Sediment Eh Research Articles

Hydrocarbons in the water and bottom sediments of Sivash Bay (the Azov Sea) during its salinization.

Sivash Bay is a unique hypersaline lagoon located in the northern part of the Crimean Peninsula. In 2014, due to political events in connection with the closure of the North Crimean Canal, the inflow of fresh water to Sivash Bay has been significantly reduced. As a result, there has been a steady increase in salinity since 2014 to the present. The main purpose of this work was to determine the spatial distribution and qualitative composition of hydrocarbons (aliphatic hydrocarbons, n-alkanes, polycyclic aromatic hydrocarbons) in the water and bottom sediments of the hypersaline Sivash Bay under increasing water salinity. The analysis of the physico-chemical parameters of Sivash Bay in 2020 showed the continued salinization and change of physico-chemical conditions of the lagoon. At the same time, spatially, the change in salinity affected only the total content and qualitative composition of hydrocarbons in the water. The content of the studied classes of hydrocarbons in the bottom sediments did not demonstrate a reliable correlation with the concentration of salts. There was also no statistically significant dependence of Eh and pH of bottom sediments on salinity. In accordance with the composition of n-alkanes and polyaromatic hydrocarbons, as well as on the basis of PCA analysis, it is possible to make a conclusion on natural, mainly autochthonous, sources of this class of substances and low toxicity of bottom sediments of the bay. Low concentrations and composition of hydrocarbons indicate an insignificant input of pollutants of anthropogenic origin in Sivash Bay during its salinization.

Ecological risk assessment of trace metals in sediments under reducing conditions based on isotopically exchangeable pool

Determination of potential mobility of toxic trace metals in sediments under changing redox condition is important in ecological risk assessment. Current methods are limited in risk prediction in such dynamic environment. In this study, we have discussed the general disagreement from widely used methods (sediment quality guideline (SQGs), potential ecological risk index (PERI), risk assessment code (RAC) using BCR fraction information). In addition, the stable isotopic dilution method (IDM) was also modified to quantify metal lability in a microcosm experiment mimicking river bank sediment turning into anaerobic. The isotopically exchangeable Cd, Cu, Pb, and Zn quantified by IDM (%E incub) was used in the RAC to reveal the trend of risk during this process. Strong risks from Cd are suggested by the PERI and RAC as a result of high toxicity and mobility of the element, while SQGs suggests medium risk for Cu, Pb, and Zn in certain samples. The disagreement between the results of RAC assessed by metal lability (%E dry) and by BCR metal fractionation reflects the effect of sediment properties and source of metal contamination. The RAC based on the non-residual fractions is likely to overestimate the potential risk for most metals even there is a significant change in sediment Eh. The RAC assessed by %E incub reveals that the variability in risk in response to the reducing Eh is not consistent. Large fluctuation in %E incub for Cd (28.5%, 49.5%), Pb (27.6%, 18.2%), and Cu (14.4%, 24.7%) can shift the risks to a higher level in certain range of Eh in two sediments. In sediment with lower contents of metal binding phases (e.g. mineral oxides, organic matters), the release of metals can be more significant, thus higher ecological risk in changing redox condition.

Assessment of benthic macroinvertebrate response to anthropogenic and natural disturbances in the Kodungallur-Azhikode estuary, southwest coast of India.

Benthic biotic indices are important ecological tools extensively used to understand the ecological quality of coastal wetlands. The present study aimed to assess the ecological status of Kodungallur-Azhikode estuary for the first time by using widely used benthic indices such as species richness (S), Shannon diversity index (H'log2), BENTIX, benthic opportunistic polychaetes amphipods (BOPA), AZTI's Marine Biotic Index (AMBI) and multivariate AMBI (M-AMBI). In the canonical correspondence analysis, salinity, dissolved oxygen, organic matter, sediment Eh, sediment pH and sand were identified as important variance descriptors. A single species of an opportunist, Americorophium triaeonyx, an amphipod belonging to the ecological group (EG) III, significantly contributed to the total macrofaunal density. Other dominant opportunistic species included Obelia bidentata (EGII), Arcuatula senhousia (EGIII), Cirolana fluviatilis (EGII), Prionospio cirrifera (EGIV) and Capitella sp. (EGV). The overall assessment indicated a 'good to moderate' condition in AMBI, 'good to poor' condition in M-AMBI, 'high to moderate' condition in BENTIX, 'high to poor' condition in BOPA and 'moderate to poor' condition in univariate Shannon diversity index. All the multivariate indices tested in the study were correlated with each other except BOPA and M-AMBI. The group of stations dominated with a sandy substrate and a moderate level of organic content indicated high to good conditions while other stations demonstrated moderate to poor conditions. However, no significant variation in indices tested between seasons was observed. The present study recommends long-term monitoring of benthic macroinvertebrate assemblages with proper taxonomic identification and functional trait analysis for better calibration of indices, which is the key factor for getting better results.

Redox potential and uranium sorption onto sediments: kinetic and thermodynamic characteristics

ABSTRACTThe effects of the flooding and initial Eh of sediments on the sorption of uranium onto the sediments were analysed by flooding and static experiments. The changes in uranium species with Eh and kinetic and thermodynamic characteristics of the uranium sorption onto the sediments were investigated. The flooding experiment indicates that the initial Eh of the sediment gradually decreased with the increase in flooding time. Based on the redox potential in the flooding experiment, simulation results obtained using the geochemical simulation software PHREEQC show that the concentration of U (VI) decreased. In contrast, the concentrations of U (III), U (IV), and U (V) gradually increased. The pseudo-second-order kinetic model well fitted the experimental data, which shows that the sorption was mainly chemical sorption. The thermodynamic parameters suggest that the entropy and enthalpy under the used conditions were positive and that ΔGθ was negative. A thermodynamic analysis shows that the sorption was endothermic and spontaneous. These results are useful for the understanding of the sorption mechanism and migration of uranium onto the sediment under different initial sediment redox potentials and provide a good theoretical foundation for radioactive pollution remediation.

Impact of methane occurrence on iron speciation in the sediments of the Gdansk Basin (Southern Baltic Sea)

Due to changing climate conditions, such inland seas as the Baltic are expected to become more eutrophicated and less saline (causing lower availability of sulphates). This may promote methanogenesis as the main process of organic matter (OM) degradation in marine sediments. Presence of methane, in turn, may affect biogeochemical cycling of many elements, including iron. Thus, in the present study we attempted to investigate the influence of CH4 on the Fe forms in marine sediments. Sediment cores were collected from three physico-chemically different stations within the Southern Baltic, taking into account such parameters as water depth, OM content, bottom zone oxygenation and distance from the Vistula River (main source of anthropogenic material). At two sampling stations methane was present in shallow sediments (P1, MET2) and at one station (W6) any traces of this gas were not determined. Iron species (FeCARB, FeOX1, FeOX2, FeMAG, FePRS, FeT, FeFeS2, FeFeS) in sediments were investigated using the sequential extraction. Pore water was analysed to obtain vertical profiles of hydrogen sulphide, sulphate and dissolved inorganic carbon (DIC). Additionally, such parameters as water and OM content, CH4 and Eh in sediments were determined. Results showed that methanogenesis affects the biogeochemical iron cycling in sediments of the Southern Baltic, leading to the increase of carbonates containing ferrous iron (as a result of DIC production during intensive OSR, AOM and methanogenesis) and decrease of ferric iron compounds used in the AOM. Moreover, in the areas of lower salinity, pyrite formation is limited by an insufficient amount of hydrogen sulphide, leading to the situation when a significant part of Fe is accumulated in the form of monosulphides. In turn, in the areas of higher salinity, where oxygen deficiencies occur more often, more hydrogen sulphide is present in pore water. Pyrite formation is then limited by iron, not by sulphur.

Arsenic distribution and speciation in multiphase media of a lake basin, Tibet: The influences of environmental factors on arsenic biogeochemical behavior in the cold arid plateau lake

Widespread arsenic (As) has been found in surface media on the Tibetan Plateau, but few studies of As species have been performed because of the difficult sampling conditions. In this study, As distribution and speciation in multiphase media (including surface water, interstitial water, and sediment) in Lake Yamdrok Basin in Tibet in wet and dry seasons were investigated to allow the biogeochemical behavior of As in the cold arid plateau lake to be understood. The total As (TAs) (mainly containing arsenate (As(V)) and arsenite (As(III))) concentrations were generally higher in surface and interstitial water in the lake zones than in Inflowing rivers. Among the four lake zones, significantly higher As concentrations were found in Chen Co, Yamdrok Tso, and Kongmo Co than in Bajiu Co, and surface sediments from the former three lake zones contained relatively high concentrations of the labile As. Redox potential (Eh) in sediments and HCO3 concentration in surface water primarily controlled labile As mobilization through reductive dissolution of As-bearing iron (hydr)oxides and oxyanion competition for As adsorption sites, and therefore affected the As distributions in aqueous phases. As(III) concentrations in interstitial water accounted for 41%±33% of TAs, and positively correlated with the arsenate-reducing microbe population in sediments. In contrast, As(V) was predominant in surface water (accounting for 95%±8% of TAs), and even trace amounts of dimethylarsinic acid (DMA) was found in wet season. Notably, lower Eh values in dry season triggered a marked increase in the As concentrations in interstitial water, and this probably increased the risk of As contamination of surface water.

Mechanism of phosphorus mobility in sediments with larval (Propsilocerus akamusi) bioturbation.

The effects of chironomid larval (Propsilocerus akamusi) bioturbation on sediment phosphorus (P) mobility were studied over the course of 34days using the indoor larval cultivation method on in situ sediment cores. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to record fine-scale changes of soluble and DGT-labile P and iron (Fe) concentrations in the sediment. The larval-driven irrigation of the overlying water into their burrows significantly increased the oxygen penetration depth (OPD) and redox state (Eh) in sediments. In addition, the soluble and DGT-labile P and Fe decreased with the increase of OPD and Eh in larval-bioturbated sediments. The greatest decrease in the mean concentration of SRP, soluble Fe, and DGT-labile P in the Propsilocerus group was observed on Day 15 of the experiment, with a decrease by over half of the mean concentration of the control group. Furthermore, two-dimensional measurements of DGT-labile P concentration showed notable reductions of DGT-labile P around larval burrows. The DGT-induced fluxes in sediments (DIFS) model also exhibited a much longer response time (420s) and a much higher rate of P adsorption (0.002s-1) in the bioturbation sediments than those in the control sediments (116s and 0.009s-1, respectively). A significant correlation was shown for DGT-labile P and DGT-labile Fe. We conclude that Fe(II) oxidation and its enhanced adsorption were the major mechanisms responsible for the decrease of soluble and DGT-labile P in sediments.

Bioecology of macrobenthic communities in the microtidal monsoonal Kodungallur–Azhikode Estuary, southwest coast of India

AbstractA comprehensive study of the macrobenthic communities in the Kodungallur–Azhikode Estuary (KAE) was conducted during 2009–2011 period. A total of 18,846 organisms were collected, with 60% being malacostracans, followed by polychaetes (20%), molluscs (9%) and ‘others’ group (11%). A total of 79 species in 71 genera belonging to 49 families were identified, with 33 spp. being polychaetes, 26 spp. being malacostracans, 11 spp. being molluscs and 9 spp. being in the ‘others’ group. A single species of opportunistic amphipod (Americorophium triaeonyx) comprised more than 62.05% of the total numerical abundance of macrobenthos. The other dominant species were Obelia bidentata, Arcuatula senhousia, Cirolana fluviatilis, Prionospio cirrifera and Capitella sp. Canonical correspondence analysis (CCA) results indicated environmental parameters such as water column salinity, turbidity, sediment Eh, substratum type, chlorophyll‐a concentration, depth and organic matter in sediment were the significant factors influencing the distribution of macrobenthic species in the KAE. The present study provides baseline information for future KAE benthic studies.

Distribution Characteristics and Release Fluxes of Phosphorus Forms in Xiangxi Bay Sediments in the Three Gorges Reservoir Before and After Impoundment

The sediment storage environment in tributaries has been altered by impoundment of water in the Three Gorges Reservoir area, affecting the distribution of phosphorus forms in sediment and processes at the sediment-water interface. Through collection of sediment and overlying water samples in Xiangxi Bay in August 2016 (before impoundment) and October (after impoundment), the distribution characteristics of sedimentary phosphorus and the environmental conditions of storage before and after impoundment were analyzed. Fluxes of PO43--P at the sediment-water interface were also estimated. Results show that pH increased, alkalinity and reducibility were enhanced, and Eh in sediments decreased after impoundment. The relative content of phosphorus in sediments changed as follows:NaOH-P > HCl-P > OP to HCl-P > OP > NaOH-P; this could be attributed to changes in the depositional environment. Compared to pre-impoundment values, TP values after impoundment in sediment, overlying water ρ(PO43--P), and interstitial water ρ (PO43--P) were 1.3 times, 3.7 times, and 8.3 times higher, increasing the risk of nutrient release in sediments of Xiangxi Bay. The manifestation of PO43--P in sendiments of Xiangxi River generally is "source" pre-impoundment and post-impoundment, but the PO43--P diffusive flux increased from -0.0029-0.0059 mg·(m2·d)-1 pre-impoundment to 0.0067-0.1071 mg·(m2·d)-1 post-impoundment. The release of phosphorus from sediments at the bottom of Xiangxi Bay increased after impoundment.

黄河下峪口段表层土壤pH、Eh分布及其主要影响因素

通过对黄河下峪口段河道表层淤积物和沙洲表层土壤pH和Eh的样品采集，分析了该段表层土壤pH及Eh的分布状况，并从底质特性、周边环境和生物扰动等方面深入探讨了pH、Eh的分布特征、影响因素和影响机理。研究表明：黄河下峪口段河道沉积物及沙洲表土呈中性–弱碱性环境，其中pH值变化于7.54~8.12之间，空间分布上具有分带性和不均一性特征，总体表现为河道沉积物pH值低于沙洲表土pH值。黄河下峪口段河道沉积物和沙洲表土总体尚未氧化环境，Eh变化分布于413~521 mV之间，大部分在483 mV左右，空间分布也具有弱分带性和不均一的特点，表现为河道沉积物Eh低于沙洲表土，也就是说河道沉积物氧化性强于沙洲表土。该区域土壤pH和Eh主要受到水动力作用、河道底质特性以及土壤有机质含量的影响，pH和Eh值呈负相关关系。 Based on the collection of surface sediment and the surface pH and Eh of the surface soil of the Xiayukou section of the Yellow River, we analyzed the distribution of the segment of the surface soil pH and Eh, and discussed from the aspects of sediment quality, surrounding environment and bioturbation. PH, Eh distribution characteristics, influencing factors and influencing mechanism. The results show that the sediment in the lower reaches of the Yellow River and the top-soil of the sandbank are neutral-weakly alkaline. The pH value varies between 7.54 and 8.12, and the spatial distribution is characterized by zonality and heterogeneity. The pH value of the sediment is lower than that of the sand surface. The Eh changes are distributed between 413 and 521 mV, most of them are about 483 mV, and the spatial distribution is also characterized by weakly zoning and heterogeneity, which is characterized by the river sediment Eh is lower than the sand surface, that is, the river sediment is stronger than the sand surface. The soil pH and Eh were mainly affected by hydrodynamic action, river sediment quality and soil organic matter content, and the pH and Eh were negatively correlated.

Internal phosphorus load in a Mexican reservoir: Forecast and validation

To determine the internal phosphorus load (IPL) as a function of redox potential (Eh) in a Mexican reservoir, the results from a phosphorus (P) release experiment were extrapolated to temporal and spatial variations of Eh in sediments, and an IPL-Eh of 24.2 ± 2.5 t/yr was obtained. This result is compared with the P mass balance (MB) in the reservoir, where the IPL-MB is determined as the difference between P inputs to the reservoir and the outputs. Inputs of P are the sum of the external P load from the hydrological basin, the IPL, and P in atmospheric precipitation; outputs of P are the sum of sedimented P, and the removal of P in water and biomass, and the resulting IPL-MB, is 26.4 ± 4.9 t/yr. In addition, P concentrations in sediment cores (SCs) are analyzed, and the historical release of P from sediments determined, resulting in an IPL-SC of 23.5 ± 1.4 t/yr. The different IPL results are similar, as average values are within the standard deviation of IPL-MB. It is concluded that analysis of the variations in Eh in sediments allows determination of the reservoir's IPL. Six-weekly IPL-Eh and IPL-MB values are analyzed, and it can be seen that IPL occurs mainly during the period from May to August, when the water column is thermally stratified.

Effects of Bioavailable Heavy Metal Species, Arsenic, and Acid Drainage from Mine Tailings on a Microbial Community Sampled Along a Pollution Gradient in a Freshwater Ecosystem

Sediment and water samples representing a pollution gradient in a long, narrow lake polluted at one end by heavy metals, arsenic, and acid drainage from mine tailings, together with samples from an unpolluted reference lake, were analyzed to determine effects of pollutants on the microbial community of the polluted lake. Ribosomal ribonucleic acid, fatty acid, and phospholipid analyses, along with assays of CO2 production, denitrification, and enzyme activities, were performed to characterize the microflora; and environmental conditions were defined by various physicochemical analyses, including determination of bioavailable metal species. Mine waste pollution fostered the growth of Holophagal Acidobacteria, green sulphur bacteria, and α-Proteobacteria but inhibited numerous other types of microorganisms, reducing the overall productivity, biomass, and biodiversity of the microflora. The beneficial effects imply toleration of pollutants, suppression of competing or antagonistic species, and utilization of biogenic sulphide; and the toxic effects are attributable to bioavailable metals, arsenic, and sulphuric acid produced by oxidation of sulphides. The bioavailability and toxicity of sediment-bound metals were evidently increased by acidification, elevation of sediment Eh, and inhibition of metal-immobilizing bacteria by pollutants but were decreased by metal-scavenging oxyhydroxides, sulphide, and organic matter. Metal toxicity also depended on specific metal properties (e.g., electronegativity), providing a basis for inferring mechanisms of toxicity and oxidation states of metals and explaining differences in relative toxicity. The pollutants harmed the ecosystem as a whole by inhibiting microorganisms that performed crucial ecological functions, notably oxygen-releasing photosynthesis, decomposition and humification of organic matter, nutrient recycling, and control of metal availability.

Floating Treatment Wetland influences on the fate of metals in road runoff retention ponds

A field trial comparing the fate of metals in two parallel stormwater retention ponds, one of which was retrofitted with a Floating Treatment Wetland (FTW), was carried out near Auckland, New Zealand. Results suggest that the FTW increased metal accumulation in the pond sediment especially in summer due to lower sediment Eh, more anoxic water column, neutral pH and greater source of organic matter (OM) induced by the FTW. These factors combined with higher temperature enhanced metal sorption onto OM, flocculation of particulate pollutants, metal sulphide formation and reduced OM degradation and thus limited release of metals. Unlike Zn, Cu speciation in the pond sediment was relatively unchanged under various sediment Eh conditions due to its strong binding property with sulphide and OM. Occasional moderate metal release was detected from the FTW pond sediment likely due to aerobic OM degradation at the beginning of spring and/or hydroxides reduction when sediments became reduced later in the season. No release was noticed from the conventional pond sediment likely due to biosorption and/or uptake by algae which developed in the conventional pond and settled on the bottom sediment. Direct uptake by the plants of the FTW and sorption onto root plaques are not thought to be significant removal pathways. Nevertheless roots play a major role in trapping particulate pollutants, eventually sloughing off to settle on the bottom of the pond, and provide an adequate substrate for bacterial development due to release of organic compounds which are both essential for dissolved metal sorption and metal sulphide formation.

Physico-chemical and nutrient variable stratifications in the water column and in macroalgal thalli as a result of high biomass mats in a non-tidal shallow-water lagoon

A field study to check parameter stratification during high density growth of four opportunistic macroalgae was carried out in Orbetello lagoon (Italy). The effects of macroalgal masses were compared with a seagrass meadow and two lagoon areas with bare bottoms as controls for pH, temperature, dissolved oxygen, salinity, nitrite, nitrate ammonium and orthophosphate. The nutrient content of thalli and sediment redox were measured. Macroalgae showed differences in stratification of thalli nutrient content. Mat with low density and high volume produced stratifications in the water column, but it did not produce nutrient release by sediment. In contrast, high density and low and high volume mats led to sharp falls in dissolved oxygen, with negative values of sediment Eh and anoxic trigger processes that presumably led to release of sediment nutrients. This depended on thallus type: heavy thalli compacted the mat and light ones distributed more widely in the water column.

Acidification process in the area of the abandoned Ľubietová - Podlipa CU-deposit, Slovakia

Abstract Acidity in surroundings of the abandoned Ľubietová-Podlipa Cu-deposit depends predominantly on the geochemical behaviour (weathering) of particular minerals (mainly pyrite). The article presents the results of measurements of basic physicochemical parameters - pH and Eh in technogenic sediments and in surface and drainage water of a dump-field. Although the dump material shows a significant amount of mobility-able metals and thus also a potential to form acidity, a massive AMD formation in future is not probable.

Contribution of bacteria to redox potential (E h) measurements in sediments

Increasing evidence suggests that bacteria are capable of creating specific redox conditions which are visible as species-specific continuous redox potential (Eh) measurements. It has been demonstrated that continuous measurements of Eh are valuable for tracking bacterial metabolic activities of bacterial species in liquid cultures. However, it remains uncertain whether this phenomenon is widespread among bacterial species and whether Eh mea- surements reflect similar mechanisms in more complex systems such as soils and sediments. The purpose of this study is therefore to evaluate whether bacteria that natu- rally occur in sediments have the capacity to control Eh and assess the relative partitioning of biological processes involved in Eh in natural sediments. To this end, continu- ous Eh measurements are linked to growth of bacteria in liquid cultures and bacterial metabolic activity in aquatic sediment microcosms containing the bioturbator Tubifex spp., in which we evaluate bacterial partitioning in microcosms treated with the bacteriocide formalin. The tested bacterial species (Micrococcus luteus, Paracoccus pantotrophus and Aminobacter aminovarans) appeared to have specific stable Eh signals during linear-exponential growth phase, suggesting that these species are capable of exerting an extracellular control on Eh measurements, thereby supporting the notion that species-specific Eh signals may be widespread among bacterial species. For- malin treatment reduced temporal variability of Eh in sediment microcosms. This outcome suggests that bacterial metabolism and inherent relative contributions of members of bacterial community principally determine development of Eh in sediment systems and that quantitation of sediment electrochemical properties may offer a potential indicator that characterizes bacterial processes.

Benthic mats in Antarctica: biophysical coupling of sea-bed hypoxia and sediment communities

Transient white and grey mats were observed in depressions and enclosed basins in marine sediment in the Windmill Islands, East Antarctica. These patches have not been described in the Antarctic marine environment previously although a similar phenomenon has been described in the Arctic. Our aim was to describe the sediment geochemical and biological properties inside the patches and to determine their similarity to each other. We compared the benthic infaunal communities and the chemical properties of the sediment in the white patches to nearby sediment without white mats. We observed differences in sediment pH, Eh and elemental concentrations inside and outside patches. The benthic infaunal communities inside the patches were significantly different, lower in abundance and diversity, compared to outside the patches. The structure of the microbial communities within the mats was described by constructing clone libraries from four different patches. These clone libraries were dominated by bacteria from the bacteroidetes phylum. Clones closely related to sulphur-oxidising bacteria from the gammaproteobacteria and/or the epsilonproteobacteria were present in all libraries. This is the first detailed description of these patches in the Antarctic and demonstrates the link between physico-chemical factors and microbial and infaunal community structure. It appears that this phenomenon may be driven by the formation and persistence of sea-ice, and as both the spatial extent of sea-ice and its persistence in polar regions are likely to change under predicted climate change scenarios, we suggest this is a previously undocumented mechanism for climate change to impact polar ecosystems.

New series of corers for taking undisturbed vertical samples of soft bottom sediments

A new series of sediment corers--a Single Tube Gravity Corer and a Multiple Tube Gravity Corer has been developed for taking undisturbed vertical soft sediment samples, and a Hydraulic Pressure Multiple Tube Corer has been preliminarily designed. The Single Tube Gravity Corer can be operated by hand easily without using a winch, and it might be the simplest and the cheapest core sampler available. The Multiple Tube Gravity Corer was developed based on the Single Corer, which normally consists of three coring tubes, however, it is possible to install up to six tubes. The Hydraulic Pressure Multiple Tube Corer has been designed based on the Multiple Corer, which would take undisturbed sediment samples, and would be controlled by a computer and monitored by an underwater video camera. The pore water collection and the sediment Eh measurement-subsampling process through the transparent plastic tube wall is straightforward. Compared to other types of coring devices, such as the famous KC Kajak sediment core sampler made in Denmark; these corers have greater reliability, being simple in structure, relatively light weight, easy and efficient to operate, highly adaptable, and relatively inexpensive to produce.

Mangrove reforestation in Vietnam: the effect of sediment physicochemical properties on nutrient cycling

Sediment physicochemical properties most likely control the reforestation success on degraded mangrove sites. Our objectives were (1) to determine the nutritional status of reforested mangrove stands; (2) to investigate the effects of the redox potential (Eh) and pH on phosphorus (P) and nitrogen (N) cycling; and (3) to assess the effect of pH on P speciation. Five transects were studied in replanted stands of the Saigon River Delta, Vietnam. Spatial gradients of sediment Eh and pH were affected by the tidal regime and pyrite oxidation. Sediment Al/Fe-P correlated with Eh or pH, depending on the sediment layer, whereas Ca-P, Morgan-P (available P) and leaf P were influenced by the pH. The highest concentrations of Al/Fe-P were recorded at pH 6.5 probably due to adsorption effects. Sediment Ca-P increased strongly at pH below 4 and above 6, reflecting the different pH-dependent solubilities of individual Ca-P mineral species as shown by a newly developed method for P species quantification. A strong increase of available P and leaf P above pH 6, and positive correlations of available P vs Ca-P (P < 0.001) and leaf P (P = 0.004) suggest that the Morgan’s reagent for available P determination solubilizes mainly thermodynamically less stable Ca-P compounds, which seem to fuel P plant uptake. Since foliar and sediment N:P ratios were influenced mainly by pH, shifts in pH likely cause limitation transitions. Overall, sediment pH rather than Eh was found to control the nutrient status.

Effect of Nitrate on Phosphorus Mobilization from Bottom Sediment in Shallow Eutrophic Lakes

Seasonal variation in phosphorus release potential from bottom sediment and environmental factors affecting phosphorus mobilization in shallow eutrophic lakes, Lakes Nishiura and Kitaura in Japan, were investigated. At uplake sites, the fluxes were low and showed no seasonal variation. The Eh in the sediments at these sites was positive and in an oxidizing state, whereas sediments at other sites were in a reducing state. The oxidation/reduction states were not affected by the presence of DO in the boundary layer water. DO in the boundary layer only oxidized the surface of sediment layer and affected DIP release from the sediment. The oxidation of sediment was due to nitrate input from inflowing rivers. At mid-lake sites, where the DIP release flux was high, the Eh in sediments was increased by nitrate input. Nitrate, which is an electron acceptor in the boundary layer, diffused into the sediment and oxidized the sediment layer in contrast to the definitive oxidation attributed to DO. The DIP release flux decreased with increasing nitrate supply rates. The sediment record shows that the oxidation state was maintained by nitrate supply, which is affected by the decomposition rate of nitrate and strongly affects DIP release from lake sediments under anaerobic conditions.