Dissolved and particulate trace metal geochemistry in the scheldt estuary,S. W. Netherlands (water column and sediments)

The geochemistry of dissolved and particulate trace metals has been studied in the water column and the sediments of the Scheldt estuary between 1987 and 1990. A strong seasonal influence on the behaviour of dissolved Cd, Cu and Zn is observed, related to the redox conditions in the upper estuary and phytoplankton activity in the lower estuary (which are both seasonally dependent variables). The dissolved trace metal concentrations in the fresh water end-member are remarkably low during spring and summer, due to metal sulphide precipitation in the anoxic Scheldt river. However, the dissolved concentrations increase rapidly with increasing salinity, due to oxidation of metal sulphides that are present in the suspended matter, accompanied by (e.g. chloro-)complexation of the released metals. Readsorption of Cd and Zn occurs in the lower estuary during the spring phytoplankton bloom. During winter, when the Scheldt river is not completely anoxic, much higher dissolved trace metal concentrations are observed in the fresh water end-member since metal sulphide precipitation in the water column is precluded. Rapid trace metal removal is observed in the low salinity, high turbidity zone, due to adsorption onto suspended matter and freshly precipitated iron and manganese oxyhydroxides. Upon further mixing, desorption is apparent, due to a similar oxidation-complexation mechanism as observed during spring and summer. Pore water infusion may also contribute to the enrichment of dissolved Cd, Cu and Zn in the mid-estuarine region. The trace metal contents of the suspended matter and the sediments show a continuous decrease with increasing salinity. This behaviour is to a very large extent due to physical mixing of contaminated fluvial particulates and relatively unpolluted marine particulates. Desorption of Cd, Cu and Zn can be identified but is of minor importance compared to the conservative mixing process. The distribution of dissolved Cd, Cu and Zn in the pore waters of the mid-estuarine region reflects the impact of early diagenetic processes. Trace metal peaks are observed near the sediment-water interface, and at greater depth in the manganese and iron reduction zones. These peaks are attributed to oxidation of reduced trace metal compounds (e.g. sulphides) and reduction of the (iron and manganese) oxide carrier phases, respectively. At greater depth, the dissolved trace metal concentrations are much lower due to metal sulphide precipitation in the sulphate reduction zone. Analysis of a large sediment dataset indicates severe trace metal pollution of the Scheldt estuary at the end of the fifties. A major reduction of the pollution by As, Cr, Hg, Pb, and Zn has occurred in the seventies, and of Cd and Cu in the eighties. The Ni pollution has increased over the time period considered. In spite of this improvement, the present-day pollution status of the Scheldt estuary is still reason for concern.     

Heavy metals in the Rosetta estuary of the Nile and the adjoining Mediterranean waters: evidence of removal of dissolved heavy metals from waters as a result of possible binding to suspended matter

The Rosetta estuary was partially separated from the Rosetta branch of the Nile by Edfina Barrage, which controls the Nile discharge into the Mediterranean Sea. The study area covers the Rosetta estuary (lotic environment) and the adjoining seawaters (lentic environment) to investigate the local and seasonal distribution of dissolved and particulate copper and zinc, as well as dissolved cadmium in this estuary and to illustrate its influence on the distribution of these metal forms in the inshore seawaters. Besides, emphasis on the removal of dissolved heavy metals from waters by their adsorption onto suspended matter (SM) was also considered. Contrary to particulate copper (PCu), the vertical values of dissolved copper (DCu) decreased generally with depth. Planktonic scavenging and regeneration processes might determine the vertical profiles of the copper forms. The copper data suggest that the surface sources of DCu exceeded the bottom sources, contrary to the sources of PCu. The markedly high and maximum seasonal averages of DCu in the estuary and inshore seawater in July inspite of the high uptake in summer possibly reflect higher amounts of humic materials. The lowest seasonal average value of DCu in the inshore seawater in January suggests removal of copper in presence of maximum value of SM during highest discharge. The highest regional average of DCu at the estuarine mouth coincided with desorption process during mixing of the fresh and salt waters. The vertical values of dissolved zinc (DZn) and particulate zinc (PZn) showed irregular variations and their high concentrations in the surface of the estuarine and inshore seawater indicate possible land-based sources. The high bottom DZn values, however, resulted from its contribution from the interstitial water of the sediments. The data suggest that the surface sources of DZn exceeded the bottom sources and PZn showed the opposite trend in the open sea area. In the estuary, the maximum seasonal average value of DZn accompanied by the lowest seasonal average of PZn in April inspite of the high uptake in spring suggest that desorption was the dominant process. The minimum regional averages of both zinc forms at the estuarine opening and the highest average of PZn near Edfina Barage are correlated with the amounts of SM, which decreased toward the estuarine mouth. The vertical values of dissolved cadmium (DCd) were much lower in the estuary than the other metals. They showed in both environments irregular variations with depth. The bottom maximum value of DCd can be attributed mainly to contamination from the sediments. There was a distinct seasonal variation of DCd. The minimum seasonal average value of DCd in the estuary in April seems to be caused by its specific binding to living plankton found in abundance. The minimum regional average of DCd value was found at the estuarine mouth. The decrease in Cd concentration due to removal from dissolved state is most pronounced in the early stage of mixing. The correlation coefficients of DCd were positive with salinity and negative with SM, indicating that Cd increased seaward. Statistical correlation between Cd and Zn concludes that the factors affecting their distribution are generally the same.     

Dissolved organic matter status in Damietta estuary of the Nile

The seasonal distribution of dissolved organic matter (DOM) in Damietta estuary of the Nile was investigated to show the influence of organic pollution. The DOM values increased generally with depth. The vertical values of DOM varied from 0.23-9.97 mg O/l. The average DOM values gave noticeable local variations in each month and pronounced monthly variations at each station. The maximum monthly average value in August coincided principally with elevation of temperature and the minimum in December is due mainly to the drop in temperature. The direct effect of pollution, in increasing the DOM content in the estuary, decreases relatively towards the sea, as indicated by the minimum regional average DOM value at the mouth of Damietta estuary. An inverse relationship was found generally between the averages of DOM and those of dissolved oxygen.     

The role of mineral compounds and chemical conditions in the binding of phosphate in the Ems estuary

In the Ems estuary the gradients in dissolved phosphorus and iron are non-conservative and opposite. The relations between different mineral compounds, environmental conditions and phosphate concentrations were studied, in an attempt to explain this. Laboratory experiments on water samples from the reaches with high concentrations of phosphate and of suspended matter revealed a low concentration of calcite-bound phosphorus in the middle reaches of the estuary (only 0.07%). Therefore, it is concluded that calcite possibly plays only a modest role in phosphate distribution in the Ems estuary. Further experiments revealed that nearly 70% of the particulate inorganic phosphorus is iron-bound. It is demonstrated that organic coatings on minerals inhibit phosphate release at a low redox potential. Moreover, the organic matter itself contained c. 20% of particulate phosphorus. Approximately 10% of the particulate phosphorus may be associated with clay minerals. It is concluded that in addition to biological processes, the phosphorus associated with ironoxyhydroxides may be crucial for the seasonal variation of the phosphate concentrations in the water.     

Interannual variability in dissolved inorganic nutrients in northern San Francisco Bay estuary

Nearly two decades of seasonal dissolved inorganic nutrient-salinity distributions in northern San Francisco Bay estuary (1960–1980) illustrate interannual variations in effects of river flow (a nutrient source) and phytoplankton productivity (a nutrient sink). During winter, nutrient sources dominate the nutrient-salinity distribution patterns (nutrients are at or exceed conservative mixing concentrations). During summer, however, the sources and sinks are in close competition. In summers of wet years, the effects of increased river flow often dominate the nutrient distributions (nutrients are at or less than conservative mixing concentrations), whereas in summers of dry years, phytoplankton productivity dominates (the very dry years 1976–1977 were an exception for reasons not yet clearly known). Such source/sink effects also vary with chemical species. During summer the control of phytoplankton on nutrient distributions is apparently strongest for ammonium, less so for nitrate and silica, and is the least for phosphate. Furthermore, the strength of the silica sink (diatom productivity) is at a maximum at intermediate river flows. This relation, which is in agreement with other studies based on phytoplankton abundance and enumeration, is significant to the extent that diatoms are an important food source for herbivores.The balance or lack of balance between nutrient sources and sinks varies from one estuary to another just as it can from one year to another within the same estuary. At one extreme, in some estuaries river flow dominates the estuarine dissolved inorganic nutrient distributions throughout most of the year. At the other extreme, phytoplankton productivity dominates. In northern San Francisco Bay, for example, the phytoplankton nutrient sink is not as strong as in less turbid estuaries. In this estuary, however, river effects, which produce or are associated with near-conservative nutrient distributions, are strong even at flows less than mean-annual flow. Thus, northern San Francisco Bay appears to be an estuary in between the two extremes and is shifted closer to one extreme or the other depending on interannual variations in river flow.     

The mutual relationship between the monitoring and modelling of estuarine ecosystems

A general simulation model developed for the Ems estuary (a part of the Wadden Sea), to describe the main carbon flow through the foodweb, was applied for monitoring purposes. In this model, the estuary is divided into five compartments, in each of which a pelagic, an epibenthic and a benthic submodel operates. A transport model regulates the distribution of dissolved and particulate substances over the area. Two parameters that partly regulate the carbon flow are the concentrations of phosphate and of the particles (turbidity) in the water. From the sensitivity analyses for phosphate and turbidity it can be concluded that the relative effects of changes in turbidity and changes in the influx of phosphate from the rivers are large in the upper reaches and small in the lowest reaches of the estuary. This is due to the increasing influence of the coastal waters in the lower reaches. These results indicate that the reaches, which represent the beginning and the end of a gradient in the estuary, are also very suitable for monitoring the quality status of this estuary in terms of production and standing stock of groups of organisms. The need to use monitored data from the boundaries (sea and rivers) of those ecosystems as input in simulation models is discussed. Moreover, the possibilities of using simulation models to generate new ideas on the functioning of estuarine ecosystems under changing environmental conditions and to help administrators to decide on specific future management strategies are also discussed. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988).     

Biogeochemical behaviour of Cd, Cu, Pb and Zn in the Scheldt estuary during the period 1981–1983

Cu, Zn, Cd and Pb have been analysed in suspendedmatter and water samples from the Scheldt estuary,collected during five cruises between May 1982 and May1983. In order to evaluate the overall metal behaviourin the estuary, continuous longitudinal profiles ofthe total, the particulate and the dissolved metalconcentrations are discussed in relation to variousspecific physico-chemical parameters. By means ofcorrelation and multi-regression techniques, severalestuarine processes could be identified which dominateand/or control the behaviour of the individual tracemetals in three distinct areas of the Scheldt estuary. The behaviour of the four selected metals was foundto show some common features, but is besides this alsocharacterised by many individual and specificproperties. The total metal concentrations seem toco-vary well with the turbidity in the maximumturbidity zone between salinities 2 and 10 psu. In thedownstream area a dilution profile is observed for thetotal metal concentrations of Cu and Cd as well as forthe dissolved concentrations of all metals. Dissolved metal production rates are high for Cu andCd, but much lower for Zn and Pb, confirming thedifferent longitudinal dissolved metal profiles. ForCu and Cd these dissolved metal profiles arecharacterised by a broad mobilisation area, coveringthe whole middle estuary. Pb on the other hand, showsa more confined mobilisation area, and for Zn no sucharea was found. During winter the dissolved metalproduction rates seem to be controlled, predominantly,by desorption processes. During summer dissolved metalproduction is correlated mainly to the dissolvedoxygen content, suggesting the domination of redoxprocesses solely or in combination with desorptionprocesses. In some cases, co-existent with nearly zerooxygen concentrations, removal of dissolved metals wasobserved, possibly reflecting precipitation reactions. Longitudinal particulate metal profiles all show anegative deviation versus the dilution profile. Cu andCd provide the higher deviations, characterised by anarrow enrichment zone preceding a broad mobilisationarea. Finally, all particulate metal profiles appearedto correlate well with POC, when expressed involumetric units. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modelling Pb and Cd dynamics in the Seine estuary

The Seine estuary (France) is currently one of the world’s most contaminated estuaries, due in particular to its high cadmium and lead content. Proper understanding of contaminant transfer, transformation and retention mechanisms throughout the estuary and up to the adjacent marine zone require a range of studies involving data collections, experiments, and modeling tool. A multivariable transport model (SiAM-3D) was used to simulate dissolved and particulate transport and it is applied to several calculation grids; a speciation model (MOCO) was used to select key species and obtain a schematic system representation. The coupled model for Cd and Pb was compared with field measurements. The complementary character of various tools (model applied to annual time scale, in situ measurements and experiments) allowed to explore and quantify various hypotheses on the high dissolved cadmium concentrations observed during low river flow. The target is to achieve a compromise between acceptable computing times and adequate result accuracy. Although particle and contaminant behaviour is globally well-reproduced by the coarse grid, calculation errors relating to bayward fluxes and stocks deposited inside the estuary were highlighted after comparison with the fine grid.     

中国主要入海河流河口集水区划分与分类

采用GIS技术开展中国主要入海河流的流域边界及其汇水单元、河口集水区的划分及其分类。基于ArcGIS水文分析模块划分出15条入海河流的汇水单元,并通过5种不同汇水单元面积等级的划分来表达各入海河流汇水单元的详细程度;划分的流域面积与相关文献资料对比偏差在10%以内;在流域与汇水单元划分的基础上,基于感潮河段的数据收集,确定出88个中国主要入海河流的河口集水区。通过建立分类指标体系,采用聚类分析方法,划分出5类河口:第一类EDA主要分布在环渤海地区、山东半岛和广东沿海;第二类EDA主要分布在长江口和杭州湾周围;第三类EDA主要分布在江苏北部沿海和辽东半岛;第四类包括山东江苏交界处沿海、江苏南部沿海;第五类主要分布在福建浙江沿海、珠江口、长江口、海河口附近、辽河河口附近,并利用快速生物评价法验证河口分类的合理性。研究成果可为进一步开展我国近海河口和海域营养盐基准制订与生态分区提供基础空间数据与方法借鉴。     

Particulate phosphorus in the Humber estuary

The biogeochemical controls on particulate phosphorus distribution in the Humber estuary have been investigated with regard to salinity and season. Surveys of the Humber were conducted in August 1991 and January, March and June 1992. Samples were collected for particulate phosphorus, dissolved phosphorus, suspended load and salinity analysis. Particulate phosphorus forms 16 to 93% of the total phosphorus present in the water column of the Humber estuary. Total particulate phosphorus concentrations in the Humber ranged from 1 to 45 μmol l⁻¹. This range is comparable to that observed in similar sized industrialized estuaries of the U.S.A. and northern Europe. Flocculation at low salinities was found to be a major influence on particulate phosphorus concentrations. Highest concentrations of particulate phosphorus were found in reaches below a salinity of 5 and may result in phosphorus being trapped in the sediments of the estuary. There is evidence that this phosphorus may be released as oxygen concentrations decrease during the summer. Suspended load, adsorption/desorption equilibrium and external inputs were also found to influence the particulate phosphorus concentrations.     

亚热带河口陆基养虾塘水体溶解性碳浓度及沉积物-水界面碳通量时空动态特征

以福建闽江和九龙江河口陆基养虾塘为研究对象,通过野外原位观测和室内模拟培养实验,开展了河口陆基养虾塘养殖期间水体溶解性有机碳(DOC)和溶解性无机碳(DIC)及养虾塘沉积物-水界面碳交换通量变化特征的研究。结果表明:时间变化上,养虾塘水体溶解性碳浓度及沉积物-水界面碳通量在闽江河口呈现8月中旬10月中旬6月中旬的特征,在九龙江河口表现为随养殖阶段推移而增加的趋势;空间变化上,闽江河口养虾塘水体溶解性碳浓度及沉积物-水界面碳通量显著高于九龙江河口;沉积物释放溶解性碳速率与水体溶解性碳浓度呈现显著正相关关系,沉积物碳释放过程是引起养虾塘水体溶解性碳浓度时空变化的重要因素。表明河口区水产养虾塘碳循环研究时需考虑不同形态碳生物地球化学循环的时空差异性。     

Bacterially mediated iron cycling and associated biogeochemical processes in a subtropical shallow coastal aquifer: implications for groundwater quality

Bacterially mediated iron redox cycling exerts a strong influence on groundwater geochemistry, but few studies have investigated iron biogeochemical processes in coastal alluvial aquifers from a microbiological viewpoint. The shallow alluvial aquifer located adjacent to Poona estuary on the subtropical Southeast Queensland coast represents a redox-stratified system where iron biogeochemical cycling potentially affects water quality. Using a 300 m transect of monitoring wells perpendicular to the estuary, we examined groundwater physico-chemical conditions and the occurrence of cultivable bacterial populations involved in iron (and manganese, sulfur) redox reactions in this aquifer. Results showed slightly acidic and near-neutral pH, suboxic conditions and an abundance of dissolved iron consisting primarily of iron(II) in the majority of wells. The highest level of dissolved iron(III) was found in a well proximal to the estuary most likely a result of iron curtain effects due to tidal intrusion. A number of cultivable, (an)aerobic bacterial populations capable of diverse carbon, iron, or sulfur metabolism coexisted in groundwater redox transition zones. Our findings indicated aerobic, heterotrophic respiration and bacterially mediated iron/sulfur redox reactions were integral to carbon cycling in the aquifer. High abundances of dissolved iron and cultivable iron and sulfur bacterial populations in estuary-adjacent aquifers have implications for iron transport to marine waters. This study demonstrated bacterially mediated iron redox cycling and associated biogeochemical processes in subtropical coastal groundwaters using culture-based methods.     

Cesium isotope-activity in the Weser ecosystem three years after the disaster at Chernobyl

In 1989, three years after the disaster at Chernobyl, the Cesium (Cs-137 and Cs-134) activity of water, sediments, suspended solids, algae, and crustaceans were studied in five sections of the Weser estuary covering the nontidal and tidal freshwater region down to the brackish area. The total water activities in the freshwater regions are similar, but there are differences in the Cs distribution between the dissolved and the particulate phase depending on the concentration of suspended solids and abiotic water parameters. The Cs activity derived from Chernobyl is only detectable in sediments and suspended solids. Specific differences in Cs accumulation were found for crustaceans but not for algae.     

Overview of trace metal contamination in the Scheldt estuary and effect of regulatory measures

Seasonally, dissolved and particulate metal concentrations in the Scheldt estuary were assessed over a period of 4 years (1995–1998). High quality data were obtained following stringent analytical protocols for each step: sampling, sample treatment, sample storage and analysis. Of the 5 trace metals, Ni showed the most conservative behaviour, while Cd and Cu were clearly transferred from the particulate to the dissolved phase in the middle estuary. A substantial part of the particulate metals entering the estuarine system are lost through sedimentation. General seasonal patterns are the following: lower concentrations in spring and higher ones in winter (sometimes late fall/early winter) for dissolved metals, while in summer a pronounced rise of the longitudinal concentration profile is observed for the particulate metals. A comparison of the trace metal concentrations (dissolved and particulate) at the mouth of the estuary in 1995–1998 with those from 1981 to 1983, reveal reductions between 30 and 58%. Reductions based on direct emission measurements for almost the same period suggest reductions (dissolved + particulate) between 42 and 64%. Biomagnification (BMF) is the accumulation of a compound through the food-chain. It is in our case expressed as the ratio of the metal concentration in the organism (g g^–1, d.w.)/the metal concentration in total suspended matter (g g^–1, d.w.). Almost all BMF-values of Periwinkle, Nereis diversicolor and Macoma balthica (3 bottom organisms in the Scheldt estuary) are negative meaning that these organisms contain less heavy metals than the particulate suspended matter. For all organisms log BMFs for Pb, respectively Ni, are around –1.8, respectively –0.7. For Cd, Periwinkle shows slight enrichment (0.05) and for Cu even more (0.45), while negative values were observed for Nereis diversicolour and Macoma balthica. The latter organisms are more enriched in Zn (–0.09) than Periwinkle (–0.43).     

The behaviour of copper and zinc in the Scheldt estuary

The behaviour of copper and zinc in the Scheldt estuary, both in the particulate and the dissolved phase, is discussed for the period 1982–1983. The longitudinal particulate copper and zinc profiles fall below the dilution line, which is attributed to mixing and mobilization processes. In the dissolved phase both metals show a production region in the end member of the estuary, for copper a pronounced mid estuarine maximum is observed. Study of the metal speciation suggested that organic ligands might keep heavy metals in solution, especially in area of intensive production, and promote the subsequent export of these metals downstream the estuary.     

181 Two Lineages Within Griffithsia (Ceramiaceae, Rhodophyta) Based on Plastid Protein-Coding psaA, psbA and rbcL Genes

"Green tides" or blooms of ulvoid green algae are frequent in Yaquina Bay estuary on the central Oregon coast, USA. Measurements of their biomass were made from late spring to early winter in 1999 at six intertidal sites in the estuary, and were continued through the winter of 2002 at two sites that showed the greatest accumulation. The dominant blooming species were the Enteromorpha linza complex, Ulva fenestrata, E. flexuosa , and E. intestinalis . Red and brown algal abundance was negligable. Nutrients in the central channel were monitored, along with incident light and water column absorbence, and sediment temperature. The abundance of benthic amphipods was negatively correlated with the concentration of dissolved sulfides in porewater. However, dissolved sulfide levels were not correlated with green macroalgae biomass. Additional correlations were sought between the variables measured in an effort to suggest or negate major cause and effect relationships. This research is relevant to discerning the effects of eutrophication as a stressor on estuarine processes and could contribute toward an understanding of anthropogenic impacts on biological communities in coastal ecosystems.     

SEASONAL ABUNDANCE,HABITAT SELECTION AND ENERGY CONSUMPTION OF WATERBIRDS AT THE BERG RIVER ESTUARY,SOUTH AFRICA

Velasquez, C.R., Kalejta, B. & Hockey, P.A.R. 1991. Seasonal abundance, habitat selection and energy consumption of waterbirds at the Berg River estuary, South Africa. Ostrich 62:109-123.

The distribution and abundance of waterbirds at the Berg River estuary were studied between September 1987 and April 1989. The estuary supports an unusually high density of waterbirds, especially of Pale-arctic migrant waders, and is a site of subregional importance for at least nine species. Intertidal mudflats are the favoured feeding habitat of the majority of species on the estuary during the low tide period. Low tide feeding densities on saltmarshes are mud less than on mudflats, but saltmarshes are important as roost sites, high tide feeding sites, and in counteracting the negative hydrological consequences of development. The current conservation status of the estuary is not commensurate with its importance as a waterbird habitat and, given the current threats facing the estuary, enhanced protection at the national level is considered a greater priority than registration with, for example, the RAMSAR Convention.     

Nutrient biogeochemistry in the water column (N,P, Si) and porewater (N) of sandy sediment of the Scheldt estuary (SW-Netherlands)

The Scheldt river drains a densely populated and industrialized area in northern France, western Belgium and the south-west Netherlands. Mineralization of the high organic load carried by the river leads to oxygen depletion in the water column and high concentrations of dissolved nitrogen and phosphorus compounds. Upon estuarine mixing, dissolved oxygen concentrations are gradually restored due to reaeration and dilution with sea water. The longitudinal redox gradient present in the Scheldt estuary strongly affects the geochemistry of nutrients. Dissolved nutrients in the water column and dissolved nitrogen species in sediment porewaters were determined for a typical summer and winter situation. Water column concentration-salinity plots showed conservative behaviour of dissolved Si during winter. During summer (and spring) dissolved Si may be completely removed from solution due to uptake by diatoms. The geochemistry of phosphorus was governed by inorganic and biological processes. The behaviour of nitrogen was controlled by denitrification in the anoxic fluvial estuary, followed by nitrification in the upper estuary (prior to oxygen regeneration). In addition, nitrogen was taken up during phytoplankton blooms in the lower estuary. Dissolved inorganic nitrogen species in porewaters from the upper 20 cm of sediments were obtained from a subtidal site in the middle of the lower estuary. Dissolved nutrient concentrations were low in the upper 10–15 cm of the sandy and organic poor (<1% POC) sediments mainly as a result of strong sediment mixing. The porewater profiles of ammonium and nitrate were evaluated quantitatively, using a one-dimensional steady-state diagenetic model. This coupled ammonium-nitrate model showed ammonification of organic matter to be restricted to the upper 4 to 7 cm of the sediments. Total nitrification ranged from 3.7–18.1 mmol m^?2 d^?1, converting all ammonium produced by ammonification. The net balance between nitrification and denitrification depended on the season. Nitrate was released from the sediments during winter but is taken up from the water column during summer. These results are in good agreement with data obtained from the independently calibrated water column model for the Scheldt Estuary (VAN GILSet al., 1993).     

Typical features of particulate phosphorus in the Seine estuary (France)

During the 2001–2002 hydrological cycle, 8,000 tons P year⁻¹ (44% as particulate phosphorus) originating from the Seine basin entered the Seine estuary. P content in suspended sediments (SS) is 2.9 g P kg⁻¹ (80% as inorganic form) at Poses (the upstream limit of the Seine estuary). During the transfer from Poses to Caudebec (the limit of saline water intrusion), Particulate Inorganic Phosphorus (PIP) in SS decreases by 40% whereas Particulate Organic Phosphorus (POP) remains stable. This decrease is explained as the result of (i) a dilution by SS poorer in P, originating from storage zones (mudflats) within the estuary, and (ii) a loss of P by sedimentation, especially in the Rouen harbour where 15% to 20% of SS are yearly trapped and extracted. Downstream, in the turbidity maximum of the estuary, P content in SS is twice as low as at Poses (1.5 g P kg⁻¹, 70% as inorganic form). PIP content is fairly homogeneous both vertically and throughout an annual survey. On the other hand, POP varies by season. Higher POP content is observed during vernal period with phytoplankton accumulation. In a salinity gradient from 0 to 30 (PSS78), PIP content further decreases by 30%. POP does not vary much in this gradient, leading to the assumption that PIP is submitted to desorption in response to the dilution by marine waters (poor in orthophosphates). Using ³²P isotopic method, we establish a mathematical formulation of P exchangeable between the suspended solid and water phases of the Seine estuary. Particulate P in the turbidity maximum zone is shown to represent a possible source of dissolved P, available for algal growth in the Seine Bight. Instead of playing a role of nutrient retention, the turbidity maximum zone of the Seine estuary could favour coastal eutrophication.