Oxic Surface Layer Research Articles

Oxygen dynamics in permeable sediments with wave‐driven pore water exchange

The effects of advective pore water exchange driven by shallow water waves on the oxygen distribution in a permeable (k = 3.3 x 10−12 to 4.9 x 10−11 m2) natural sediment were studied with a planar oxygen optode in a wave tank. Our experiments demonstrate that pore water flow driven by the interaction of sediment topography and oscillating boundary flow changes the spatial and temporal oxygen distribution in the upper sediment layer. Oxygenated water intruding in the ripple troughs and deep anoxic pore water drawn to the surface under the ripple crests create an undulating oxic‐anoxic boundary within the upper sediment layer, mirroring the topographical features of the sediment bed. Anoxic upwelling zones under ripple crests can separate the oxic sediment areas of neighboring ripple troughs with steep horizontal oxygen concentration gradients. The optode showed that migrating wave ripples are trailed by their pore water flow field, alternately exposing sediment volumes to oxic and anoxic pore water, which can be a mechanism for remobilizing particulate oxidized metal precipitates and for promoting coupled nitrification‐denitrification. More rapid ripple migration (experimental threshold ~20 cm h−1) produces a continuous oxic surface layer that inhibits the release of reduced substances from the bed, which under slowly moving ripples is possible through the anoxic vertical upwelling zones. Swift, dramatic changes in oxygen concentration in the upper layers of permeable seabeds because of surface gravity waves require that sediment‐dwelling organisms are tolerant to anoxia or highly mobile and enhance organic matter mineralization.

Temporal variation of nitrification rates in experimental freshwater sediments enriched with ammonia or nitrite

Two freshwater sediments (organic-poor and organic-rich) that contained their distinct natural microbial communities were incubated in experimental microcosms with either NH 4 + or NO 2 − in the overlying water. Microsensor measurements revealed the thin oxic surface layer as a site of initially high rates of nitrification, i.e. O 2, NH 4 +, and NO 2 − consumption, and NO 3 − production. Unexpectedly, during the subsequent 4-week incubation NH 4 + consumption decreased in both sediment types and NO 2 − consumption decreased in the organic-rich sediment. In the organic-rich sediment O 2 consumption and NO 3 − production paralleled these decreases, i.e. the reduced NH 4 + and NO 2 − consumption rates were most probably due to reduced activity of nitrifiers. These microsensor data imply factors other than frequently suggested competition between nitrifiers and heterotrophs for NH 4 +, NO 2 − or O 2 as causes for the loss of nitrification activity. We hypothesize that experimental manipulations (e.g. removal of macrofauna, redistribution of particulate organic matter, permanent nutrient enrichment) rendered the performance of the microbial community unstable. It is thus recommendable to restrict experiments in such commonly used model systems to the period of highest stability.

Early diagenesis and isotopic composition of lead in Lake Laisan, northern Sweden

Water column (dissolved/suspended phase, sediment traps) and sediment data (pore-water, solid-phase sediment) were combined with stable Pb and 210Pb isotope data to trace the early diagenetic behaviour and geochemical cycling of Pb in Lake Laisan, a lake which has received large quantities of anthropogenic Pb since the early 1940s. Early diagenetic remobilisation of Pb is indicated by a subsurface pore-water Pb maximum (120 μg l −1) in the oxic surface layer of the sediment, where the solid-phase Pb concentration is 3400–4600 μg g −1. The remobilisation of Pb appears to be caused by a pH-controlled desorption of Pb from solid-phase sediment, which is consistent with a model describing surface complexation of Pb(II) on hydrous goethite surfaces. The diffusive Pb flux from the subsurface pore-water maximum towards the sediment surface (36 μg cm −2 year −1) exceeds the depositional Pb flux (8.6 μg cm −2 year −1) by approximately a factor of four, indicating that Pb is highly mobile in the sediment. Stable Pb isotope data and a mass balance calculation suggest that Pb diffusing upwards is, to a large extent, trapped in the surface sediment. Lead that may diffuse into the slightly alkaline lake water appears to be efficiently sorbed to suspended particulate matter, resulting in low dissolved Pb concentrations in the water column (0.040–0.046 μg l −1). Sorption of Pb to suspended particulate matter is consistent with the elevated suspended particulate Pb concentrations in the hypolimnion (3800–4000 μg g −1), and the fact that the stable Pb isotopic compositions of suspended matter and pore-water are similar.

Small-scale distribution of interstitial nitrite in freshwater sediment microcosms: the role of nitrate and oxygen availability, and sediment permeability.

The spatial distribution of interstitial NO2(-) concentrations was studied in NO3(-)-exposed freshwater sediment microcosms, using pore water extractions as well as ion-selective microsensors. Porewater extractions revealed ecotoxicologically critical NO2(-) concentrations in hypoxic and anoxic sediment layers in which significant NO3(-) consumption took place. In contrast, the use of ion-selective microsensors demonstrated the high capacity of the thin oxic surface layer of the sediments to consume NO2(-) and to produce NO3(-). Two modes of NO3(-) supply to the sediments were compared: In treatments with NO3(-) supply to the overlying water, a subsurface maximum of NO2(-) concentration was observed, coinciding with the site of maximum NO3(-) consumption. When NO3(-) was perfused up through the sediment cores, however, NO2(-) accumulated throughout the entire sediment column. Such spatially extensive NO2(-) accumulations were only observed in sediments poor in organic matter with a relatively high permeability. By manipulating the O2 content of the overlying water, the release of NO2(-) from the sediments could be influenced: In treatments with air-saturated overlying water, the sediments did not release detectable amounts of NO2(-) into the water phase. When kept hypoxic (25% air saturation) instead, significant NO2(-) accumulations were recorded in the overlying water. These findings suggest that in treatments with air-saturated overlying water, NO2(-) that was produced in deeper sediment layers (denitrifying conditions) was completely consumed at the oxic sediment surface (nitrifying conditions) before it could reach the overlying water.

A geochemical model of the Peru Basin deep-sea floor—and the response of the system to technical impacts

A geochemical model of the Peru Basin deep-sea floor, based on an extensive set of field data as well as on numerical simulations, is presented. The model takes into account the vertical oscillations of the redox zonation that occur in response to both long-term (glacial/interglacial) and short-term (El Nino Southern Oscillation (ENSO) time scale) variations in the depositional flux of organic matter. Field evidence of reaction between the pore water NO3− and an oxidizable fraction of the structural Fe(II) in the clay mineral content of the deep-sea sediments is provided. The conditions of formation and destruction of reactive clay Fe(II) layers in the sea floor are defined, whereby a new paleo-redox proxy is established. Transitional NO3− profile shapes are explained by periodic contractions and expansions of the oxic zone (ocean bottom respiration) on the ENSO time scale. The near-surface oscillations of the oxic–suboxic boundary constitute a redox pump mechanism of major importance with respect to diagenetic trace metal enrichments and manganese nodule formation, which may account for the particularly high nodule growth rates in this ocean basin. These conditions are due to the similar depth ranges of both the O2 penetration in the sea floor and the bioturbated high reactivity surface layer (HRSL), all against the background of ENSO-related large variations in depositional Corg flux. Removal of the HRSL in the course of deep-sea mining would result in a massive expansion of the oxic surface layer and, thus, the shut down of the near-surface redox pump for centuries, which is demonstrated by numerical modeling.

The different diffusive transport behaviours of some metals in layers of Peru Basin surface sediment

Abstract Diffusion experiments with a duration of 90 days were carried out with radio tracers in an oxic and a suboxic sediment layer from two deep-sea sediment cores. Diffusion coefficients, breakthrough and time-lag values were determined and the results checked by modelling. Only small differences of diffusive transport were observed for Na and Cs during the four experiments; Na behaved as a perfect conservative tracer, and the somewhat stronger uptake of Cs in one sample was caused by preferential absorption by illite that was enriched in this sample. The significant differences between the four experiments for the diffusion parameters of Mn, Zn, Co, and Cd correlate with the differing chemical compositions of the sediment layers; higher Mn oxide concentrations in the sample led to slower diffusion through the sediment sample and to significant sorption of the diffusing heavy metals. This result confirms the dominating role of Mn oxide phases in controlling the diffusive exchange of heavy metals between deep-sea sediment and bottom water. Sediment resuspension of the oxic surface layer, such as during potential ferromanganese nodule mining in the deep-sea, would be followed by increased diffusive flux of dissolved heavy metals, especially Mn, from suboxic sediment layers to the new disturbed sediment–bottom-water interface. According to our modelling, the Mn flux from the oxic sediment surface of 0.3 mg m−2 yr−1 in the undisturbed state would increase to up to several mg m−2 yr−1 within the first few weeks after disturbance. Depending on the time necessary until a stable oxic and Mn oxide rich layer is again re-established, a total Mn2+ release of 54 mg m−2 was calculated if the original state is reached within 100 yr, in contrast to only 3.5 mg m−2 for 5 yr of re-establishment. The fluxes of the other heavy metals would be smaller, corresponding to their lower pore water concentrations in the suboxic layer compared to Mn.

Nitrification and denitrification in the rhizosphere of rice: the detection of processes by a new multi-channel electrode

N turnover in flooded rice soils is characterized by a tight coupling between nitrification and denitrification. Nitrification is restricted to the millimetre-thin oxic surface layer while denitrification occurs in the adjacent anoxic soil. However, in planted rice soil O2 released from the rice roots may also support nitrification within the otherwise anoxic bulk soil. To locate root-associated nitrification and denitrification we constructed a new multi-channel microelectrode that measures NH4+, NO2–, and NO3– at the same point. Unfertilized, unplanted rice microcosms developed an oxic-anoxic interface with nitrification taking place above and denitrification below ca. 1 mm depth. In unfertilized microcosms with rice plants, NH4+, NO2– and NO3– could not be detected in the rhizosphere. Assimilation by the rice roots reduced the available N to a level where nitrification and denitrification virtually could not occur. However, a few hours after injecting (NH4)2HPO4 or urea, a high nitrification activity could be detected in the surface layer of planted microcosms and in a depth of 20–30 mm in the rooted soil. O2 concentrations of up to 150 μM were measured at the same depth, indicating O2 release from the rice roots. Nitrification occurred at a distance of 0–2 mm from the surface around individual roots, and denitrification occurred at a distance of 1.5–5.0 mm. Addition of urea to the floodwater of planted rice microcosms stimulated nitrification. Transpiration of the rice plants caused percolation of water resulting in a mass flow of NH4+ towards the roots, thus supporting nitrification.

Organic matter diagenesis at the oxic/anoxic interface in coastal marine sediments, with emphasis on the role of burrowing animals

The present paper reviews the current knowledge on diagenetic carbon transformations at the oxic/anoxic interface in coastal marine sediments. Oxygen microelectrodes have revealed that most coastal sediments are covered only by a thin oxic surface layer. The penetration depth of oxygen into sediments is controlled by the balance between downward transport and consumption processes. Consumption of oxygen is directly or indirectly caused by respiration of benthic organisms. Aerobic organisms have the enzymatic capacity for complete oxidation of organic carbon. Anaerobic decay occurs stepwise, involving several types of bacteria. Large organic molecules are first fermented into small moieties. These are then oxidized completely by anaerobic respirers using a sequence of electron acceptors: Mn4+, NO3-, Fe3+, SO42- and CO2. The quantitative role of each electron acceptor depends on the sediment type and water depth. Since most of the sediment oxygen uptake is due to reoxidation of reduced metabolites, aerobic respiration is of limited importance. It has been suggested that sediments contain three major organic fractions: (1) fresh material that is oxidized regardless of oxygen conditions; (2) oxygen sensitive material that is only degraded in the presence of oxygen; and (3) totally refractory organic matter. Processes occurring at the oxic/anoxic boundaries are controlled by a number of factors. The most important are: (1) temperature, (2) organic supply, (3) light, (4) water currents, and (5) bioturbation. The role of bioturbation is important because the infauna creates a three-dimensional mosaic of oxic/anoxic interfaces in sediments. The volume of oxic burrow walls may be several times the volume of oxic surface sediment. The infauna increases the capacity, but not the overall organic matter decay in sediments, thus decreasing the pool of reactive organic matter. The increase in decay capacity is partly caused by injection of oxygen into the sediment, and thereby enhancing the decay of old, oxygen sensitive organic matter several fold. Finally, some future research directions to improve our understanding of diagenetic processes at the oxic/anoxic interface are suggested.

Manganese and Iron Oxidation During Benthic Oxygenic Photosynthesis

The effect of benthic oxygenic photosynthesis on sediment-water fluxes of manganese and iron was studied for an intertidal sediment. Undisturbed sediments were incubated at an incident surface irradiance of 250μEm−2s−1at 26°C. Oxygenic photosynthesis was selectively inhibited by adding [3-(3,4-dichloro)-1,1-dimethyl-urea] (DCMU). Benthic fluxes were determined experimentally from the change in manganese and iron concentrations in the overlying water, and were predicted from the pore water concentration gradients at the sediment-water interface assuming molecular diffusion as the transport mechanism. The experimental fluxes of manganese and iron in DCMU-treated cores amounted to −0·84 and −0·59mmolm−2day−1, respectively, and were directed from the sediment towards the overlying water. In the control cores, showing high rates of benthic oxygenic photosynthesis, the fluxes of manganese and iron were directed towards the sediment, 0·06 and 0·01mmolm−2day−1, respectively. Mass balances for the 0·1–0·14cm thick oxic zone, calculated from the experimental fluxes and the predicted fluxes, suggest a minimum areal reoxidation of 0·6mmolm−2day−1for manganese and of 0·48mmolm−2day−1for iron in cores showing benthic photosynthesis. The estimated turnover times for dissolved Mn2+and dissolved Fe2+in the oxic surface layer during benthic photosynthesis were 0·8 and 0·25h, respectively. Sediment oxygen microprofiles and the sediment pH profiles suggest that chemical precipitation and reoxidation dominates the retention of manganese and iron during benthic oxygenic photosynthesis in shallow intertidal sediments.

Sulfate-Reducing Bacteria and Their Activities in Cyanobacterial Mats of Solar Lake (Sinai, Egypt)

The sulfate-reducing bacteria within the surface layer of the hypersaline cyanobacterial mat of Solar Lake (Sinai, Egypt) were investigated with combined microbiological, molecular, and biogeochemical approaches. The diurnally oxic surface layer contained between 10(6) and 10(7) cultivable sulfate-reducing bacteria ml-1 and showed sulfate reduction rates between 1,000 and 2, 200 nmol ml-1 day-1, both in the same range as and sometimes higher than those in anaerobic deeper mat layers. In the oxic surface layer and in the mat layers below, filamentous sulfate-reducing Desulfonema bacteria were found in variable densities of 10(4) to 10(6) cells ml-1. A Desulfonema-related, diurnally migrating bacterium was detected with PCR and denaturing gradient gel electrophoresis within and below the oxic surface layer. Facultative aerobic respiration, filamentous morphology, motility, diurnal migration, and aggregate formation were the most conspicuous adaptations of Solar Lake sulfate-reducing bacteria to the mat matrix and to diurnal oxygen stress. A comparison of sulfate reduction rates within the mat and previously published photosynthesis rates showed that CO2 from sulfate reduction in the upper 5 mm accounted for 7 to 8% of the total photosynthetic CO2 demand of the mat.

High Resolution Profiles and Fluxes of Redox Species in Continental Margin Sediments: Implications for the Cycles of Nitrogen, Iodide, Manganese, and Iron

The preferential use of the electron acceptor that yields the highest amount of free energy in the bacterially mediated oxidation of organic matter constitutes a long-standing paradigm in biogeochemistry. In marine sediments, this leads to a well-established vertical sequence of redox reactions in which oxygen is reduced near the sediment-water interface, followed in turn by the reduction of nitrate and manganese oxide, iron oxide, sulphate, and finally carbon dioxide (e.g. Froelich et al., 1979). The early expressions of the paradigm were limited to these major redox elements, but thermodynamic calculations and experimental observations have shown that other redox elements (for example I, Se, Cr) can also be involved in organic carbon oxidation, either directly or by responding to changes in redox potential. Progress in understanding the zonation of redox reactions in sediments is closely tied to the ability to make high resolution measurements. Here we report new measurements of redox species in undisturbed sediment cores from 100m, 200 m and 325 m depth along a cross-channel section in the Laurentian Trough in the Gulf of St. Lawrence. The vertical distributions of 02, Mn(II), Fe(II), I(-I), and HS(-I) were measured with millimeter resolution in the top several cm of the cores using a voltammetric Hg/Au amalgam microelectrode (Brendel and Luther, 1995). The distributions of NO3, NH2 and HPO]were measured colorimetrically, using porewater samples obtained by conventional slicing and centrifugation of replicate cores. Solid-phase Mn was extracted with HC1 and measured by AA-spectrometry. Benthic fluxes of 02, NO3 and NH~ were measured by incubating cores at the in s i tu temperature. The vertical distributions we measured were qualitatively similar at the three sites. In all cores, 02 disappeared at about 4 mm depth, followed by the successive appearance of I(I-), Mn(II) and Fe(II). The appearance of Fe(II) was very abrupt and was accompanied by a broad peak in the voltammogram consistent with the presence of colloidal Fe(III). As in our previous studies with this microelectrode, there was a depth interval in which neither 02 nor Mn(II) could be detected. The concentration of iodide increased sharply below the sediment surface and displayed a sharp subsurface maximum of about 5 gM at the very depth where Mn(II) was first detected. NO3 decreased rapidly across the sediment-water interface to levels below detection limits at 10 mm depth. Below this depth NO3 increased again and displayed a broad concentration maximum with concentrations as high as 15-40 gM. These data are consistent with the successive use of electron acceptors during organic carbon degradation according to free energy yield. However, a close examination of the data reveals new details about diagenetic processes. Thus, a nitrate minimum located near the base of the oxic sediment layer supports a flux of nitrate from the water column into the sediment which agrees with the measured nitrate flux. This is consistent with classical anaerobic denitrification. However, a sharp ammonium gradient across the oxic surface layer is not reflected in a corresponding release of ammonia from the sediment, indicating there is an important ammonium sink within the oxic layer. A possible sink could be the coupled nitrification-denitrification within this layer; this would require the presence of

Redox cycling of iron and manganese in sediments of the Kalix River estuary, Northern Sweden

Iron and manganese redox cycling in the sediment — water interface region in the Kalix River estuary was investigated by using sediment trap data, pore-water and solid-phase sediment data. Nondetrital phases (presumably reactive Fe and Mn oxides) form substantial fractions of the total settling flux of Fe and Mn (51% of Fetotal and 84% of Mntotal). A steady-state box model reveals that nondetrital Fe and Mn differ considerably in reactivity during post-depositional redox cycling in the sediment. The production rate of dissolved Mn (1.6 mmol m−2 d−1) exceeded the depositional flux of nondetrital Mn (0.27 mmol m−2 d−1) by a factor of about 6. In contrast, the production rate of upwardly diffusing pore-water Fe (0.77 mmol m−2 d−1) amounted to only 22% of the depositional flux of nondetrital Fe (3.5 mmol m−2 d−1). Upwardly diffusing pore-water Fe and Mn are effectively oxidized and trapped in the oxic surface layer of the sediment, resulting in negligible benthic effluxes of Fe and Mn. Consequently, the concentrations of nondetrital Fe and Mn in permanently deposited, anoxic sediment are similar to those in the settling material. Reactive Fe oxides appear to form a substantial fraction of this buried, non-detrital Fe. The in-situ oxidation rates of Fe and Mn are tentatively estimated to be 0.51 and 0.16–1.7 μmol cm−3 d−1, respectively.

Biogeochemical cycling of nutrients and trace metals in anoxic freshwater sediments of the Neckar River, Germany

Pore-water components (SO42-, Fe2+, Mn2+), including nutrients (NH4+:, NO3-, PO43-), alkalinity and pH were determined at three sites in the Neckar River. Sequential extraction procedures and trace metals in both pore waters and sediments are reported in order to evaluate the mobility of trace metals in contaminated sediments. The results show that the mineralization of organic matter plays an important role in the cycling of nutrients and trace metals. Pore-water profiles (Zn, Cu, Pb, Cd) suggest that the element maximums at the sediment-water interface are caused by the decomposition of biomass. Low concentrations of dissolved Zn, Cu, Pb and Cd in the anoxic sediments can be explained by a sharp decrease of SO42- in pore water concomitant with HS- production. This leads to the formation of highly insoluble metal sulfides. Solubility calculations show that the sediments act as a sink for trace metals with respect to trace metal sulfides. The organic/sulfidic-bound fraction accounts for 64-81% of Cd in the sediment cores, 36-67% of Pb and 51-69% of Cu. In contrast, Cr in pore water increases with depth because of its release from Fe/Mn oxides. NH4+ and PO43- are also released into the pore water owing to the mineralization of organic matter. No significant fluxes of NH4+ and PO43- into overlying water were found because of the existence of an oxic surface layer.

238U decay series nuclides in the northeastern Arabian Sea: Scavenging rates and cycling processes

The extent of radioactive disequilibrium between 234Th- 238U, 210Po- 210Pb and 210Pb- 226Ra has been measured in four vertical profiles from the northeastern Arabian Sea, a region characterized by oxygen deficient and denitrification layers at intermediate depths. The most common feature of the data is that the dissolved concentrations of the daughter nuclides are considerably less than those of their parents, exceptions being the near equilibrium or excess concentrations of 210Po (relative to 210Pb) at ∼100m and that of 210Pb relative to 226Ra in surface waters. These results suggest that scavenging of dissolved 234Th, 210Po and 210Pb occurs throughout the water column in the northeastern Arabian Sea. In the mixed layer, the scavenging residence time of Th is ∼1–2months and that of Po and Pb is ∼4–8months. At depths greater than ∼300m, the mean dissolved [ 210Po/ 210Pb] and [ 210Pb/ 226Ra] are ∼0.75 and ∼0.4, respectively,representing some of the most pronounced disequilibria observed in the deep sea. The particle reactivity of the measured nuclides increases as Th > Po > Pb. The 238U concentration and the extent of 234Th- 238U disequilibrium are similar in the oxic surface layers and su☐ic intermediate waters, indicating that U and Th distributions are not significantly affected by oxygen depletion and denitrification processes. On the contrary, both 210Po- 210Pb and 210Pb- 226Ra disequilibria show enhanced scavenging of 210Po and 210Pb at stations that are characterized by intense denitrification. Modelling of dissolved 210Po profile in the surface ∼100m shows its potential to yield eddy diffusion coefficients in surface waters and to place constraints on the new production rates. Results obtained in this study, although encouraging, stress the need for further work to confirm some of the assumptions of the model.

Thiosulfate and sulfite distributions in porewater of marine sediments related to manganese, iron, and sulfur geochemistry

Depth distributions of thiosulfate (S 2O 3 2−) and sulfite (SO 3 2−) were measured in the porewaters of a Danish salt marsh and subtidal marine sediments by HPLC analysis after derivatization with DTNP [2,2'-dithiobis(5-nitropyridine)]. The distributions were compared to the redox zonation as indicated by Eh and Mn 2+, Fe 2+ and H 2S distributions. Concentrations of S 2O 3 2− varied from below detection (<50 nM) to 600 nM while SO 3 2− concentrations generally were 2–3 times higher, 100–1500 nM. Depth distributions of the two species were roughly similar. Lowest concentrations were found in the oxidized zone, including both the oxic surface layer and the suboxic zone of intense manganese and iron reduction, and concentrations tended to increase through the suboxic and into the reduced, sulfidic zone. The similarity of SO 3 2− and S 2O 3 2− profiles suggested a close coupling of the cycling of the two species. Rates of consumption were suggested as the main factor governing their distribution. Rapid turnover times for S 2O 3 2− and H 2S of 4 and 1.1 h, respectively, were estimated for the upper 0–1 cm of a subtidal sediment.

Distribution of sulfate-reducing bacteria, O2, and H2S in photosynthetic biofilms determined by oligonucleotide probes and microelectrodes.

The vertical distribution of sulfate-reducing bacteria (SRB) in photosynthetic biofilms from the trickling filter of a sewage treatment plant was investigated with oligonucleotide probes binding to 16S rRNA. To demonstrate the effect of daylight and photosynthesis and thereby of increased oxygen penetration, we incubated two 4-mm-thick biofilm samples in darkness or exposed to light at natural intensity. Gradients of O2, H2S, and pH were examined with microelectrodes during incubation. The samples were subsequently frozen with liquid nitrogen and sliced on a cryomicrotome in 20-microns vertical slices. Fluorescent-dye-conjugated oligonucleotides were used as "phylogenetic" probes to identify single cells in the slices. Oligonucleotide sequences were selected which were complementary to short sequence elements (16 to 20 nucleotides) within the 16S rRNA of sulfate-reducing bacteria. The probes were labeled with fluorescein or rhodamine derivatives for subsequent visualization by epifluorescence microscopy. Five probes were synthesized for eukaryotes, eubacteria, SRB (including most species of the delta group of purple bacteria), Desulfobacter spp., and a nonhybridizing control. The SRB were unevenly distributed in the biofilm, being present in all states from single scattered cells to dense clusters of several thousand cells. To quantify the vertical distribution of SRB, we counted cells along vertical transects through the biofilm. This was done in a blind experiment to ascertain the reliability of the staining. A negative correlation between the vertical distribution of positively stained SRB cells and the measured O2 profiles was found. The distribution differed in light- and dark-incubated samples presumably because of the different extensions of the oxic surface layer. In both cases the SRB were largely restricted to anoxic layers.

Stromatolitoid microbial nodules from Bermuda ?a special micro habitat for meiofauna

In Bermuda, stromatolitoid microbial nodules are found in dense groups in seagrass beds and on subtidal sandy or rocky bottoms. They develop between January and the beginning of August and may reach 15 cm in diameter and 6 cm in height. Nodules grow on top of the sediment surface and consist of convex interwoven mats of the cyanobacterium Phormidium corium and fine sediment particles trapped in these mats. Nodules were studied in the field and laboratory (in 1989–1992) with respect to their structure, microbial community, sediment chemistry and associated meiofauna. Vertical profiles taken with microelectrodes showed steep gradients of oxygen and sulfide. While free oxygen was only detected in the upper 2 mm, the sulfide concentrations increased with depth and reached maximal 1250 μmoll-1 inside the nodules. On the basis of microscopic observations and sediment chemistry, vertical sections through a nodule reveal four distinct layers: (A) an oxic green surface layer of growing cyanobacteria mats, (B) an anoxic yellow-white laminated layer of sediment particles, mucus and unpigmented cyanobacteria sheaths, (C) an anoxic sulfidic (sulfide <50 μmoll-1) interface where colour changes from yellow to gray and (D) an anoxic sulfidic (sulfide can increase to 1250 μmoll-1) gray core composed of sediment and decomposing cyanobacteria. The following meiofauna taxa were found in the nodules: Ciliata, Turbellaria, Gnathostomulida, Gastrotricha, Nematoda, Kinorhyncha, Polychaeta, Ostracoda and Harpacticoida. The distribution of the meiofauna was significantly different (p<0.05) within the four layers. The highest density of individuals was found in the sulfidic interface and core. Nematodes represented the dominant group in general. Thiobiotic organisms, such as the Gnathostomulida, Solenofilomorphidae, and Stilbonematinae were primarily found in the anoxic sulfidic layers of the nodules.

The impact of desiccation of a freshwater marsh (Garcines Nord, Camargue, France) on sediment-water-vegetation interactions

The impact of desiccation on a marsh sediment was studied both in the laboratory and in the field. Changes in the sediment chemistry of a homogenized sediment suspension during desiccation were studied in the laboratory. FeS was oxidized completely. A considerable mineralization of organic phosphate took place, from both the acid soluble organic phosphate fraction and from the residual organic phosphate fraction, but no significant mineralization of organic matter was observed. The o-P formed during the mineralization was recovered partly in the Fe(OOH) ≈P fraction and partly in the CaC3≈P fraction. An upward flux was found. During spring and summer 1990 the water inlet to a shallow permanent freshwater marsh with a surface of about 1.5 hectares was blocked, in order to desiccate the marsh by evaporation. The sediments initially consisted of a black anoxic organic top layer and a less organic anoxic gray layer. During the desiccation of the sediment a brown oxic surface layer was formed from the black layer and an increase of pH and Eh occurred. Subsequent rainfall made the Eh increase further but caused a decrease in pH indicating an increase in bacterial activity. A progressive oxidation of FeS was observed. An increase in Tot-P in the surface layer and a decrease in the gray and the black layer of the sediment occurred, probably due to a capillary upward flux. A mineralization of organic matter was observed in the two deeper layers. In the upper brown layer this mineralization was less evident, probably because it has been masked by the capillary movement. A net C loss of 40% was calculated to have occurred in the layer 0–40 cm. In the deeper layers a decrease in Tot-N was observed, whereas no important increase occurred in the top layer. Over a sediment layer of 40 cm a N loss of 50% was calculated. C- and N losses occurred simultaneously, suggesting the importance of mineralization as a source of inorg-N for denitrification. The chemical and physical changes in the sediment during desiccation affected layers down to 40 cm. This means that not only the top layer of a sediment but also deeper layers are active in systems of which part of the sediment dries occasionally. Fractionation of the surface sediment phosphate showed an increase of Fe(OOH) ≈ P in the top layer due to the oxidation of FeS to Fe(OOH), enlarging the P-adsorption capacity of the sediment. A mineralization of about 50% of acid soluble organic phosphate occurred. After rainfall, a net increase in residual organic phosphate occured presumably due to an increase of bacterial activity. Drying may therefore be utilized as a tool, in wetland management, to eliminate organic nitrogen and carbon from the sediment. In rice culture, it may be used to make part of the organic nitrogen available to the rice.

Microelectrode studies of seasonal oxygen uptake in a coastal sediment: role of molecular diffusion

Oxygen uptake rates and microdistributions of 0, in sediment of Aarhus Bay, Denmark. were studied over a year. Oxygen uptake was determined in the laboratory by 3 methods: (a) total O2 uptake in closed cores, (b) O2 flux through the diffusive boundary layer and (c) O2 consumption within the oxic surface zone. The O2 consumption rates in the oxic surface layer, calculated from (b) and (C) using molecular diffusion coefficients of 02, closely agreed and accounted for 70 % of the total O2 uptake. The O2 uptake rates strongly depended on in situ O2 concentrations in the overlying seawater. which in the bay varied from 100 % air saturation during winter to 23 'X, during summer. The potentially higher O2 respiration during the summer months was counteracted by a lower O2 concentration in the bottom water, which led to a small seasonal variation in 0, uptake. Depth of the oxic sediment zone reached 5.1 mm during winter, narrowed down rapidly to a few mm upon settling of a spring phytoplankton bloom, and was only 1.2 mm during summer. Modeling of O2 consumption from O2 microprofiles showed zero-order kinetics, i.e. constant 0, consumption rates throughout the oxic zone during winter. Enhanced O2 consumption was found at the oxic-anoxic interface during summer, presumably due to reducing solutes which diffused up from anoxic layers Sediment 0, uptake was impeded by 3 to 5 % during winter and by 12 to 16 L during summer due to transport resistance through the 300 pm thick diffusive boundary layer. Results demonstrate the importance of the boundary layer for sediment O2 uptake and its regulation.

Oxygen-consuming processes at the profundal and littoral sediment-water interface of a small meso-eutrophic lake (Lake Vechten, The Netherlands)

Oxygen consumption in the profundal gyttja and littoral sands of Lake Vechten was determined with pore-water analyses and core incubations. Methane (58–64%), ammonium (10–13%), iron (2–6%), and sulfide (<12%) oxidation accounted for ∼75% of the oxygen consumed in the profundal sediments. Almost the complete diffusive flux of CH4 out of the anoxic layer into the oxic surface layer (7.9–9.4 mmol m‒2 d‒1) was oxidized, while only 27–36% of the NH4+ flux (5.1–5.2 mmol m‒2d‒1) was oxidized in the oxic surface layer and 64–73% diffused into the overlying water. In contrast to the profundal sediments, oxidation of reduced end products of anoxic respiration was of minor importance (<15%) in littoral sandy sediments. Also, sediment oxygen consumption rate was higher in the profundal gyttja (29 mmol m‒2 d‒1 at 7°C) during overturn than in the littoral sandy sediments in both winter (8.7 mmol m‒2 d‒1 at 7°C) and summer (18.6 mmol m‒2 d‒1 at 22°C). Poisoning the sediment cores with formaldehyde stopped bacterial activity, but oxygen profile measurements showed that this treatment does not reveal the bacterial or chemical nature of the oxidative processes.