Sediment Redox Conditions Research Articles

Mineralogical and geochemical controls of arsenic speciation and mobility under different redox conditions in soil, sediment and water at the Mokrsko-West gold deposit, Czech Republic

Naturally contaminated soil, sediment and water at the Mokrsko-West gold deposit, Central Bohemia, have been studied in order to determine the processes that lead to release of As into water and to control its speciation under various redox conditions. In soils, As is bonded mainly to secondary arseniosiderite, pharmacosiderite and Fe oxyhydroxides and, rarely, to scorodite; in sediments, As is bonded mainly to Fe oxyhydroxides and rarely to arsenate minerals. The highest concentrations of dissolved As were found in groundwater (up to 1141 μg L − 1 ), which mostly represented a redox transition zone where neither sulphide minerals nor Fe oxyhydroxide are stable. The main processes releasing dissolved As in this zone are attributed to the reductive dissolution of Fe oxyhydroxides and arsenate minerals, resulting in a substantial decrease in their amounts below the groundwater level. Some shallow subsurface environments with high organic matter contents were characterized by reducing conditions that indicated a relatively high amount of S − 2,0 in the solid phase and a lower dissolved As concentration (70–80 μg L − 1 ) in the pore water. These findings are attributed to the formation of Fe(II) sulphides with the sorbed As. Under oxidizing conditions, surface waters were undersaturated with respect to arsenate minerals and this promoted the dissolution of secondary arsenates and increased the As concentrations in the water to characteristic values from 300 to 450 μg L − 1 in the stream and fishpond waters. The levels of dissolved As(III) often predominate over As(V) levels, both in groundwaters and in surface waters. The As(III)/As(V) ratio is closely related to the DOC concentration and this could support the assumption of a key role of microbial processes in transformations of aqueous As species as well as in the mobility of As.

Functional diversity of macrophyte communities within and between Pyrenean lakes

Submersed vegetation is a common feature in about 70% Pyrenean high mountain (>1500 m a.s.l.) lakes. Isoetids and soft-water elodeids are common elements of this underwater flora and can form distinct vegetation units (i.e. patches of vegetation dominated by different species) within complex mosaics of vegetation in shallow waters (<7 m). Since isoetids exert a strong influence on sediment biogeochemistry due to high radial oxygen loss, we examined the small scale characteristics of the lake environment (water and sediment) associated to vegetation patches in order to ascertain potential functional differences among them. To do so, we characterised the species composition and biomass of the main vegetation units from 11 lakes, defined plant communities based on biomass data, and then related each community with sediment properties (redox and dissolved nutrient concentration in the pore water) and water nutrient concentration within plant canopy. We also characterised lake water and sediment in areas without vegetation as a reference. A total of twenty-one vegetation units were identified, ranging from one to five per lake. A cluster analysis on biomass species composition suggested seven different macrophyte communities that were named after the most dominant species: Nitella sp., Potamogeton praelongus, Myriophyllum alterniflorum, Sparganium angustifolium, Isoetes echinospora, Isoetes lacustris and Carex rostrata. Coupling between macrophyte communities and their immediate environment (overlying water and sediment) was manifested mainly as variation in sediment redox conditions and the dominant form of inorganic nitrogen in pore-water. These effects depended on the specific composition of the community, and on the allocation between above- and belowground biomass, and could be predicted with a model relating the average and standard deviation of sediment redox potential from 0 down to -20 cm, across macrophyte communities. Differences in pore-water total dissolved phosphorus were related to the trophic state of the lakes. There was no correlation between sediment and water column dissolved nutrients. However, nitrate concentrations tended to be lower in the water overlaying isoetid communities, in apparent contradiction to the patterns of dissolved nitrates in the pore-water. These tendencies were robust even when comparing the water overlaying communities within the same lake, thus pointing towards a potential effect of isoetids in reducing dissolved nitrogen in the lakes.

Tectonic Background of Ordos Basin and its Controlling Role for Basin Evolution and Energy Mineral Deposits

In order to study the relationship of energy mineral deposits and tectonic movements in Ordos basin, more than 82 samples were taken for determination of apatite and zircon fission tracks. Furthermore, 93 apatite and zircon fission tracks have been selected from other publications. According to these data and other geological background, the evolution of Ordos basin can be divided into 4 stages, and the reformation of the basin can be divided into 5 stages. The basin formation and development were controlled by regional geodynamics. Oil, coal, nature gas and uranium deposits were formed in deferent stages. In the same time, 140 samples were taken and analyzed by organic and inorganic geochemical methods. The results indicate that coal and source rocks generate oil and natural gas, which migrate through reservoir rocks. Oil and natural gas changed the redox conditions of sediments and let uranium accumulate in the sandstones.

Cadmium concentration in sea bottom sediment and its potential risk in the upper Gulf of Thailand

Untreated or partially treated waste water discharge from industrial and domestic sources entering the Upper Gulf of Thailand have been reported to increase cadmium concentration in bottom sediment. This study was directed at providing a better understanding of cadmium transformation in the sediment from the area. Sediment samples collected from Chao Phraya River mouth (CPY), Bang Pakong River mouth (BPK) and Klong Dan estuary (KD) located in the Upper Gulf of Thailand were analyzed for cadmium concentration in various sediment particle size fractions. Using laboratory microcosms, cadmium release from sediment as affected by salinity and sediment redox condition was studied. A higher concentration of cadmium (0.2–0.6 μ g/g dry weight) was measured in finer sediment particle size fractions (< 0.075 mm) as compared to courser fractions at all sampling stations. Cadmium release from the sediment to water was influenced by both salinity and redox condition. Sediment was spiked with 10 ppm cadmium which is the cadmium level in sediment adopted by the Australia and New Zealand sediment quality guidelines which can cause adverse environmental impacts. Experiments conducted showed soluble cadmium concentration at sediment oxidation reduction conditions representative of bottom sediment were at levels that can adversely impact aquatic organisms, according to the PCADMIUM water quality guideline. In Thailand, there is no sediment quality guideline. Based on these experiments, 10 ppm of cadmium in sediment was recommended as a regulatory guidelines for allowable levels of cadmium in sediment in the study area.

The accumulation of silver in marine sediments: A link to biogenic Ba and marine productivity

The concentrations of Ag and a suite of redox‐sensitive trace metals (Re, Cd, and Mo) were measured in surface sediments from the Western Canadian, Mexican, Peruvian, and Chilean continental margins. In all regions, Ag content increases from ∼80 ng g−1 (i.e., lithogenic values) on the shelf up to as high as 1483 ng g−1 on the lower slope. However, the trend of increasing Ag with increasing water depth breaks down at deepwater sites (&gt;2500 m) where only lithogenic concentrations are documented. Silver content does not correlate with the distributions of redox‐sensitive trace metals, suggesting that sedimentary redox conditions are not the primary control on Ag accumulation. Instead, a positive correlation between Ag and Ba in surface and near‐surface sediments suggests that Ag is scavenged by and delivered to the sediment with the organic particle flux. Scavenging probably results from the precipitation of Ag2S within the organic particles due to the development of anoxia and sulfate reduction. If this hypothesis is correct, then Ag has the potential to be a paleoproductivity proxy.

Determining the post-glacial evolution of a northeast Pacific coastal fjord using a multiproxy geochemical approachThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.

A 40.32 m piston core recovered from Effingham Inlet, on the west coast of Vancouver Island, provides the basis for a high-resolution geochemical study of the last deglaciation and the Holocene. Glacial retreat, basin isolation, sea-level rise, and productivity variations are determined using proxies for sediment composition (K/Al, Fe/Al, Mg/Al), grain size (Ti/Al, Zr/Al), sedimentary redox conditions (Mo/Al, U/Al), and productivity (wt.% organic carbon, wt.% opal). As local ice retreated and marine waters inundated the basin, coarse-grained glacimarine sediments were replaced by finer grained, laminated, opal-rich sediments. During meltwater pulse-1a, the dominance of local crustal rise over eustatic sea-level rise resulted in the progressive restriction of ocean circulation in Effingham Inlet and the formation of a temporary freshwater lake. The transition into stable Holocene conditions was initiated at ∼12 700 BP, which corresponds to the onset of the Younger Dryas, as identified by the Greenland Ice core Project (GRIP) ice core δ18O record and was completed by 10 700 BP, ∼800 years after the GRIP ice core record stabilized. Holocene Mo/Al and U/Al ratios range between 12–35 (×104) and 1–3.4 (×104), respectively, indicating that although large-amplitude, high-frequency fluctuations occur, the sediments of Effingham Inlet inner basin have remained organic rich and oxygen depleted for the entire Holocene period. The combination of anoxic bottom waters and a Holocene sedimentation rate of 217 cm/ka have preserved a high-resolution record of environmental change in the northeast Pacific over the last 11 000 years.

Phosphorus dynamics in shallow eutrophic lakes: an example from Zeekoevlei, South Africa

Zeekoevlei is the largest freshwater lake in South Africa and has been suffering from hyper-eutrophic conditions since last few decades. We have used total P (TP), dissolved phosphate (PO4 3−), organic P (OP), calcium (Ca) and iron (Fe) bound P fractions to investigate the relevant physical, chemical and biological processes responsible for sedimentation and retention of P and to study phosphorus (P) dynamics in this shallow lake. In addition, redox proxies (V/Cr and Th/U ratios) are used to study the prevailing redox conditions in sediments. Adsorption by CaCO3 and planktonic assimilation of P are found to control P sedimentation in Zeekoevlei. Low concentration of the labile OP fraction in surface sediments restricts the release of P by bacterial remineralisation. Low molar Ca/P and Fe/P ratios indicate low P retention capacity of sediments, and P is most likely released by desorption from wind-induced resuspended sediments and mixing of pore water with the overlying water column.

Spatial changes in a macrozoobenthic community along environmental gradients in a shallow brackish lagoon facing Sendai Bay, Japan

The spatial distribution, abundance, and assemblage structure of macrozoobenthos were examined at 45 stations in a brackish lagoon (Idoura Lagoon, Japan) to examine the animal–environmental relations in estuarine soft-bottom habitats. We found a total of 23 taxa; the polychaetes Heteromastus sp., Hediste spp., and Prionospio japonica and the isopod Cyathura muromiensis numerically dominated the community. Cluster analysis and one-way analysis of similarity (ANOSIM) identified seven groups of stations that had significantly different macrozoobenthic communities; these were subsequently consolidated into five habitat groups according to their association with environmental characteristics. Canonical correspondence analysis (CCA) showed that salinity, silt-clay content, and the oxidation–reduction potential (ORP) of the sediment strongly affected the macrozoobenthos distribution pattern in the lagoon, whereas other factors (e.g., relative elevation of the habitat and sediment organic content) had much weaker effects. Similarity percentages (SIMPER) procedures indicated that the polychaete Notomastus sp. and the bivalve Macoma contabulata were specific to habitats with low salinity and reduced mud, whereas the bivalve Nuttallia olivacea was specific to sandy bottoms. Heteromastus sp. and Hediste spp. achieved their highest densities in rather oxidized sediments. The acid-volatile sulfide (AVS) content in the sediment was suggested as another possible factor affecting macrozoobenthic density. Our results clearly demonstrate that macrozoobenthic assemblages in estuarine soft-bottoms have high spatial heterogeneity on a small scale (e.g., hundreds of meters) related to physical and chemical environmental changes. Our data also suggested the importance of sediment redox condition (e.g., ORP and AVS content) and sediment grain size as structuring factors in estuarine soft-bottom communities as well as the salinity in the habitat.

Trace element compositions of iridium enriched illite-smectite assemblages from a K/Pg boundary section in the Anjar area of the Deccan volcanic province: palaeoenvironmental implications

Clay minerals in the Cretaceous-Palaeogene (K/Pg) boundary section of the Anjar area comprise smectite, palygorskite, sepiolite, illite, and mixed layer illite – smectite. Elemental data plots show fractionation of heavy from light Rare Earth Elements (REE). The palygorskite-dominated upper succession shows high light REE concentrations compared to the illite-smectite-rich lower succession. REE patterns exhibit lower values in the lower and middle part of the succession, possibly related to leaching of the REE inter-element fractionation during weathering. Minor release of trace elements is coincident with an Ir-rich layer and anoxic conditions at the time of deposition. REE patterns in Ir-rich layers show a negative Eu anomaly. La/Yb ratios reflect heavy REE enrichment relative to light REE, and negative correlation with illite content in mixed layer illite-smectite. The increase in δCe is simultaneous with Ir spikes in the basal limestone, indicative of anoxic conditions. In the upper part of the succession, δCe shows more or less constant values, signifying an oxic environment of deposition. δCe and δEu are elevated in sepiolite-rich clays and negative in the palygorskite-dominated lower and upper successions respectively. The positive Ce anomalies and inter-element ratios of the illite-smectite reveal high Ir concentrations at three levels which have been attributed to abrupt changes in sedimentary redox conditions. It is inferred that the physico-chemical conditions during deposition of the Ir layers were different from those of other sedimentary units, as well as the Ir hosted fossiliferous siltstone.

Uranium mobility in subsurface aqueous systems: the influence of redox conditions

AbstractUranium is a redox-sensitive element and can be immobilized by reduction from soluble U(VI) to insoluble U(IV). By performing flow-through experiments, uranium mobility was observed under different redox conditions. Inflow solutions with different electron acceptors, nitrate and sulphate, and a control inflow solution were used to obtain different sedimentary redox conditions. Uranium was about one order more mobile when nitrate was used than when sulphate or the control was used. The difference in uranium mobility is attributed to the reduction of uranium. Even though uranium mobility is heavily dependent on the redox state of uranium, sedimentary concentrations of organic matter argue that organic matter is the most important complexing agent and that this determines the retardation of uranium.

Sediment and nutrient accumulation rates in sediments of twelve New Zealand lakes: influence of lake morphology, catchment characteristics and trophic state

Intact sediment cores were collected from the deepest basins of 12 lakes in the Rotorua District, New Zealand, to test for effects of morphological features, catchment characteristics and lake trophic state on net sedimentation rates and sediment nutrient concentrations. Multiple linear regression was used to show that 68% of the variation in net sedimentation rates across the lakes could be explained by lake trophic state and catchment area. Comparison of 2006 data with results from a survey in 1995 showed that surficial sediment (0–2 cm) total phosphorus concentrations (TP) have increased in three of the 12 lakes, at rates ranging from 27.5 to 114.4 mg P kg–1 dry wt y–1. Total nitrogen (TN) concentrations in surficial sediments have increased in nine of the 12 lakes at rates ranging from 51.8 to 869.2 mg N kg–1 dry wt y–1. Temporal changes in sediment TP and TN concentrations were not significantly linearly related (P = 0.12–0.88) to catchment area or different water column indices considered to reflect lake trophic state, including annual mean water column concentrations of TP, TN or chlorophyll a. It is concluded that between-lake variations in sediment TP and TN concentrations are influenced by a range of complex interacting factors, such as sediment redox conditions (and periodic anoxia in the hypolimnion of some lakes) as well as variations in sediment mineral composition (which influences retention and release of various sediment phosphorus and nitrogen species). Subsequently, these factors cause sediment TP and TN concentrations across the 12 lakes to respond differently to temporal changes in water column TP and TN concentrations.

A method for cultivation of microorganisms oxidizing iron and manganese in bottom sediments of Lake Baikal

Methods for cultivation of microorganisms oxidizing iron and manganese in bottom sediments of Lake Baikal were tested. Bacteria were grown on selective media with a characteristic accumulation of metal oxides. The morphology and developmental patterns of the isolated microbial group were studied. The studied sediment core was used to isolate 42 strains of bacteria oxidizing iron and manganese. The cultivated bacteria were confined to the upper sediment layers (down to 11 cm) and their abundance depended on sediment type and redox conditions.

Sedimentary redox conditions in continental margin sediments (N.E. Pacific) — Influence on the accumulation of redox-sensitive trace metals

Redox conditions in near-surface sediments from the continental margin off western Canada were characterized to determine how they influence the accumulation of redox-sensitive trace metals. Despite the development of suboxic conditions within millimeters of the sediment–water interface there is little enrichment of Re, U, Cd or Mo in the upper 10 to 15 cm of the sediment column. This observation is consistent with the low extractible solid Mn concentrations but high I/C org ratios that imply that near-surface sediment redox conditions are poised between Mn and I reduction. In contrast, below ∼ 15 cm Re, and to a lesser extent Cd and U, are enriched (e.g., up to 65 ng/g Re) but only in those cores collected from within and below the oxygen minimum zone. Molybdenum concentrations remain low (< 1.5 μg/g) in all cores indicating that these near-surface sediments never became fully anoxic. A combination of low sedimentation rates on the slope (< 1 to 6 cm/kyr) and/or intense bioturbation at all locations, including on the shelf where the sedimentation rate is substantially higher (40 cm/kyr), has maintained suboxic conditions in these sediments. Suboxic conditions exist despite a high organic carbon flux to the sediment and low bottom water oxygen concentrations, both of which should favour the development of anoxic conditions close to the sediment–water interface. The enrichment of Re, U and Cd that is observed occurs well below the sediment–water interface, overprinting and thus potentially compromising any paleo-signal. Accordingly, caution is necessary when using redox-sensitive trace metal concentrations as paleo-proxies in environments characterized by low sedimentation rates and/or deep bioturbation.

Transformation and mobilization of arsenic in the historic Cobalt mining camp, Ontario, Canada

More than eight decades of silver mining in the Cobalt mining camp of northern Ontario, Canada, have left large volumes of As-bearing mine wastes widely distributed in and along watercourses in the Cobalt area. Metal leaching from these mine wastes has led to the contamination of the area drainage with dissolved As concentrations at least an order of magnitude higher than the Canadian drinking water criterion of 0.025 mg l − 1 . To clarify the transformation and mobilization of arsenic in the historic mining camp, a portion of an extensive wetland located in northeast Cobalt and partially filled with historic tailings has been sampled for detailed characterization, chemical analysis and extraction tests. Field deployment of anionic exchange membranes, As sorption isotherm and desorption analyses in conjunction with chemical and mineralogical analyses indicate that: (1) the submerged tailings are likely a source instead of a sink of arsenic to the local streams; and (2) Al-minerals are the main sorbents for As with significant P competing for the available sorbing sites. Subjecting selected samples to a laboratory redox experiment complemented with X-ray absorption spectroscopic analyses confirms that changes in arsenic speciation readily occur with changes in redox conditions in the surface sediments, resulting in rapid mobilization of arsenic. Preliminary enumeration of iron- and sulfur-reducing bacteria at selected sites coupled with scanning electron microscopic analyses show that microbial sulfate reduction occurs locally in the wetland, possibly leading to co-precipitation of arsenic as a sulfide in associated with framboidal pyrite. Further detailed study of the bacteria responsible for the arsenic transformation in conjunction with arsenic speciation analysis is recommended.

Influence of sediment redox conditions on release/solubility of metals and nutrients in a Louisiana Mississippi River deltaic plain freshwater lake

The influence of sediment redox conditions on solubility of selected metals and nutrients in sediment from a coastal Louisiana freshwater lake (Lake Cataouatche) receiving diverted Mississippi River water was quantified. Sediment redox was cycled step wise in 50 mV increments between oxidized (-200 to +500 mV) and reduced (+500 to -200 mV) conditions. Changes in sediment oxidation/reduction status and pH influenced solubility of both metals and nutrients. When redox potential (Eh) was increased from -200 to +500 mV, sediment pH decreased from 7.1 to 5.7. When the sediment Eh decreased from +500 to -200 mV, pH increased from 5.7 to 7.1. The increase in sediment acidity upon oxidation resulted in the release of the Pb, Ca, Mg, Al, and Zn into solution. The solution concentration of these elements was inversely proportional to Eh (P</=0.05). The concentration of Fe, Mn, and P in sediment suspension was strongly governed by changes in oxidation-reduction status of sediment. The oxidation of reduced sediment resulted in a decrease in amount of Fe and Mn in solution, a result of the conversion of soluble ferrous and manganous form to less soluble ferric and manganic form. Following reduction of oxidized sediment, Fe and Mn became more soluble because the ferric iron and manganic manganese form changed to ferrous and manganous form. Phosphorus behavior as influenced by oxidation/reduction status was closely related to Fe and Mn chemistry with an increase in phosphate following iron reduction. This study demonstrated that sediment redox potential is an important parameter affecting metal and nutrient solubility and mobility in Louisiana coastal freshwater lake sediment. Reduction or aeration status of sediment should be considered in predicting the release of these elements into the aquatic environment.

Phosphorus burial as a function of paleoproductivity and redox conditions in Arabian Sea sediments

In this study the response of sedimentary phosphorus (P) burial to changes in primary productivity and bottom water oxygen concentrations during the Late Quaternary is investigated, using two sediment cores from the Arabian Sea, one recovered from the continental slope and the other from the deep basin. The average solid-phase P speciation in both cores is similar, authigenic and biogenic (fish debris) apatite make up the bulk of the P inventory (ca. 70%);whereas P adsorbed to iron oxides, organic P, and detrital apatite constitute minor fractions. Postdepositional redistribution has not significantly altered the downcore distribution of total solid-phase P. Phosphorus burial efficiencies are generally lower during periods of increased paleoproductivity. This is caused by (a) partial decoupling of the P export flux, consisting primarily of particulate organic P, and the P burial flux, consisting primarily of biogenic and authigenic apatite; and (b) the lack of increased rates of authigenic CFA formation during periods of higher P deposition. In addition, fluctuations in bottom water oxygen concentrations may have affected P burial in continental slope sediments. The results of this study indicate that higher primary productivity induces more efficient P cycling. On time scales exceeding the oceanic P residence time, this process may induce higher surface water productivity, thus creating a positive feedback loop. In the Arabian Sea, this feedback mechanism may have contributed to changes in sea surface productivity on sub-Milankovitch time scales because P, regenerated on the continental slopes of the Oman and Somalian coastal upwelling zones, is reintroduced into the photic zone relatively fast.

Relationship of Sediment Redox Conditions to Methyl Mercury in Surface Sediment of Louisiana Lakes

Surface sediment from three Louisiana Lakes containing overlying water layer spiked with 2 µg/g (2 ppm) mercury were incubated under oxygenated (air) and nonoxygenated (N2) conditions for determining the impact of oxygen status of overlying water on methylation of Hg in surface sediment from these lakes. The added mercury resulted in a greater than ten fold increase in methyl mercury (MeHg) as compared to native concentration of MeHg. The increase in methyl Hg production was less in sediment in which overlying water was exposed to oxygen rather than nitrogen. Results suggest that methyl Hg production would be less in lakes containing an oxygenated water column. In parallel microcosm studies without added mercury, MeHg decreased in sediment when redox potential of sediment suspension was increased from −200 mV to + 50 mV. Results of these studies demonstrate the importance of oxygenation or redox condition of surface sediment on mercury methylation and demethylation. Sediment conditions, which either reduce methylation or enhance demethylation in surface sediment, will limit the bioavailability of MeHg to the aquatic environment.

Alteration to lake trophic status as a means to control arsenic mobility in a mine-impacted lake

The relationship between lake trophic status, sedimentary redox conditions and As mobility was examined in mine-impacted Balmer Lake, Canada. Under the current redox regime, the reductive dissolution of As-bearing Fe(III) oxyhydroxides occurs in close proximity to the sediment-water interface, resulting in the remobilization of dissolved As in the shallow porewaters to values as high as 8.5 mg L(-1). The shallow depth of the oxic zone limits the extent to which As can be re-sorbed in the interfacial horizons, and as a result, a proportion of the remobilized As escapes into the water column where it poses a water quality concern. Examination of the relationship between summer average chlorophyll a and total P at spring overturn in the lake water column demonstrates that Balmer Lake is currently eutrophic as a result of mining-derived inputs of P (domestic waters) and N (blasting residues and cyanide breakdown products). The results suggest that actively pushing the system towards oligotrophy by reducing non-natural P loadings to the system will decrease rates of in situ production and associated sediment oxygen demand, which will in turn result in increased thickness of the aerobic zone and enhanced As scavenging. Such conclusions are supported by porewater data which indicate that the flux of As to the water column is significantly reduced when the Fe(III) redox cline is situated at deeper sediment depths. In the absence of detailed P-loading data, it is recommended that P inputs be reduced to approximately 10% of the estimated pre-mining P loading of approximately 200 kg yr(-1). This implies reducing the collective P-loadings from the two mine sites adjacent to the lake from the approximate current value (approximately 150 kg yr(-1)) to approximately 20 kg yr(-1). It is proposed that establishment of oligotrophy in the lake should significantly mitigate the current level of dissolved As in lake waters.

Nutrient inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact

Nutrient input through submarine groundwater discharge (SGD) rivals river inputs in certain regions and may play a significant role in nutrient cycling and primary productivity in the coastal ocean. In this paper, we review the key factors determining the fluxes of nitrogen (N) and phosphorus (P) associated with SGD and present a compilation of measured rates. We show that, in particular, the water residence time and the redox conditions in coastal aquifers and sediments determine fluxes and ratios of N and P in SGD. In many coastal groundwater systems, and especially in contaminated aquifers, N/P ratios exceed those in river water and are higher than the Redfield ratio. Thus, anthropogenically driven increases in SGD of nutrients have the potential to drive the N-limited coastal primary production to P-limitation. River input of N and P to the coastal ocean has doubled over the past 50 yr. Results of a dynamic biogeochemical model for the C, N and P cycles of the global proximal coastal ocean (which includes large bays, the open water part of estuaries, deltas, inland seas and salt marshes), suggest that this has led to a factor 2 increase in primary production and biomass and a decline in water column N/P ratios, i.e. the system has become more N-limiting. With the same model, we show that an increase of SGD-N fluxes to ∼0.7–1.1 Tmol yr −1 (with a SGD N/P ratio of 100; equal to ∼45–70% of pre-human riverine N-inputs) is required to drive the coastal ocean to P-limitation within the next 50 yr.

Adaptation of sympatric Achromatium spp. to different redox conditions as a mechanism for coexistence of functionally similar sulphur bacteria.

Changes in the abundance of sympatric Achromatium spp. in response to the artificial manipulation of redox conditions in sediment microcosms was determined by fluorescence in situ hybridization (FISH). Adaptation to different redox conditions was shown to be one mechanism that supported the coexistence of functionally similar Achromatium spp. In sediment microcosms, in which the overlying water was oxygenated, Achromatium community size and composition remained unchanged over time. However, imposition of anoxic conditions induced changes in community structure. Anoxia caused a reduction in the relative abundance of Achromatium sp. RY8 (72 +/- 4% to 49 +/- 2%) and an increase in Achromatium sp. RY5 (19 +/- 5% to 32 +/- 3%) and a newly identified Achromatium sp., RYKS (14 +/- 4% to 27 +/- 2%). In anoxic microcosms supplemented with a single addition of nitrate at different initial concentrations the relative decline in Achromatium sp. RY8 was dependent on the initial nitrate concentration. In these experiments nitrate was rapidly removed. In contrast, when high levels of nitrate were maintained by periodic replacement of the overlying water with nitrate supplemented anoxic water, the composition of the Achromatium community remained stable over time. This suggested that all of the coexisting Achromatium spp. are obligate or facultative anaerobes, but, Achromatium sp. RY8 was more sensitive to sediment redox conditions than the other Achromatium species. Given the heterogeneous nature of sedimentary environments, redox-related niche differentiation may promote coexistence of sympatric Achromatium spp.