Sequential fractionation of sediment phosphate

By means of sequential extractions with Ca-NTA and EDTA, a separation was performed between Fe(OOH) P and CaC0₃P in a few sediments; the remaining fraction, considered to be organic phosphate, was quantified as well. We found that with the commonly used method of extraction with NaOH and H₂S0₄, less Fe(OOH) P and much more CaC0₃ P was found than with the chelating extractants. The organic phosphate pool in live and dead algal material and in some mud samples was partly hydrolysed and therefore recovered as inorganic phosphates with classical extractions. The difference between chelating extractants and the classical ones is discussed.Abbreviations o-P: ortho phosphate (or its concentration) - org-P: organic phosphate - extr-P: extractable sediment bound phosphate - extr-Fe: extractable sediment bound iron - Fe(OOH) P: iron bound, sediment phosphate - CaCO₃ P: calcium bound, sediment phosphate - org-C: organic sediment bound carbon     

Binding of phosphate in sediment accumulation areas of the eastern Gulf of Finland,Baltic Sea

The relationships between P and components binding P were studied by analysing the concentrations of N, P, Fe, Mn, Ca and Al in sediments and pore water along the estuarine transect of the River Neva in August 1995. The high sediment organic matter concentration resulted in low surface redox potential and high pore-water o-P concentration, whereas the abundance of amphipods resulted in high surface redox potentials and low pore-water o-P concentration. However, despite the variation in sediment organic matter and the abundance of amphipods, very reduced conditions and slightly variable concentrations of Tot-P (0.7–1.1 mg g^–1 DW) were observed in the 10–15 cm sediment depth along the estuarine gradient, indicating that the pools of mobile P were largely depleted within the depth of 0–15 cm. Multiple regression analysis demonstrated that organic matter and Tot-Fe concentration of the sediment were closely related to the variation in Tot-P concentration of the sediments (r ² = 0.817, n=32). In addition, the high total Fe:P ratio suggested that there is enough Fe to bind P in sediments along the estuarine gradient. However, low Fe_diss concentrations in the pore water of reduced sediment (redox-potential <–50 mV) indicated efficient precipitation of FeS (FeS and FeS₂), incapable to efficiently bind P. Consequently, the low Fe_diss:o-P ratio (< 1) recorded in pore water in late summer implied that Fe³⁺ oxides formed by diffusing Fe_diss in the oxic zone of the sediments were insufficient to bind the diffusing o-P completely. The measured high o-P concentrations in the near-bottom water are consistent with this conclusion. However, there was enough Fe_diss in pore water to form Fe³⁺ oxides to bind upwards diffusing P in the oxic sediment layer of the innermost Neva estuary and the areas bioturbated by abundant amphipods.     

Characterization of phosphorus fractions in the sediments of a tropical intertidal mangrove ecosystem

Solid phases of phosphorus fractions in the surface and core sediments were studied to understand the biogeochemical cycling and bioavailability of phosphorus in the Pichavaram intertidal mangrove sediments of India. Total P in surface and core sediments ranged between 451–552 and 459–736 μg g⁻¹ respectively and Fe bound P was the dominant fraction. Low levels of Fe bound P in the mangrove zone than the two estuarine zones may be because of high salinity inhibition of phosphate adsorption onto the Fe-oxides/hydroxides. Post-depositional reorganization of P was observed in surface sediments, converting organic P and Fe bound P into the authigenic P. High levels of organic P in the mangrove zone is primarily due to intensive cycling and degradation of organic matter and adsorption of phosphate on the organic molecules. The burial rates and regeneration efficiency of P in the intertidal mangrove ecosystem ranged from 5.41 to 7.27 μmol P cm⁻² year⁻¹ and 0.122 to 0.233 μmol P cm⁻² year⁻¹, respectively. High burial efficiency (≈99%) of P proves the earlier observation of limiting nature of P for the biological productivity. Further, bioavailable P (exchangeable P + Fe bound P + organic P) constituted a considerable proportion of sedimentary P pool of which an average accounted for 55 and 50% in surface and core sediments respectively. The results indicate that significant amount of P is locked in sediments in the form of authigenic P and detrital P which makes P as a limiting nutrient for the biological productivity.     

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. Handling editor: J. Saros     

Variability of the sediment phosphate composition of a temporary pond (Doñana National Park,SW Spain)

This study reports on the spatial and temporal variability of the phosphate composition in the sediment of a temporary pond over a period of 3 years using the EDTA-method for P-fractionation. Sediment samples were collected at three different sites (open-water, littoral and flood plain) to compare the effect of the length of the wet/dry phase on the sediment phosphate composition, with special emphasis on the potential bioavailability of the P-fractions.Fine sediments (<0.1 mm) were rich in organic matter (9–25%) and contained high mean concentrations of Tot-P (182–655 mg kg^–1 d.w.), especially in the flood plain sediment. The sediment P composition was dominated by P-organic fractions at all sites (64–94%). The average C/N ratios were 8.8, 6.0 and 5.9 for sediments of the flood plain, littoral and open-water sites, respectively. The flood plain sediment was significantly poorer in iron-bound P (FeOOHP), but richer in the P-organic fractions extracted by EDTA than the sediment of the open-water site (P<0.01). The percentage of organic matter increased significantly in the sediment of the open-water site at the end of each dry season (P<0.05), while it decreased in the sediment of the flood plain site (P<0.01). In all sediments, the fraction of Fe(OOH)P decreased at the end of each dry season and some of these changes were significant (P<0.05). The decrease in the fraction of Fe(OOH)P was not related to changes in the sediment redox potential. Although the flood plain site was dry longer than the open-water site during the study period, the differences between the sediment composition of both sites were probably due to the effect of plant growth on the dry sediments of the flood plain site rather than to a direct effect of desiccation.     

The role of the ironhydroxide-phosphate-sulphide system in the phosphate exchange between sediments and overlying water

The accumulation of inorganic phosphate in lake sediments and a possible following release is due to the adsorption of phosphate onto Fe(OOH) and, especially in hard waters, to the precipitation of apatite. Attempts are made to quantify both processes.For the quantification of the P adsorbed, P_ads, onto Fe(OOH) the Freundlich adsorption isotherm, P_ads=A(o-P)^B, gave good results. The constants A and B could be quantified. Constant A appeared to depend on the pH and the Ca²⁺ and Mg²⁺ concentrations in the water. Constant B appeared to approach 0.333. The full equation becomes then: % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiaadcfadaWgaa% WcbaGaamyyaiaadsgacaWGZbaabeaakiabg2da9iaaikdacaaIZaGa% aGOnaiaaicdacaaIWaGaaiOlaiaacIcacaaIXaGaaGimamaaCaaale% qabaGaaGimaiaac6cacaaI0aacbiGaa8hCaiaa-HeaaaGccaGGPaGa% aiikaiaaikdacaGGUaGaaG4naiaaiEdacqGHsislcaaIXaGaaiOlai% aaiEdacaaI3aGaai4oaiaadwgadaahaaWcbeqaaiabgkHiTiaa-nea% caWFHbaaaOGaaiykamaakeaabaGaam4BaiabgkHiTiaadcfaaSqaai% aaiodaaaaaaa!57AF!\[P_{ads} = 23600.(10^{0.4pH} )(2.77 - 1.77;e^{ - Ca} )\sqrt[3]{{o - P}}\]. with the Ca concentration in mmol l^–1 and the o-P and P_ads concentrations in mg l^–1.For the quantification of the solubility of calcium-bound phosphate the solubility product of apatite being 10^–50, as found in the two hard water rivers Rhine and Rhone, was used. With this solubility product the solubility of o-P can be calculated as function of the Ca²⁺ concentration and the pH. The two equations, for adsorption and precipitation, are put together in a so-called solubility diagramme, which describes the o-P concentration as function of the Fe(OOH) concentration in the sediments, and the pH and the Ca²⁺ concentration in the overlying water.The release of phosphate from the Fe(OOH)P complex under anoxic conditions after adding H₂S in inorganic suspensions was shown to be limited. Only when a large excess of H₂S was added there was some release, but if less than 75% of the Fe(OOH) was converted into FeS, there was no release. The possibility of organic phosphate as the source of phosphate release under anoxic conditions is discussed. For a full understanding of this possibility, fractionation of sediment bound phosphate must be carried out in such a way, that these organic phosphates are not hydrolysed.This article is dedicated to the memory of Dr Kees de Groot, who died on 21 September 1994. He was a young enthusiastic, promising scientist who will be missed by all who have known him.     

Phosphorus speciation in Myall Lake sediment, NSW, Australia

The amount of phosphorus and its fractions in the sediment of Lake Myall, NSW, Australia, was assessed using a sequential extraction technique. Five sedimentary phosphorus reservoirs were measured, namely loosely sorbed phosphorus (NH₄Cl–P), iron associated phosphorus (BD–P), calcium bound phosphorus (HCl–P), metal oxide bound phosphorus (NaOH–P) and residual phosphorus (organic and refractory P, Res-P). Samples were taken from the deep and shallow sites of the lake. During the analysis, the average concentrations of each fraction of phosphorus was calculated. The results depicted that the total phosphorus (TP) content and chemically extractable phosphorus in both fine and coarse sediment fractions from the deep sites of the lake were significantly higher than those of the shallow sites, except for HCl–P extracted from the fine sediment fraction. Sediment TP was also strongly and positively correlated to sediment Fe. The phosphorus in the sediment mainly consisted of BD–P and Res-P, while NH₄Cl–P and HCl–P only contributed a minor part. The rank order of the different phosphorus extracts was similar for the two sites, namely Residual-P > BD–P > NaOH–P > HCl–P > NH₄Cl–P.     

Sequential extraction of trace metals and particle size distribution studies of Kainji Lake sediment,Nigeria

Abstract

Sequential extraction was carried out to determine the concentrations of some trace metals (Mn, Cu, Fe, Pb and Cd) in the Kainji lake sediments from seven different locations in four fractions: exchangeable, bound to iron and manganese oxide, bound to organic matters and residual. This was to undertaken to assess the environmental fate of these trace metals. The BCR technique, a modified form of the Tessier method of sequential extraction was used.

The proportion of the mean metal concentrations of the bioavailable metals follows the order Fe>Cu>Pb>Mn>Cd. Generally, Fe was most abundant metal in the sediment and about 40% was found in its bioavailable form. Although Cd contributed least to the bioavailable content, a greater percentage (~60%) was found in the bioavailable fraction. This suggests that Cd is highly mobile and, since it is known to be toxic, its concentration in the bioavailable form constitutes an environmental threat.

The contribution of metals bound to organic matter was found to be high and of the same magnitude as those bound to oxides and constitutes about 70% of the non-bioavailable metal contents. The petrological analysis of the sediments revealed that the sediments were predominantly quartz and rock clays with percussion marks and indentations.     

Factors influencing fractional phosphorus composition in sediments of Spanish reservoirs

Total phosphorus in sediment (P_sed) and its fractional composition (reactive phosphate extracted with NaOH, NaOH-RP, reactive phosphate extracted with HCl, HCl-RP, and residual phosphate, residual-P) have been determined in superficial sediments of 43 Spanish reservoirs located in different limnological regions and with different trophic states. Data were evaluated by statistical analysis to examine the influence of regional distribution and trophic status. Relations with calcium, manganese, iron and aluminium contents have also been studied.In the western part of Spain, reservoirs presented the highest values on average of P_sed, NaOH-RP and residual-P (1296, 328 and 877 µg g^–1 dw., respectively) and the lowest values of HCl-RP (91.0 µg g^–1 dw.). The main phosphorus fractions were residual-P (> 50%) and NaOH-RP (>10%). In the eastern area, P_sed NaOH-RP and residual-P attained the lowest values on average (502, 4 and 330 µg g^–1 dw., respectively), whereas HCl-RP presented the highest values (167 µg g^–1 dw.). The main fractions were residual-P (> 50%) and HCl-RP (> 25%).Trophic status seemed to be a secondary factor controlling P_sed. The highest contents of P_sed were found in eutroohic reservoirs, but only when those of the same region were compared, and the statistical significance (ANOVA F test) of the observed differences was very small (p < 0.057).     

Influence of outbreak of macroalgal blooms on phosphorus release from the sediments in Swan Lake Wetland,China

Macroalgal blooms have occurred worldwide frequently in coastal areas in recent decades, which dramatically modify phosphorus (P) cycle in water column and the sediments. Rongcheng Swan Lake Wetland, a coastal wetland in China, is suffering from extensive macroalgal blooms. In order to verify the influence of macroalgal growth on sediment P release, the sediments and filamentous Chaetomorpha spp. were incubated in the laboratory to investigate the changes of water quality parameters, P levels in overlying water, and sediments during the growth period. In addition, algal biomass and tissue P concentration were determined. In general, Chaetomorpha biomasses were much higher in high P treatments than in low P treatments. Compared with algae+low P water treatment, the addition of sediments increased the algal growth rate and P accumulation amount. During the algal growth, water pH increased greatly, which showed significant correlation with algal biomass in treatments with high P (P < 0.05). P fractions in the sediments showed that Fe/Al–P and organic P concentrations declined during the algal growth, and great changes were observed in algae+low P water+sediment treatment for both. As a whole, the sediments can supply P for Chaetomorpha growth when water P level was low, and the probable mechanism was the release of Fe/Al–P at high pH condition induced by intensive Chaetomorpha blooms.     

Spatial variation of phosphorus fractions in bottom sediments and the potential contributions to eutrophication in shallow lakes

Spatial variation of phosphorus fractions in bottom sediment, pore water and overlying water in three shallow eutrophic lakes, Nishiura, Kitaura and Sotonasakaura, Japan, and the contributions of the fractional P to mobilization of phosphorus from sediment were examined in this study. The vertical distributions of dissolved inorganic phosphorus (DIP) concentrations in overlying and pore water differed with lake and sampling site. In particular, DIP was high in pore water in the surface layer of the sediment for the middle to downlake areas of Lake Kitaura. DIP release flux calculated from a gradient of the concentrations at the sediment–water interface was high compared with other sites. The distribution of fractional P content in sediments was highly variable. The citrate–dithionite–bicarbonate–non-reactive phosphorus (CDB–NRP) fraction, in particular, differed greatly among the three lakes. According to correlation in the ratios between CDB–NRP and loss on ignition, sediments of these lakes were classified in three clusters. The CDB–NRP fraction was suggested to play a role in DIP release from sediment. The possibility of nitrate concentration playing a role in the control of DIP release was considered.     

Geochemical composition,phosphorus speciation and mass transport of fine-grained sediment in two Lake Erie tributaries

The concentration of major elements (Si, Al, Ca, Mg, Na, K, Fe, Ti, Mn and P), particulate phosphorus forms (NH₄Cl-RP, BD-RP, NaOH-RP, HCl-RP and NaOH₍₈₅₎-RP) and carbon content were determined in six size fractions (<8, 8–12, 12–19, 19–31, 31–42 and 42–<60 µm) of sediment collected at gauging stations located in two Lake Erie tributaries (Big Creek and Big Otter Creek). Concentrations of major elements and phosphorus forms were remarkably similar in sediment size fractions from both rivers. Nonapatite inorganic P (NAIP) and organic P (OP) concentrations increased with decreasing grain size while apatite inorganic P (AIP) content decreased with decreasing grain size. Results of phosphorus fractionation studies were combined with historical (particle size) and hydrometric data to simulate the export of particle P on tributary sediment < 63 µm. AIP represents 67 and 70% of the calculated particulate P mass while NAIP accounts for 26 and 23% of sediment-bound P transported in Big Otter Creek and Big Creek, respectively. The < 8 µm size fraction of tributary sediment is the most significant for the potential release of bioavailable P into the water column.     

Influence of nitrogen species (NH4 + and NO3 −) on the dynamics of P in water–sediment–Salvinia herzogii systems

Water – Salvinia herzogii – sediment systems were exposed to different phosphorus and nitrogen combinations in outdoor experiments. The aim was to estimate the amounts of P immobilized in macrophytes and sediments, as well as to elucidate whether or not the presence of N affects the retention of P. The following components were added: o-P, o-P + NH₄ ⁺, o-P + NO₃ ^– + NH₄ ⁺, o-P + NO₃ ^–. The concentration of nutrients was periodically determined throughout the experiment (28 days). The concentrations of P and N in plant tissues and sediments were determined at the beginning and the end of the experiment. Sequential extractions of P-fractions in sediment were performed using the EDTA method (Golterman, 1996). The removal efficiency of P in water was 95–99%. The removal of NH₄ ⁺ (97–98%) was more effective than that of NO₃ ^– (44–86%). The presence of nitrogen species increased the removal velocity of o-P from water, NH₄ ⁺ was the most effective species. Sediments not only had higher P removal rates than macrophytes but, in the control treatment without macrophytes, they reached the values obtained by macrophytes plus sediments in the other treatments. The adsorption of P takes place at the surface layer of the sediment (1 cm). Most of the P incorporated into the sediment during the experiment was sorbed by the fraction Fe(OOH)P. The addition of nutrients to water modified the leaves/lacinias weight ratio.     

Effects of temporary desiccation on the mobility of phosphorus and metals in sulphur-rich fens: differential responses of sediments and consequences for water table management

In the Netherlands, permanent damming of sulphate (SO₄ ^2–)-rich surface water, in order to rewet desiccated wetlands, has resulted in stagnation and eutrophication of surface water. Permanent damming of surface water prevents periodic drought during summer and leads to suppressed iron (Fe)-rich groundwater input and to stimulated SO₄ ^2– reduction, all likely stimulating depletion of reducible Fe in the sediment. A laboratory experiment was conducted to assess the importance of temporary desiccation, its differential effects on various sediment types and the consequences for water table management. Permanent high SO₄ ^2–-rich surface water tables above sediments that are indirectly affected by shallow groundwater flows, resulted in severe eutrophication. The effect of temporary desiccation on phosphorus (P) mobilization and immobilization strongly depended on the sediment Fe and P pools in combination with the buffering capacity of the sediment. Desiccation of sediment that is indirectly affected by shallow groundwater flows, led to a long-lasting reduction in phosphate (o-PO₄ ^3–) release from the sediment because a reduced Fe pool is present, resulting in the release of Fe upon oxidation. Formerly dry sediments that have not been influenced by groundwater for a long time do not possess such a reduced Fe pool and desiccation did not reduce P-release from these sediments resulting in considerable eutrophication of the water layer immediately after rewetting. In sediment of seepage zones that are directly and permanently influenced by deeper groundwater, reduced Fe and calcium levels are so high that o-PO₄ ^3– was effectively immobilized under oxidized as well as reduced conditions. The results indicate that restoration of desiccated wetlands can not be achieved by simply retaining water by means of constructed dams. If water retention is artificially increased, temporary drops in water level during the summer are necessary to recharge the reducible P-binding Fe pool in large zones of the wetlands in order to prevent eutrophication.     

Phosphorus fractions and alkaline phosphatase activity in sediments of a large eutrophic Chinese lake (Lake Taihu)

Spatial, vertical, and seasonal variations in phosphorus fractions and in alkaline phosphatase activity (APA) were investigated in sediments in a large-shallow eutrophic Chinese lake (Lake Taihu) in 2003–2004. The phosphorus content was highest in the most seriously polluted lake area. Iron-bound phosphorus (Fe(OOH)～P) dominated (47% on average) among the phosphorus fractions determined according to Golterman (Hydrobiologia 335:87–95, 1996). Notably, organically-bound P comprised a further significant additional portion (acid-soluble + hot NaOH-extractable organic P = 25%), which was highest at the most polluted sites. The Fe(OOH)～P content was the lowest in spring (April, 2004), suggesting that degradation of organic matter led to the release of iron-bound phosphates. Sediment APA showed a significant positive relationship with both organically-bound P and Fe(OOH)～P. Consequently, organically-bound P is an important portion of the sediment phosphorus in Lake Taihu. It is mainly derived from freshly-settled autochthonous particles and from external discharges. Organically-bound P induces APA and may lead to the release of bioavailable phosphates from the organic sediments, thereby accelerating lake eutrophication.     

Sedimentary phosphorus fractions and bioavailability as influenced by repeated sediment resuspension

Concentrations of phosphorus (P) fractions and changes in their bioavailability in the sediments as influenced by repeated resuspension were determined by sequential fractionation in laboratory experiments. The water and sediment samples used were taken from the campus canal. Sequential fractionation indicated that the concentrations of the iron bound P (BD–P) were predominant, consisting of over 50% of total P (Tot-P) in the sediments that did and did not undergo resuspension. BD–P mobility was reduced due to resuspension resulting from the decline of the proportion ratio of non-occluded Fe–P and occluded Fe–P from 0.53 to 0.29. Therefore, under sediment resuspension conditions, using the sum of loosely sorbed P (NH₄Cl–P), BD–P, aluminium bound P (Al–P), and organic-P (NaOH–nrP) to estimate bio-available P (BAP) might be problematic. However, BAP could be accurately estimated by the sum of NH₄Cl–P, % BD–P (bio-available, non-occluded Fe–P), and NaOH–nrP. By this estimation, the amount of BAP in the sediments as influenced by repeated resuspension decreased by about 10% of Tot-P, compared with the initial state (raw sediments). The results suggest that repeated resuspension could accelerate the transformation of P from mobile fractions to refractory fractions, which can be attributed to the increase of occluded Fe–P, Al–P, and calcium bound P (HCl–P).     

Dissolved iron:phosphate ratio as an indicator of phosphate release to oxic water of the inner and outer coastal Baltic Sea

Pore water concentrations and benthic fluxes of dissolved Fe, P and N were measured at two coastal basins in the Gulf of Finland, northern Baltic Sea, during a seasonal cycle. The bioturbated inner coastal basin, where exchange of near-bottom water is efficient, had a better ability to retain P in sediments than the outer basin, where near-bottom water O₂ concentration decreases during summer. Under the presence of O₂ high pore water dissolved Fe:P ratio (>3.6 w:w) in surface layer of the sediment, measured especially in winter, indicated negligible or low P-release and high N:P ratio in the efflux. On the contrary, low Fe:P ratio (<3.6), measured in summer and autumn, indicated high efflux of P and low N:P flux ratio. The low dissolved Fe:P ratio suggested that there was not enough diffusing Fe to form Fe³⁺ oxide-rich layer in the oxic surface zone of the sediments or near-bottom water to bind the P diffusing from the sediment. However, in sediments bioturbated by the abundant bivalve Macoma baltica, small efflux of P were measured almost throughout the study period. Thus, the Fe:P ratio cannot alone explain the P-release in bioturbated sediments. The low N:P ratio in the efflux measured in summer and autumn partly explains the measured low N:P ratio in the near-bottom water and thus N limitation of primary production in the Gulf. Additionally, it is evident that the release of P in the Gulf itself is of great importance for the trophic state of the Gulf of Finland.     

Natural and man-caused factors affecting the abundance and cycling of dissolved organic substances in precambrian shield lakes

A study on nutrient regeneration processes and a measure of their fluxes at the sediment-water interface was carried out in two different stations of a shallow lagoon of the Po delta river (Italy). A few parameters on the solid fraction (grain-size, porosity, C, N) and pore water profiles of o-P, NH₃, NO _inf3 ^sup– , SiO₂, Tot-CO₂, SO _inf4 ^sup2– , Fe, Mn, Ca, Mg, pH, Eh were determined. At both stations the results were typical for fine sediments rich in organic matter. The ratio of variations of sulphate (SO _inf4 ^sup2– ) to total carbonate demonstrates the main role sulphate reduction plays on the organic matter decay. The use of the ratios of variations of sulphate (SO _inf4 ^sup2– ) to ammonia (NH₃) and of sulphate (SO _inf4 ^sup2– ) to phosphate (o-P) in pore waters enabled us to calculate the C/N/P of the decomposing organic matter. Obtained C/N/P indicated an enrichment of N and P with regard to C/N/P ratios of the solid fraction, due to the selective stripping of N and P during organic matter mineralization. This phenomenon decreases with depth, where organic matter becomes more refractory. Calculations on saturation degrees have shown the possibility of authigenic calcite, apatite and rhodochrosite precipitation in sediments. Nutrient fluxes were estimated for SiO₂, NH₃ and o-P by means of benthic chambers and modelling the pore water profiles. The model used for the calculation of fluxes allowed us to account for the bioturbation-irrigation influence near the interface, by means of a cumulative diffusion coefficient. Directly measured fluxes proved to be always significantly greater than the theoretical ones. These differences seem to be due to surface processes which do not affect pore water concentrations (degradation of fresh materials at the interface; micro-bioturbation by small gasteropoda such as Hydrobia ventrosa) and/or to the different concept of the two methods in time and space. Number, size and biomass of macrobenthic species living in the sediment underneath the benthic chambers were determined. The comparison between data on macrobenthic populations and flux values showed a good relationship between the number of organisms and benthic fluxes within each station. However, flux variations between stations are to be attributed mainly to the different arrangement of the tubes of the polychaetes Polydora ciliata in the sediment.     

The labyrinth of nutrient cycles and buffers in wetlands: results based on research in the Camargue (southern France)

Wetlands, especially in the Mediterranean area, are subject to severe eutrophication. This may upset the equilibrium between phytoplankton production in undesirable quantities and a quantitatively desirable macrophyte production. In order to manage this equilibrium, a quantitative knowledge of nutrient input and fluxes is essential and the role of sediments in these processes must be understood. This knowledge can be useful even for agriculture, e.g. rice cultivation, where optimal utilization of fertilizers can lead to an economic benefit.In this article different aspects of nutrient cycles are discussed in view of approaching a sufficiently precise quantification. The nutrient input balance of the Camargue was therefore measured which showed that the input of nutrients with the irrigation water, taken from the river Rhone, roughly equals the quantity of fertilizers added.Phytoplankton growth can be approached reasonably with the Monod model, although there are still many practical problems, such as the influence of the pH on P uptake and the problem of measuring P uptake in the field. The situation is worse for macrophyte growth; quantitative data are scarce and studies have often been carried out with unrealistic nutrient concentrations or without addressing the influence of the sediment. This influence can also include negative factors, such as high concentrations of Fe²⁺, H₂S or FeS, but cannot yet be quantified.The nitrogen cycle in wetlands is dominated by denitrification. Most wetlands have sediments with high concentrations of organic matter, therefore with a large reducing capacity. Besides this process, we have shown that denitrification can also be controlled by FeS. In the Camargue sediments this denitrification is mediated by bacteria from the sulfur cycle; this appeared to be the major pathway. It was shown that a stoicheiometric relation exists between nitrate reduced and sulphate produced. The influence of the temperature was quantified and appeared to be stronger at high organic matter concentrations than at lower ones. Denitrification with FeS means that the bacteria use nitrate also for their N demands, while this is not necessarily the case during denitrification with organic matter.Mineralization of macrophytes is a much slower process than that of phytoplankton, probably because of their high C/N ratio. We could, however, not confirm the general assumption that the addition of nitrogen stimulates this mineralization. On the contrary, we found that two amino acids both with a C/N ratio of 6 had different mineralization rates. The amino acid composition of dead macrophytes and the C/N ratio may be of equal importance.Unlike nitrogen, phosphate is always strongly adsorbed onto sediments. The two mechanisms of the adsorption of inorganic phosphate onto sediments, i.e. the adsorption onto Fe(OOH) and the precipitation of apatite, have been quantified. The adsorption of phosphate onto Fe(OOH) can be satisfactory described with the Freundlich adsorption isotherm: P_ads = A^* (o-P)^B. The adsorption coefficient A depends on the pH of the system and the Ca²⁺ concentration of the overlying water and can be quantified preliminarily by A = a.10(^–0.416*pH).(2.86 – (1.86.e^–Ca2+)). B can be approached by 0.333, which means the cube root of the phosphate concentration. The second mechanism is the solubility of apatite. We found a solubility product of 10^–50 for hard waters. The two mechanisms are combined in solubility diagrams which describe equilibrium situations for specific lakes.The conversion of Fe(OOH) to FeS has a strong influence on phosphate adsorption, although the partial reduction of Fe(OOH) P by H₂S does not release significant quantities of phosphate. Even after complete conversion to FeS only a small part of the bound phosphate was released.Besides the two inorganic phosphate compounds, we established the existence of two organic pools, one soluble after extraction with strong acid (ASOP), the other one with strong alkali. The first pool is probably humic bound phosphate, while the larger part of the second pool was phytate. The ASOP was remineralized during the desiccation of a Camargue marsh; this drying up oxidized FeS, thus improving the phosphate adsorption and decreasing the denitrification capacity. It can, therefore, be an important tool for management. The phytate was strongly adsorbed onto Fe(OOH), which explains the non-bioavailability towards bacteria.The fact that the sediment phosphate concentration can be approached by multiplying the relevant sediment adsorption constant with % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOqaaeaaca% WGVbGaeyOeI0IaamiuaaWcbaGaaG4maaaaaaa!3B8D!\[\sqrt[3]{{o - P}}\] concentration has the consequence that much larger quantities of phosphate accumulate in the sediments than in the overlying water. This means that even if the phosphate input is stopped, the eutrophication will only be reversed very slowly, and not at all, if the shallow waters in wetlands have no through flow — as is often the case in many marshes in Mediterranean wetlands.Abbreviations used o-P = dissolved ortho phosphate (or its concentration) - N_part, P_part = particulate N or P - Tot-N_inorg = Total inorganic nitrogen (= NH₃ + NO ₂ ^– + NO ₃ ^– ) This paper, giving an overview of the research in the sediments of the Camargue, was read during the symposium Nutrient Cycles — A Joy Forever, on the occasion of my retirement, 19th of May 1993 at the I.H.E. in Delft (Netherlands).