N₂O) was measured monthly from September 1997 to August 1998 in the brackish Lakes Shinji and Nakaumi, Japan. N₂O (5–37 μg N l⁻¹) was supersaturated in the overlying water on lake sediments from October 1997 to January 1998. The N₂O concentration in the hypolimnion was higher than that in the epilimnion on 17 October 1997, when N₂O was first observed in a water column of Lake Nakaumi. Afterward, N₂O was almost uniform throughout the water column and then disappeared on 16 February 1998. On the one hand, large amounts of N₂O were found throughout the year in the interstitial water in Lake Shinji, where a high concentration of nitrate was discharged from the Hii River. On the other hand, in Lake Nakaumi, stratified by halocline, a high concentration of N₂O was observed in the interstitial water only from winter to spring. N₂O concentrations in the interstitial water were about 10 to 1000 times as large as those in the overlying water. These results imply that N₂O was mainly produced at the sediment-water interface and was diffused to the overlying water. It was also suggested that the accumulation of N₂O in the sediment-water system was accelerated by a high concentration of hydrogen sulfide. Received: July 6, 2000 / Accepted: November 30, 2000     

Biogeochemical evolution of a sulfur-iron rich aquatic system in a reflooded wetland environment (Lake Agmon,northern Israel)

Major biogeochemical processes in the newly created, shallow Lake Agmon (Hula Valley, northern Israel) were investigated from 1994 to 1996. Sediment cores, lake water and porewater were analyzed for chemical composition and spatial distribution. Sediment analyses revealed that Lake Agmon has two different sediment types: peat sediments in the northern and central parts, and marls in the southern part. The basic composition of the lake's water was controlled mainly by the mixing of two distinct water types (Jordan River and water drainage), and by evaporation. About 3/4 of the lake water originated from the Jordan Inlet, a quarter through the Z Canal Inlet (peat drainage) and a minor amount from groundwater seepage. Lake Agmon is unique among freshwater wetlands owing to its high SO ₄ ^2– content, which is ca. 1/3 that of sea water. This characteristic is ascribed to the dissolution of secondary gypsum, formed in the peat soils since the drainage of the historic Hula Marsh. Leaching gypsum from the shallow sediments during the first few months after flooding was followed by a later stage of constant diffusion and advection of SO ₄ ^2– from gypsum dissolution in deeper sediments. Gypsum dissolution in lake sediments contributed ca. half of the SO₄ ^2– and Ca²⁺ inputs to the lake. The concomitant increase of Ca²⁺ combined with alkalinity release due to organic matter decomposition in the sediments led to the precipitation of CaCO₃. This precipitation was enhanced by photosynthesis, particularly during summers, and consumed about a tenth of the Ca²⁺ and third of the alkalinity outputs from the lake. Iron-hydroxide was the main agent for microbial oxidation of organic matter, surpassing by far the role of sulfate, nitrate and manganese as electron acceptors. The produced Fe²⁺ was transported upward by diffusion and advection and oxidized to ferric iron at the sediment-water interface. There is evidence, however, that some sulfate was reduced, but most of the produced sulfide reacted with ferrous iron and accumulated in the sediments as FeS minerals. Therefore, despite high sulfate concentrations, the high iron availability restricted release of toxic sulfides into the water and thereby served to maintain reasonable water quality.     

Exchange of CO2, CH4 and N2O between the atmosphere and two northern boreal ponds with catchments dominated by peatlands or forests

Concentrations of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in the water column and their exchange at the water/air interface were studied during the open water period in two freshwater ponds with different catchment characteristics in the northern boreal zone in Finland; either peatlands or coniferous upland forests dominated the catchment of the ponds. Both ponds were supersaturated with dissolved CO₂ and CH₄ with respect to the equilibrium with the atmosphere, but were close to the equilibrium with N₂O. The mean CO₂ efflux from the pond was higher in the peatland-dominated catchment (22 mg m^–2 h^–1) than in the forested catchment (0.7 mg m^–2 h^–1), whereas the mean CH₄ emissions were similar (7.6 and 3.5 mg m^–2 d^–1, respectively). The fluxes of N₂O were generally negligible. The higher CO₂ concentrations and efflux in the pond with the peatland-dominated catchment were attributed to a greater input of allochthonous carbon to that pond from its catchment due to its higher water colour and higher total organic carbon (TOC) concentration. The water pH, which also differed between the ponds, could additionally affect the CO₂ dynamics. Since the catchment characteristics can regulate aquatic carbon cycles, catchment-scale studies are needed to attain a deeper understanding of the aquatic greenhouse gas dynamics.     

Exchange of phosphorus across the sediment-water interface

In this article, principles of phosphorus retention and phosphorus release at the sediment-water interface in lakes are reviewed. New results and hypotheses are discussed in relation to older models of phosphorus exchange between sediments and water. The fractional composition of sedimentary phosphorus is discussed as a tool for interpretation of different retention mechanisms. Special emphasis is given to the impact of biological, particularly microbial, processes on phosphorus exchange across the sediment-water interface and to the significance of biologically induced CaCO₃ precipitation to phosphorus retention in calcareous lakes.     

Temporal trends in N2O flux dynamics in a Danish wetland – effects of plant‐mediated gas transport of N2O and O2 following changes in water level and soil mineral‐N availability

Temporal trends of N₂O fluxes across the soil–atmosphere interface were determined using continuous flux chamber measurements over an entire growing season of a subsurface aerating macrophyte (Phalaris arundinacea) in a nonmanaged Danish wetland. Observed N₂O fluxes were linked to changes in subsurface N₂O and O₂ concentrations, water level (WL), light intensity as well as mineral‐N availability. Weekly concentration profiles showed that seasonal variations in N₂O concentrations were directly linked to the position of the WL and O₂ availability at the capillary fringe above the WL. N₂O flux measurements showed surprisingly high temporal variability with marked changes in fluxes and shifts in flux directions from net source to net sink within hours associated with changing light conditions. Systematic diurnal shifts between net N₂O emission during day time and deposition during night time were observed when max subsurface N₂O concentrations were located below the root zone. Correlation (P < 0.001) between diurnal variations in O₂ concentrations and incoming photosynthetically active radiation highlighted the importance of plant‐driven subsoil aeration of the root zone and the associated controls on coupled nitrification/denitrification. Therefore, P. arundinacea played an important role in facilitating N₂O transport from the root zone to the atmosphere, and exclusion of the aboveground biomass in flux chamber measurements may lead to significant underestimations on net ecosystem N₂O emissions. Complex interactions between seasonal changes in O₂ and mineral‐N availability following near‐surface WL fluctuations in combination with plant‐mediated gas transport by P. arundinacea controlled the subsurface N₂O concentrations and gas transport mechanisms responsible for N₂O fluxes across the soil–atmosphere interface. Results demonstrate the necessity for addressing this high temporal variability and potential plant transport of N₂O in future studies of net N₂O exchange across the soil–atmosphere interface.     

Potential impacts of climate change on nitrogen transformations and greenhouse gas fluxes in forests: a soil transfer study

Relatively little research has been conducted on how climate change may affect the structure and function of arid to semiarid ecosystems of the American Southwest. Along the slopes of the San Francisco Peaks of Arizona, USA, I transferred intact soil cores from a spruce‐fir to a ponderosa pine forest 730 m lower in elevation to assess the potential impacts of climate change on soil N cycling and trace gas fluxes. The low‐elevation site has a mean annual soil temperature about 2.5°C higher than the high‐elevation site. Net rates of N transformations and trace gas fluxes were measured in high‐elevation soil cores incubated in situ and soil cores transferred to the low‐elevation site. Over a 13‐month period, volumetric soil water content was similar in transferred soil cores relative to soil cores incubated in situ. Net N mineralization and nitrification increased over 80% in transferred soil cores compared with in situ soil cores. Soil transfer significantly increased net CO₂ efflux (120%) and net CH₄ consumption (90%) relative to fluxes of these gases from soil cores incubated in situ. Soil net N₂O fluxes were relatively low and were not generally altered by soil transfer. Although the soil microbial biomass as a whole decreased in transferred soil cores compared with in situ soil cores after the incubation period, active bacterial biomass increased. Transferring soil cores from the low‐elevation to the high‐elevation site (i.e. simulated global cooling) commonly, but not consistently, resulted in the opposite effects on soil pools and processes. In general, soil containment (root trenching) did not significantly affect soil measurements. My results suggest that small increases in mean annual temperature can have large impacts on soil N cycling, soil–atmosphere trace gas exchanges, and soil microbial communities even in ecosystems where water availability is a major limiting resource.     

Exchange of Cu and Cd across the sediment-water interface in intertidal mud flats from Ria Formosa (Portugal)

Depth profiles of Fe, Mn, (HS)_t, Cu and Cd concentrations in pore water were determined on a seasonal scale in intertidal sediments of Ria Formosa. Concentrations of Cu and Cd were also determined in near-bottom water during the short period that water inundates the sediment. A maximum near the sediment-water interface was observed in depth profiles of Mn and Fe concentrations followed by a decrease with depth. Otherwise, depth profiles of (HS⁻)_t were irregular but peak concentrations was observed below Mn and Fe maximum. Although subsurface maximum was observed at deeper layers for Cu and Cd, the profiles shape varied among sites and sampling dates. This suggests site specificity and alterations associated with early diagenetic reactions. In order to assess exchanges of Cu and Cd across the sediment water interface, diffusive fluxes and advective transport were estimated. Both contribute substantially to the daily transfer of Cd from intertidal sediments to the water column of Ria Formosa. In the case of Cu, the flux associated with tidal flooding (advective flux) was the major contributor. Presumably, the exchange of trace elements between the sediment-water interface in intertidal areas of macro- and meso-tidal systems are underestimated since do not take into consideration the pulse contribution associated with tidal flooding.     

Temperature dependence of sediment-water exchange in Lake Grevelingen,SW Netherlands

Extended abstract Lake Grevelingen is a brackish water lake in the SW Netherlands. The lake has an area of 108 km², a mean depth of 5.3 m (maximum 48 m), a mean chlorinity of 13 to 16%0 Cl⁻, and a hydraulic residence time of about 8 years. Mass budget studies have shown a consistent seasonal pattern in the phosphorus sediment-water exchange in Lake Grevelingen (Kelderman 1980). From May to August a P mobilization from the sediment takes place, estimated atca. 12.5 mg P · m⁻² · day⁻¹. The sediment accumulatesca. 5.5 mg P · m⁻² · day⁻¹ during the rest of the year. Temperature may be an important factor in establishing this pattern. Sediment-water exchange was studied by means of laboratory experiments under specified conditions. Sediment cores (30 cm depth, 11 cm diameter) were taken at four stations in the lake, with sediment types varying from medium- to muddy sand (Fig. 1). The cores with overlying water (ca. 21) were placed in the dark at 5 °C in thermostatically controlled water baths. After a week's incubation time the temperature was slowly raised, such that after three weeks eight cores (four sediment types, duplicates) were at 5 °C, eight were at 10 °C, eight at 15 °C and eight at 20 °C. The same procedure was applied to the four control cores, containing lake water.     

Extracellular phosphatase activity in sediments of the Breitenbach,a Central European mountain stream

Activity of extracellular phosphatases (phosphomonoesterases) was measured in sandy streambed sediments of the Breitenbach, a small unpolluted upland stream in Central Germany. Fluorigenic 4-methylumbelliferyl phosphate served as a model substrate. Experiments were conducted using sediment cores in a laboratory simulation of diffuse groundwater discharge through the stream bed, a natural process occurring in the Breitenbach as well as many other streams.Streambed sediments contained high levels of particulate phosphorus, but concentrations of dissolved phosphorus in the interstitial water were 3 to 4 orders of magnitude lower. These interstitial concentrations were similar to those in the stream and groundwater. Extracellular phosphatase activity was high in the streambed sediments. These enzymes probably contribute significantly to the flux of phosphorus in sediment by hydrolyzing phosphomonoesters, making free phosphate available to the sediment microorganisms.Factors influencing the kinetic parameters V _max (maximum activity) and apparent K _m (enzyme affinity) of phosphatase were discharge rates of water through the sediment, water quality (ground- or stream water), and substrate (phosphomonoesters) as well as dissolved ortho-phosphate concentrations. Enzymes are supposed to be effective at limiting substrate concentrations, where, in this study, changes in discharge rates had little influence on rates of hydrolysis. Higher V _max and lower K _m values were found during percolation of groundwater through the sediment cores, compared with stream water. This indicates that rates of hydrolysis were higher with groundwater, both at substrate limitation and at substrate saturation. This was probably a consequence of the lower levels of dissolved ortho-phosphate in the groundwater.