Chemical Buffering Capacity Research Articles

The Ocean Carbon Cycle

The ocean holds vast quantities of carbon that it continually exchanges with the atmosphere through the air-sea interface. Because of its enormous size and relatively rapid exchange of carbon with the atmosphere, the ocean controls atmospheric CO2 concentration and thereby Earth's climate on timescales of tens to thousands of years. This review examines the basic functions of the ocean's carbon cycle, demonstrating that the ocean carbon inventory is determined primarily by the mass of the ocean, by the chemical speciation of CO2 in seawater, and by the action of the solubility and biological pumps that draw carbon into the ocean's deeper layers, where it can be sequestered for decades to millennia. The ocean also plays a critical role in moderating the impacts of climate change by absorbing an amount of carbon equivalent to about 25% of anthropogenic CO2 emissions over the past several decades. However, this also leads to ocean acidification and reduces the chemical buffering capacity of the ocean and its future ability to take up CO2. This review closes with a look at the uncertain future of the ocean carbon cycle and the scientific challenges that this uncertainty brings.

Assessment of semi-dynamic leaching characteristics of lead and zinc from stabilized contaminated soil using sustainable phosphate-based binder after carbonation

Stabilized contaminated soils face long-term complex environmental effects in the process of sustainable resource reuse. Therefore, semi-dynamic leaching tests in this study were conducted to investigate the leaching characteristics of novel sustainable phosphate binder stabilized soil contaminated with lead (Pb) and zinc (Zn) after carbonation in various simulated acid rain solutions to verify the sustainability of stabilization. Immobilization mechanisms of Pb and Zn, carbonation impact, leaching concentrations, pH, electrical conductivity, desorption parameters, leaching mechanisms, observed/effective diffusion coefficient (Dobs/Deff), and leachability index were investigated. Complexation or adsorption of heavy metals can reduce the number of exchangeable ions, especially after carbonation. Carbonization showed a beneficial effect, indicating that the stabilized materials are a good carbon catcher. The leaching mechanism of Pb and Zn were all controlled by diffusion, which was caused by complex destruction and desorption. Dobs values for Pb ranged from 10−19 to 10−16 m2/s, and those for Zn from 10−17 to 10−13 m2/s. Therefore, the mobilization of Zn was more intense than that of Pb. Deff values, calculated through the desorption parameter, were ∼3–4 orders of magnitude larger than Dobs values. The leaching characteristics of Pb and Zn were influenced by leachant pH and carbonation, which were related to electrical conductivity and the cumulative leaching mass of magnesium. The effects had an influence on hydration products' chemical stability and buffering capacity. The stabilized materials can be used as recycled material in engineering, judging from the leachability index. This study can improve researchers' and engineers' understanding of the environmental leaching and toxicity of stabilized contaminated soil during reutilization, increasing reliability in reusing contaminated soil resources.

Improvement of the chemical properties and buffering capacity of coastal sandy soil as affected by clays and organic by-product application

The main problem with coastal sandy soil is its low water and nutrient retention due to its low clay and organic matter content. This study was aimed at improving the chemical properties and buffering capacity of these soils by using ameliorants of clay and organic polymers. The leaching experiment was conducted with two factors and three replications. The first factor was a clay ameliorant (5% clay, whether from the soil type Inceptisol (I) and the soil type Vertisol (V)). The second factor was a natural or synthetic organic polymer (tapioca 1% and 2% (T1 and T2), tapioca dregs 1% and 2% (TD1 and TD2), polyvinyl alcohol 0.1% and 0.2% (P1 and P2)). The leaching was carried out at 1-month intervals and the leachate was collected for the analysis of the soluble Ca, Mg, K and Na. The leaching was stopped after all the treatments reached the electrical conductivity values < 100 μS/cm. The ameliorants of clay (I or V) and natural polymer (T or TD) significantly increased the cation exchange capacity, the available cations, and the buffering capacity of the coastal sandy soil. The single treatment of I was better than V in increasing the available Mg, while the combination with organic natural polymers could increase the available Ca and K. The treatment of ITD2 was able to increase the soil buffering and maintain the soluble Ca, Mg and K in the coastal sandy soil. Therefore, TD which is a by-product of the tapioca flour industry when combined with I has the potential to be a prospective ameliorant for coastal sandy soils.

Initiation of soil formation in weathered sulfidic Cu-Pb-Zn tailings under subtropical and semi-arid climatic conditions

Field evidence has been scarce about soil (or technosol) formation and direct phytostabilization of base metal mine tailings under field conditions. The present study evaluated key attributes of soil formation in weathered and neutral Cu-Pb-Zn tailings subject to organic amendment (WC: woodchips) and colonization of pioneer native plant species (mixed native woody and grass plant species) in a 2.5-year field trial under subtropical and semi-arid climatic conditions. Key soil indicators of engineered soil formation process were characterized, including organic carbon fractions, aggregation, microbial community and key enzymatic activities. The majority (64–87%) of the OC was stabilized in microaggregate or organo-mineral complexes in the amended tailings. The levels of OC and water soluble OC were elevated by 2–3 folds across the treatments, with the highest level in the treatment of WC and plant colonization (WC+P). Specifically, the WC+P treatment increased the proportion of water stable macroaggregates. Plants further contributed to the N rich organic matter in the tailings, favouring organo-mineral interactions and organic stabilization. Besides, the plants played a major role in boosting microbial biomass and activities in the treated tailings. WC and plants enhanced the contents of organic carbon (OC) associated with aggregates (e.g., physically protected OC), formation of water-stable aggregates (e.g., micro and macroaggregates), chemical buffering capacity (e.g., cation exchange capacity). Microbial community and enzymatic activities were also stimulated in the amended tailings. The present results showed that the formation of functional technosol was initiated in the eco-engineered and weathered Cu-Pb-Zn tailings under field conditions for direct phytostabilization.

Change of ergosterol content after inorganic N fertilizer application does not affect short-term C and N mineralization patterns in a grassland soil

N released from soil organic matter and plant residues is an important source of available N in permanent grassland. Inorganic fertilizer application alters root biomass and quality and therefore changes the input and turnover of soil organic matter. Little is known about how intensification of N fertilization affects C and N mineralization processes and thus contributes to N losses and C emissions. It remains unclear if N fertilizer addition positively or negatively affects soil microbial biomass carbon and soil organic C stored in the short- and long-term. Our research investigated whether increased N fertilization of permanent grassland soil affects the microbial community and if this is reflected by a change in mineralization processes from soil organic matter and Lolium perenne roots. We performed an aerobic incubation experiment on samples from a field trial on permanent grassland with varying factors: fertilizer type and N level, and presence of roots. Inorganic fertilization and root presence caused a shift in microbial community structure with an increase of saprotrophic fungi, as indicated by ergosterol content and ergosterol-to-MBC ratio. However, no significant effects of organic or inorganic fertilizer application on mineralization processes and soil microbial biomass were observed, reflecting a strong resistance of the present grassland soil system. This is probably caused by the high level of microbial biomass and clay content, providing a large chemical and biological buffer capacity. Overall, our results indicate, that the microbial community and mineralisation processes are not affected by intensification of N fertilization.

Alteration of MX-80 bentonite backfill material by high-pH cementitious fluids under lithostatic conditions – an experimental approach using core infiltration techniques

Abstract We characterize and quantify processes at a cement–bentonite interface spatially and temporally during a long-term core infiltration experiment. A young ordinary Portland cement pore fluid (K + –Na + –OH − : pH 13.4) was infiltrated into a MX-80 bentonite core with an initial saturated density of 1920 kg m −3 that shifted to 1890–1930 kg m −3 after 761 days. A hydrostatic external pressure of 4.1 MPa and an infiltration pressure of 2.1 MPa were applied in a triaxial-type apparatus. A decrease in hydraulic conductivity from approximately 2.2×10 −13 to approximately 4.2×10 −15 m s −1 was observed passing from advection-dominated flow to a diffusion-dominated regime. Sulphate replaced chloride in the outflow during the high-pH infiltration period controlled by the dissolution of gypsum, the uptake of K + by ion exchange, and complex mineral reactions occurred near the inlet. X-ray computed tomography (CT) scans performed repeatedly during the experiment tracked a progressing hemispherical reaction plume in the first millimetres of the bentonite, revealing a zone of bulk density increase. This zone consisted of two distinct, but overlapping, zones of Mg- and Ca-enrichment related to precipitation of saponite and calcite. The experiment attested an effective chemical buffering capacity for bentonite, a progressing coupled hydraulic–chemical sealing process and also the preservation of the physical integrity of the interface region in this set-up with a total pressure boundary condition on the core sample.

Quantifying air–sea re-equilibration-implied ocean surface CO2 accumulation against recent atmospheric CO2 rise

To understand better the role of ocean surface chemical buffering capacity in mitigating the recent atmospheric CO2 rise, we investigated potentials of wintertime mixed-layer dissolved inorganic carbon (DIC) to increase after re-equilibration with decadal atmospheric CO2 rise and the corresponding anthropogenic CO2 accumulation rates, based upon carbonate system chemistry over the global ocean surface above the wintertime thermocline. This is an idealized study to quantify the isolated effect of atmospheric CO2 rise on the DIC content of the global mixed layer, assuming all else constant. Our results show that the potentials of wintertime DIC over the global open ocean surface to rise after re-equilibration with the elevated atmospheric CO2 mol fraction in a reference year 2000 ranged from 0.28 to 0.70 μmol kg−1 ppm−1 (ppm = parts of CO2 per million dry air), while the global mean wintertime sea surface DIC increase rate was close to 1.0 μmol kg−1 year−1. From 1995 to 2005, the decadal mean atmospheric CO2 rise implies an anthropogenic CO2 accumulation rate of 0.40 × 1015 g C year−1 within the global ocean surface. From the 1960s to 2000s, the air–sea re-equilibration-implied ocean surface anthropogenic CO2 accumulation rate may have increased by 46 % due to the accelerated atmospheric CO2 rise. However, the chemical buffering capacity within the ocean surface may have declined by 16 % during the same period.

Rapid acidification of mode and intermediate waters in the southwestern Atlantic Ocean

Abstract. Observations along the southwestern Atlantic WOCE A17 line made during the Dutch GEOTRACES-NL programme (2010–2011) were compared with historical data from 1994 to quantify the changes in the anthropogenic component of the total pool of dissolved inorganic carbon (ΔCant). Application of the extended multi-linear regression (eMLR) method shows that the ΔCant from 1994 to 2011 has largely remained confined to the upper 1000 dbar. The greatest changes occur in the upper 200 dbar in the Subantarctic Zone (SAZ), where a maximum increase of 37 μmol kg−1 is found. South Atlantic Central Water (SACW) experienced the highest rate of increase in Cant, at 0.99 ± 0.14 μmol kg−1 yr−1, resulting in a maximum rate of decrease in pH of 0.0016 yr−1. The highest rates of acidification relative to ΔCant, however, were found in Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The low buffering capacity of SAMW and AAIW combined with their relatively high rates of Cant, increase of 0.53 ± 0.11 and 0.36 ± 0.06 μmol kg−1 yr−1, respectively, has lead to rapid acidification in the SAZ, and will continue to do so whilst simultaneously reducing the chemical buffering capacity of this significant CO2 sink.

Recycling Wood Composite Panels: Characterizing Recycled Materials

Downgraded medium density fiberboard (MDF), particleboard (PB), and oriented strandboard (OSB) panels were individually subjected to steam explosion treatment. Downgraded MDF and PB panels were separately treated with thermal chemical impregnation using 0.5% butanetetracarboxylic acid (BTCA). And downgraded PB panels were processed with mechanical hammermilling. The pH, buffer capacity, fiber length, and particle size of these recycled materials were evaluated. After the steam explosion and thermal chemical impregnation treatments, the pH and buffer capacity of recycled urea formaldehyde resin (UF)-bonded MDF and PB furnishes increased and the fiber length decreased. The hammermilling of recycled PB was less likely to break particles down into sizes less than 1 mm2.

Spring carbonate chemistry dynamics of surface waters in the northern East China Sea: Water mixing, biological uptake of CO2, and chemical buffering capacity

AbstractWe investigated sea surface total alkalinity (TAlk), dissolved inorganic carbon (DIC), dissolved oxygen (DO), and satellite‐derived chlorophyll‐a in the connection between the Yellow Sea and the East China Sea (ECS) during April to early May 2007. In spring, Changjiang dilution water (CDW), ECS offshore water, and together with Yellow Sea water (YSW) occupied the northern ECS. Using 16 day composite satellite‐derived chlorophyll‐a images, several algal blooms were identified in the CDW and ECS offshore water. Correspondingly, biological DIC drawdown of 73 ± 20 μmol kg−1, oversaturated DO of 10–110 μmol O2 kg−1, and low fugacity of CO2 of 181–304 μatm were revealed in these two waters. YSW also showed CO2 uptake in spring, due to the very low temperature. However, its intrusion virtually counteracted CO2 uptake in the northern ECS. In the CDW and the ECS offshore water, Revelle factor was 9.3–11.7 and 8.9–10.6, respectively, while relatively high Revelle factor values of 11.4–13.0 were revealed in YSW. In the ECS offshore water, the observed relationship between DIC drawdown and oversaturated DO departed from the Redfield ratio, indicating an effect of chemical buffering capacity on the carbonate system during air‐sea reequilibration. Given the fact that the chemical buffering capacity slows down the air‐sea reequilibration of CO2, the early spring DIC drawdown may have durative effects on the sea surface carbonate system until early summer. Although our study is subject to limited temporal and spatial coverage of sampling, these insights are fundamental to understanding sea surface carbonate chemistry dynamics in this important ocean margin.

A geochemical comparison of modern and holocene mangrove sediments, Townsville region, North Queensland

The geochemistry of coastal sediment is important in terms of potential soil acidification and the resultant mobilisation of metals. This paper reports geochemical analyses for mangrove sediments from the Townsville region, North Queensland. Vibrocores were taken from modern swamp (ca. 200 B.P.), and from buried Holocene mangroves of the inner shelf of Cleveland Bay. Although the average pH (ca. 5) and Eh (ca. -250mV) is similar for both the modern and early Holocene mangrove sediments, the pH and Eh are more variable in the younger mangrove sediments. Strongly acidic conditions (ca. pH 2.5) exist in the recently buried mangrove sediments, mainly caused by the degradation of organic matter. the porewater chemistry of early Holocene mangrove sediments suggests that acid-sulphate material may be preserved under marine and anoxic conditions because of limited exchange with seawater. The clay-rich mature of the mangrove sediments in the central Great Barrier Reef region enhances their chemical buffering capacity, the main buffers being clay (silicate) minerals, carbonates, iron-oxyhydroxides and organic matter. This leads to a reduced potential for acid-sulphate soil development and metal mobilisation.

Mesh-capped probe design for direct pH measurements at an actively corroding metal surface

Quantitative characterization of surface reaction species improves understanding of reaction mechanisms, especially for heterogeneous reactions such as in corrosion and electrolysis. A universal mesh-capped surface pH probe was developed in order to investigate surface proton concentrations for aqueous metal dissolution/deposition reactions. This technique can be generally applied to characterize the surface pH either for bare metal reactions or metals corroding beneath a corrosion product film. Measurements with the new probe design have shown a good degree of reproducibility. The surface pH measurements during corrosion of mild steel have shown that a higher surface pH is achieved at the steel surface as compared with that of the bulk solution, just as predicted from theory. Chemical buffering effects on surface pH were observed during corrosion by contrasting the measurements obtained in HCl deaerated solutions with those containing CO2, i.e., carbonic acid (H2CO3) and acetic acid (CH3COOH/HAc) solutions. It was found that the surface pH was mainly controlled by the water chemistry and chemical buffer capacity. The surface pH is affected by the deposited corrosion scale porosity. A higher deviation of surface pH compared to the bulk value was observed for more dense layers with lower porosity.

Iron–sulphur–phosphorus Interactions: Implications for Sediment Buffering Capacity in a Mediterranean Eutrophic Lagoon (Sacca di Goro, Italy)

The interactions among sedimentary cycles of sulphur, iron and phosphorus were investigated in the eutrophic Sacca di Goro lagoon (Northern Adriatic coast, Italy) in order to assess the iron buffering capacity of the sediment. Three stations were chosen, which represented different primary producer communities, hydrodynamics and sediment characteristics. Station G was close to the outlet of the Po di Volano river, station 4 was in the central part of the lagoon under tidal influence and station 17, in the sheltered zone, was affected by macroalgal blooms. From January 1997 to January 1998, sediment cores were sampled approximately every 2 months. In parallel, temperature, salinity and dissolved oxygen were determined in the water column. Sedimentary profiles of Eh, pH, dissolved sulphide (DS), acid volatile sulphide (AVS), chromium reducible sulphur (CRS), iron and phosphorus pools were determined in the 0-10 cm sediment horizon. Bacterial sulphate reduction rates were measured only at station 17. Iron pools, AVS and CRS followed similar patterns at all the considered stations, with lower values in the 0-2 cm sediment horizon and peaks in the deepest sediment layer (5-10 cm horizon). Overall, the labile Fe accounted for an annual average of 115.6 ± 3.0 μmol cm -3 , with peaks of labile ferric iron up to 40 μmol cm -3 . The great iron availability and reactivity accounted for an efficient buffering capacity against sulphides, with the accumulation of AVS and CRS, specially at station G where the iron buffer was replenished by iron-rich freshwaters. At station 17, in spite of a great iron availability the buffering capacity was less efficient due to macroalgal blooms and accumulation of labile organic matter, whose decomposition stimulated bacterial sulfate reduction and the inherent sulphide production both in the pore-water (DS: 4.0 ± 3.7 mM) and in the deepest water column (DS: 85 μM). The iron and sulphur interactions had also implications for P cycling, since in summer the ferric iron reduction was accompanied by a significant increase of pore-water and exchangeable phosphates. The AVS to labile Fe and AVS to TRIS (AVS + CRS) ratios were used to assess the sediment capacity of precipitating and retaining sulphides. These ratios were then compared with the chemical buffer capacity, demonstrating their suitability as indicators of buffering capacity towards sulphides.

Identifying calcium sources at an acid deposition-impacted spruce forest: a strontium isotope, alkaline earth element multi-tracer approach

Depletion of calcium from forest soils has important implications for forest productivity and health. Ca is available to fine feeder roots from a number of soil organic and mineral sources, but identifying the primary source or changes of sources in response to environmental change is problematic. We used strontium isotope and alkaline earth element concentration ratios of trees and soils to discern the record of Ca sources for red spruce at a base-poor, acid deposition-impacted watershed. We measured 87Sr/86Sr and chemical compositions of cross-sectional stemwood cores of red spruce, other spruce tissues and sequential extracts of co-located soil samples. 87Sr/86Sr and Sr/Ba ratios together provide a tracer of alkaline earth element sources that distinguishes the plant-available fraction of the shallow organic soils from those of deeper organic and mineral soils. Ca/Sr ratios proved less diagnostic, due to within-tree processes that fractionate these elements from each other. Over the growth period from 1870 to 1960, 87Sr/86Sr and Sr/Ba ratios of stemwood samples became progressively more variable and on average trended toward values that considered together are characteristic of the uppermost forest floor. In detail the stemwood chemistry revealed an episode of simultaneous enhanced uptake of all alkaline earth elements during the growth period from 1930 to 1960, coincident with reported local and regional increases in atmospheric inputs of inorganic acidity. We attribute the temporal trends in stemwood chemistry to progressive shallowing of the effective depth of alkaline earth element uptake by fine roots over this growth period, due to preferential concentration of fine roots in the upper forest floor coupled with reduced nutrient uptake by roots in the lower organic and upper mineral soils in response to acid-induced aluminum toxicity. Although both increased atmospheric deposition and selective weathering of Ca-rich minerals such as apatite provide possible alternative explanations of aspects of the observed trends, the chemical buffering capacity of the forest floor-biomass pool limits their effectiveness as causal mechanisms.

Relationships between sediment chemical buffering capacity and H2S accumulation: comparative study in two temperate estuarine brackish lagoons

To reveal the mechanisms of sedimental H2S accumulation, annual investigations on sedimental environments were conducted in two temperate estuarine lagoons. The lagoons, Gamo and Idoura (Japan), have similar shapes, locations, and topographical properties but different degrees of H2S accumulation. Water stagnation causes a high phytoplankton biomass (Chl. a; 26–52 μg l−1) in the inner Gamo Lagoon. Gamo Lagoon sediment was characterized by high bounded sulfides (bounded S mainly FeS) and H2S contents, and low C/N ratios (mean = 10.4) and iron (reactive Fe2+ and total Fe) contents. H2S was not detected in Idoura Lagoon where phytoplankton biomass was much lower (Chl. a; 0.6–4 μg l−1). Idoura Lagoon sediment had high C/N ratios (mean=17.9) and high iron contents. The C/N ratio difference implies that organic matter in Gamo Lagoon originates mainly from more decomposable phytoplankton, while organic matter in Idoura Lagoon derives mainly from terrestrial vascular plants with lower decomposability. The excess loading of phytoplanktonic detritus in Gamo accelerates sedimentary microbial activity, including sulfate reduction (i.e., H2S production). High Fe2+and low bounded S contents in Idoura sediment indicate a high chemical buffering capacity toward H2S. In contrast, almost all Fe2+ in Gamo Lagoon had already reacted with H2S as FeS. H2S accumulation in Gamo Lagoon is caused by low sedimentary chemical buffering capacity toward H2S, as well as higher microbial H2S production, caused by the excess loading of phytoplanktonic detritus.

Geochemical evaluation of different groundwater–host rock systems for radioactive waste disposal

The geochemical suitability of a deep bedrock repository for radioactive waste disposal is determined by the composition of geomatrix and groundwater. Both influence radionuclide solubility, chemical buffer capacity and radionuclide retention. They also determine the chemical compatibility of waste forms, containers and backfill materials. Evaluation of different groundwater–host rock systems is performed by modeling the geochemical environments and the resulting radionuclide concentrations. In order to demonstrate the evaluation method, model calculations are applied to data sets available for various geological formations such as granite, clay and rocksalt. The saturation state of the groundwater–geomatrix system is found to be fundamental for the evaluation process. Hence, calculations are performed to determine if groundwater is in equilibrium with mineral phases of the geological formation. In addition, corrosion of waste forms in different groundwater is examined by means of reaction path modeling. The corrosion reactions change the solution compositions and pH, resulting in significant changes of radionuclide solubilities. The results demonstrate that geochemical modeling of saturation state and compatibility of the host formation environment with the radioactive waste proves to be a feasible tool for evaluation of various sites considered as deep underground repositories.

Chemical Characteristics and Acid Buffering Capacity of Surface Soils in Japanese Forests

To determine the geological distribution of acid buffering capacity and exchangeable Al of forest soils in Japan, surface soils under forest vegetation were collected nationwide from a total of 1,034 sites. Generally, surface soils in Japanese forests are mostly acidic and low in both exchangeable cation content and exchangeable Al. The median of soil pH(H2O), total exchangeable cations, and exchangeable Al are 5.1, 76 mmol(+)Kg−1, and 19.6 mmol(+)kg−1, respectively. Acid buffering capacities of selected soils were determined using a soil column, which was comparable to the capacity that resulted from cation exchanges with protons. Soils with high buffering capacity and low exchangeable Al are widely distributed in Japan, and overlap with the areas of Andisol distribution. Volcanogenic materials seem to mask soil characteristics derived from underlying geology even though they are not Andisols. However, central to western Honshu Island, Shikoku Island, and northern Kyushu Island showed weak acid buffering capacities with high exchangeable Al potential in surface soils.

Microbiological and environmental variables involved in the sulfide buffering capacity along a eutrophication gradient in a coastal lagoon (Bassin d'Arcachon, France)

Microbiological and environmental variables involved in the removal of free sulfide were studied along an eutrophication transect in the Bassin d'Arcachon (France). At four sites, analyses were carried out on reduced sulfur compounds, iron species and total numbers of viable sulfur bacteria (sulfide-producing bacteria, colorless sulfur bacteria and purple sulfur bacteria). In addition, the chemical buffering capacity towards free sulfide and the potential microbiological sulfide oxidation rates were determined. In the ecosystem, no free sulfide occurs in the top layers of the sediment at all four sites, despite a high nutrient load and hence favourable conditions for sulfide-producing bacteria. The explanation of this apparent discrepancy was shown to be the high biological sulfide oxidizing capacity in combination with a high chemical buffering capacity. The data presented illustrate that the buffering capacity of sediments towards free sulfide is the combined result of the chemical and biological processes. The ratio between these were found to depend on the degree of eutrophication. It was shown that the chemical buffering capacity towards sulfide is severely overestimated when based on the pool of chemically reactive iron, a more realistic value is obtained by estimating the total amount of sulfide that can be added before free sulfide can be detected. A clear difference was observed between the numbers of colorless sulfur bacteria and the activity of the entire population. For a proper quantification of the sulfide buffering capacity of sediments, it is essential to estimate the concentration of iron and sulfur compounds that actually can react with sulfide, as well as to analyze the activities of sulfide-oxidizing microbes.

Sulfide-induced release of phosphate from sediments of coastal lagoons and the possible relation to the disappearance of Ruppia sp.

The production and consumption of sulfide and its influence on phosphorous cycling were studied in a hypertrophic coastal lagoon (Valle Smarlacca, Italy). Oxygen measurements revealed that the water phase was supersaturated except for the layer directly overlying the sediment. This layer was devoid of oxygen and contained sulfide at all times. Maximal rates of sulfide production, calculated from in situ profiles, were observed in the 0 to 2 cm sediment layer and the 1 cm water layer directly above. Sediment iron data suggested a moderate chemical buffering capacity towards free sulfide; however, the in situ buffering capacity was fully exploited. Stirring increased the chemical buffer, indicating that, in situ, part of the iron did not contribute to the chemical buffer. The potential rate of biological sulfide oxidation, estimated in sediment slurries amended with oxygen, was high; however, the actual rate was low due to a shortage of oxygen in the sediments and the overlying water. Evidence was obtained for enhanced release of phosphate caused by free sulfide. Under simulated natural conditions the release of phosphate exceeded the initial concentration of Fe-bound phosphate by an order of magnitude, indicating a significant contribution of non-iron- bound phosphate. The observations in Valle Smarlacca were used to shed Light on the virtually complete disappearance of Ruppia sp, from other lagoons of the Valli di Comacchio (Northern Italy). [KEYWORDS: eutrophication; phosphate mobilization; sulfide production; Ruppia; coastal lagoons Marine-sediments; bassin-darcachon; organic-matter; phosphorus; iron; ecosystems; fractionation; reduction; exchange; ferrara]

The Buffering Capacity Towards Free Sulphide in Sediments of a Coastal Lagoon (Bassin d’Arcachon, France)—the Relative Importance of Chemical and Biological Processes

The Bassin d’Arcachon (south-west France) was chosen as a model ecosystem to study the chemical and microbiological buffering towards free sulphide. Data were collected on the vertical distribution of oxygen, sulphur and iron compounds and the vertical distribution of colourless sulphur bacteria and sulphide-producing bacteria. In addition, data on the chemical and biological buffering capacity towards free sulphide were collected in sediment slurries from defined depth layers using a biological sulphide and oxygen monitor (BOSM) equipped with electrodes for oxygen, sulphide, redox and pH. The data showed that a substantial population of aerobic sulphide-oxidizing bacteria was present, yet buffering towards free sulphide could mainly be attributed to chemical processes: in particular, reactions with iron were of importance. Interestingly, the potential microbiological rate of sulphide oxidation was orders of magnitude higher than the rate of sulphate reduction reported for this ecosystem.The ecological implications of these observations for the Bassin d’Arcachon are that the powerful biological buffering capacity towards the free sulphide present will become effective after the chemical buffering capacity has been depleted. Under such conditions the colourless sulphur bacteria will no longer face the competition with iron, and thus may be expected to proliferate. The crucial factor then becomes the availability of oxygen.