Average annual growth and condition of mussels as a function of food source

Prior to 1986, before completion of a storm surge barrier, the average annual condition of mussels at culture plots in the Eastern Scheldt was better in the western part near the North Sea, than in the central part of the estuary. No such difference was observed in 1986 and 1987.To predict the effects of the barrier on mussel culture, the condition of the mussels, which is an index of growth rate, is analysed and correlated with food sources for the period 1981–84. In the western and central part of the Eastern Scheldt, mussel condition correlates strongly with average annual primary production, but not with chlorophyll-a concentrations. This suggests a direct link between primary production and the growth of mussels.In the western part of the estuary, the relation between condition and primary production has a less steep slope than in the central part. Import of food from the North Sea is suggested to act as an additional food source in the west of the Eastern Scheldt; hence the better condition values of the mussels, also in years with relatively low primary production.The storm surge barrier reduces the water exchange with the North Sea. Reduced import of food and consequently lower mussel condition are expected in the western part of the estuary. Preliminary data from 1987 confirm this prediction.     

Elemental composition of seston and nutrient dynamics in the Sea of Marmara

The Sea of Marmara, an intercontinental basin with shallow and narrowstraits, connects the Black and Mediterranean Seas. Data obtained during1991–1996 have permitted the determination of the elementalcomposition of seston in the euphotic zone and the N:P ratio of thesubhalocline waters of the Marmara Sea. Since primary production is alwayslimited to the less saline upper layer (15–20 m), of the Marmara Sea,the subhalocline waters of Mediteranean origin are always rich in nutrients(NO₃ + NO₂ = 8–10 μm, PO₄ = 0.8–1.2 μm) but depleted in dissolvedoxygen (30–50 μm) throughout the basin, yielding an -O_{_2} : N : P ratio of 178 : 9 : 1. Pollution of the surfacewaters since the 60s has modified the subhalocline nutrient chemistryslightly. In the euphotic zone, the N : P ratio of the seston changes from5.9 to 9.5 between the less and more productive periods. Though the biologyof the Marmara has changed significantly during the previous two decades,the close relationship observed between the elemental composition of thesurface seston and the NO₃ : PO₄ ratio of thesubhalocline waters strongly suggests that during the whole year primaryproduction throughout the basin and POM export to the lower layer remainnitrogen-limited. This suggestion needs to be confirmed by bio-assays,biological studies and sediment trap data from the upper subhaloclinedepths. Nonetheless, the counterflows in the Marmara basin possessrelatively low N : P ratios in both dissolved and particulate nutrients andextend as far as the adjacent seas. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Modelling of water and sediment quality in the Scheldt estuary

In 1987 the Tidal Waters Division (Ministry of Transport, Public Works and Water Management, The Netherlands) initiated the SAWES project (Systems Analysis WEstern Scheldt). In the framework of this project, a water quality model for the Scheldt estuary was made. The model aims to establish the relation between pollution inputs and the concentration of pollutants in the abiotic compartments and to establish the fate of wasted substances. The model includes oxygen, nitrogen and trace metals. Due to the low oxygen content of the upper Scheldt estuary, a new model approach for trace metals had to be developed, taking into account precipitated metal sulphides. The calibration, verification and sensitivity analysis of the model provided a good understanding of the chemistry of the estuary. Afterwards the model was used to support policy making by computing how the water and sediment quality in the estuary respond to reduced inputs of waste.     

A box-model of metal flows through the Scheldt estuary (1981–1983 and 1992–1995)

The behaviour of Cu, Zn, Cd and Pb in the Scheldtestuary was investigated in the periods 1981 to 1983and 1995. The Hg behaviour was studied between 1992and 1994. A box model was used to establish therelationship between the trace metal inputs and thefate of these pollutants in the Scheldt estuary.Annual budgets were calculated for particulate anddissolved trace metals. For Hg, seasonal budgets ofthe various species (inorganic Hg, methylmercury,Hg °) could also be calculated. These budgetsallow an evaluation of the consistency of eachindividual flux or process, such as the inputs of thevarious metals into the North Sea, the sedimentationflux of particulate metals in the area of highturbidity, the evasional fluxes and the formationrates of Hg °, etc. in relation to the other ones. This revised version was published online in July 2006 with corrections to the Cover Date.     

Water quality modelling of the western Scheldt estuary

In 1987 the National Institute for Coastal and Marine Management (theNetherlands) initiated the SAWES project (Systems Analysis WEstern Scheldt).The main goals of the project were to acquire knowledge with respect to thepollution problems in the Scheldt estuary and to apply this knowledge tomanagement issues of the Scheldt on an operational level. The main interestwas to understand quantitatively the relation between inputs of pollutingsubstances and effects on the ecosystem. In the framework of the project, awater quality model for the Scheldt estuary was made. In 1995, the modelarea was expanded to include the Belgian coastal region at the mouth of theestuary. The model calculates the fate of discharged substances andestablishes the relation between pollution inputs and the resulting waterquality, including general water quality (oxygen, pH, alkalinity, major ionchemistry and nutrients) as well as pollutant concentrations. The modelincorporates all chemical processes which affect these concentrations,including the precipitation/dissolution of metal sulphides which to a largeextent controls the fate of trace metals. Based on calibration andverification exercises, it has been shown that the model provides a goodrepresentation of the physical and chemical processes taking place withinthe estuary and can therefore be used to support policy development for theestuary. For example, the model can compute how the water and sedimentquality in the estuary will respond to reduced inputs of waste or to suchhuman activities as dredging. This revised version was published online in July 2006 with corrections to the Cover Date.     

The environmental risks of pollution in the Scheldt estuary

The ecotoxicological effects of pollution in the Scheldt estuary is evaluated for several routes of exposure by comparing observed concentrations of micro-contaminants in water and tissues of biota with toxicity data in literature. Ecological risks are estimated for dissolved trace metals according to the method of VAN STRAALEN (1990) using published data on NOECs from aquatic toxicity tests concerning reproduction, mortality and growth for cold blooded marine and estuarine organisms. Of eight investigated trace metals, zinc and nickel cause the highest risks, both affecting up to 4% of the aquatic species. The occurrence of bioaccumulation of PCBs and cadmium is investigated in several trophic levels. High concentrations of PCBs are observed in tissues of biota and may indicate adverse effects on top predators (seals, terns), especially in the brackish zone. Environmental risks of other organic micro-contaminants have not been indicated due to the limited number of data on toxicity or environmental concentrations. The effects of a mixture of pollutants in sediments from the Scheldt estuary were investigated with an oyster larvae bioassay showing an increased toxicity in upstream direction. Moreover, in the upper part of the estuary, preconditions regarding the oxygen concentration are not met and overshadow the toxic effects of pollution with micro-contaminants. Especially in the less contaminated part of the estuary more ecotoxicolgical research is required to perform an integral risk evaluation. At present, still little is known about effects that may occur in the field.     

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).     

Cadmium and copper accumulation in the common mussel Mytilus edulis in the Western Scheldt estuary: a model approach

The Western Scheldt of the Dutch Delta area is severely contaminated with trace metals. Accumulation models of trace metals in the mussel Mytilus edulis are required to predict the biological efficiency of reductions in the metal and organic matter load. Two models are constructed: a black-box model and a physiologically structured model. The black-box model predicts metal accumulation in mussels from uptake and elimination parameters. The physiological model attempts to improve predictions by taking into account the kinetics of individual uptake and elimination routes. These in turn, are taken as depending upon two more general physiological processes, the ventilation rate and the metabolic rate. Metal uptake via food and water are expressed as relative fractions. Metal input is differentiated into particulate adsorbed, and dissolved species.The reliability of the two models is evaluated by comparing predicted concentrations for mussels with measurements. Model predictions for copper deviate less than 100% from measured concentrations, but neither model appears to predict cadmium concentration with sufficient accuracy since deviations of more than 100% occured. The introduction of physiological refinements did not improve performance. Food mediated contributions for cadmium and copper to total body burden had been overestimated in the model by a factor of 100 when compared to literature values. The physiological model did predict that the ratio of food mediated contribution to total body burden is probably different for cadmium and copper and decreases with increasing salinity for both. As yet there are no measurements available to confirm such predictions.We conclude that additional laboratory experiments should be done for a better understanding of why there is poor agreement between the few field observations and the simulations. In these experiments mussels grown under different environmental condition can be tested for their accumulation capacity of trace metals. More field observations are needed.     

Review on the effects of pollution on marine fish life and fisheries in the North Sea

Information on possible effects of pollution on fish life is reviewed with special emphasis to the North Sea. Concentrations of heavy metals are high in onshore waters, sediments and organisms. Offshore regions are considered to be not endangered. In estuaries concentrations of certain heavy metals are often surpassing thresholds, which have been experimentally demonstrated to produce sublethal effects on marine organisms. Also mussel culture areas had to be closed due to excess of cadmium contamination. Mercury in fillets of cod in the southern North Sea sometimes reaches 0.3 ppm representing 1/3 of the legal limit. Reductions of inputs are necessary due to the bioaccumulative and toxic potential of heavy metals. Organochlorine contamination of water, sediments and organisms is higher in the southern than in the northern North Sea. In offshore areas clearly elevated levels of PCB's, which are the dominating organochlorine compound in marine organisms, have been analyzed. At present only a minor fraction of total organochlorine residues accumulated in marine biota is analyzed. Margins between experimentally derived lethal levels and those occurring in marine biota are narrow suggesting that sublethal effects might occur. One of the special concerns in the context of organochlorine contamination has to be seen in the potential of these substances affecting the reproductive capacity of marine organisms. Reduction measures are urgently necessary and should include substances with bioaccumulative and toxicological potential, which are not presently included in Marine Monitoring Programmes. Impact of oil pollution is considered to be a regional problem, although especially in estuaries concentrations in the water column might be high. Input through oil platforms has led to local reductions in benthic communities. Measures have to be taken, to further reduce these inputs also in view to avoid contamination of fish in the vicinity of oil fields. Oxygen depletion in German and Danish coastal waters has been shown to be a severe degradation in coastal water quality resulting in significant impact on fish populations. Indications for large-scale avoidance reactions of low DO-areas and mortalities of fish and benthos organisms have been demonstrated. It is suspected that the low DO-situations are related to increased nutrients input and high phytoplankton production. Improvement of the situation can only be expected, if input of nutrients is reduced. Reasons for the occurrence of toxic plankton blooms are not understood. An intensified research is necessary, to avoid damage to fisheries and mariculture. Although activity of radionucleids in North Sea waters is increased due to input of radioactive material from La Hague and Sellafield reprocessing units, the present levels of activity are not considered to be a threat for marine life. The same holds for thermal pollution, which is, depending on the recipient area, expected to have only local effects. Sewage sludge dumping and discharge of untreated sewage introduce significant amounts of accompanying heavy metals, which can be expected to have a negative influence on the total metal budget of recipient waters. Nutrient input through sewage will exhibit only local effects. Dumping of wastes from titanium dioxide industry are suspected to be causally linked to the occurrence of increased prevalences of certain diseases of dab in the German Bight. Present management strategies should take account of the fact that through dumping of waste from titanium dioxide production significant quantities of heavy metal are introduced into the southern North Sea. Also the incineration of chemical wastes at sea should be reduced due to uncertainties concerning quantities and toxicity of incineration products and their potential impact on marine life. It is concluded that clear evidence exists for pollution impact in estuarine regions. Most of the major estuaries at the North Sea coast show benthos reductions, which can be related to water quality and it has to be assumed that areas of a sublethal effect are located in the vicinity of those areas of well demonstrated effects. They are of unknown size and attempts have to be undertaken to quantify areas of sublethal impact. The occurrence of fish diseases is interpreted as an expression of degraded water quality. Some of the diseases detected can produce mortality, thereby having an as yet unquantified impact on fish populations. The presence of fish diseases also reduces the marketability of catches. It cannot be excluded that changes in species composition of fish populations in German Wadden Sea areas are related to pollution. Concentrations of organochlorine substances in fishes of the southern North Sea give rise to the concern that their reproductive capability might be impaired. Future work has to be directed to this problem. Impact of pollution on populations of marine organisms has so far not been demonstrated.     

Microelement composition of the green alga Ulva fenestrata from Peter the Great Bay, Sea of Japan

Concentrations of heavy metals Fe, Mn, Cu, Zn, Pb, Cd, and Ni were determined in the thalluses of the green alga Ulva fenestrata sampled from different locations in Peter the Great Bay (Sea of Japan). According to the metal concentrations in Ulva, the degree of pollution of the surveyed areas in Peter the Great Bay decreases in the following series: Amur Bay > Ussuri Bay > Nakhodka Bay > Vostok Bay > the water area of Far Eastern State Marine Nature Biosphere Reserve. The microelement composition of Ulva from open-shore stations reflects the heavy metal pollution level of water areas as a whole. The concentrations of trace elements in U. fenestrata from closed coastal areas are indicative of marine coastal water pollution from local sources. Generally, metal concentrations in U. fenestrata from Peter the Great Bay are similar to heavy metal levels in non-polluted or weakly polluted coastal areas of the world.     

Behaviour of selected PCBs,PAHs and γ-HCH in the Scheldt estuary,S.W. Netherlands

The behaviour of some organic micropollutants in the abiotic compartments water, particulate matter and bottom sediments of the Scheldt estuary, S.W. Netherlands, was studied between 1986 and 1989. Special attention was given to two individual PolyChlorinated Biphenyl congeners (PCB 52 and PCB 153), two Polycyclic Aromatic Hydrocarbons (PAHs) fluoranthene and benzo(a)pyrene and γ-HCH. Dissolved PCBs and γ-HCH behave conservatively during estuarine mixing. The concentration of γ-HCH in the fresh-water is extremely high in spring compared to the other seasons. Dissolved fluoranthene does not behave conservatively. The concentration of dissolved fluoranthene in the freshwater endmember shows a seasonal dependence with highest concentrations in winter and lowest in summer. Benzo(a)pyrene could hardly be detected in the dissolved phase with the methods used. PCBs and PAHs in particulate matter and sediments behave essentially conservatively as a result of the mixing of riverine particulates with high, and marine particulates with low organic micropollutant contents. Particulate fluoranthene is removed at low salinities under (nearly) anoxic conditions probably by microbial degradation. The rivers Scheldt and Rupel appear to be the major sources for the compounds studied. However, in particular the organic micropollutant contents of the sediments indicate that important emissions along the estuary were or are still present. Measurements of individual PCBs and PAHs in the <63 μm fraction of a dated sediment core from a salt marsh in the eastern part of the Scheldt estuary show that the recent input of PCBs and PAHs into the estuaryis probably 2–3 times lower than the maximum input in the mid-1960s. The PCB profile shows postdepositional congener-selective mobilization caused by advective transport. The PAH assemblage is remarkably uniform in the sediment core. The PAH ratios indicate that combustion of coal is the main source of PAHs in the Scheldt estuary.     

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.     

Accumulation of organochlorines and mercury in flounder — an approach to pollution assessments

Levels of PCBs, HCB and Hg were determined in the muscle tissue of flounder (Platichthys flesus L.), sampled at two sites of the highly industrialized Elbe estuary which is characterized by a decreasing gradient of contaminant levels in water and sediments towards the mouth of the river. The contaminant levels were compared to those found in muscle tissue of flounders caught in an unpolluted reference area, the Eider estuary. Juvenile flounders (1–2 cm total length) caught during their migration from the southern North Sea coast into the Elbe estuary in early summer showed a significantly higher level of contamination at both Elbe stations than those from the reference area. In the following spring, the flounders, having grown to a length of 12–14 cm, reflected the gradient of contaminants measured in water and sediment. However, during summer and late autumn a drastic increase in average contaminant concentration was observed in the muscle tissue of flounders taken from the less polluted estuarine site. It became evident that severe oxygen depletion in the heavily polluted area near Hamburg had provoked the migration of highly contaminated fish downstream to the less polluted sampling area; thus intermingling with the less contaminated flounder population of this region had occurred. The results obtained indicate that careful analysis of environmentally controlled behavioural patterns in the species used for monitoring purposes as well as an appropriate statistical treatment of the data gained from chemical analysis are essential for assessing the contaminant concentrations in organisms in relation to the pollution load of their habitats.     

Genotoxicity biomarkers in Mytilus galloprovincialis: wild versus caged mussels

A biomonitoring programme of wild and caged mussels (Mytilus galloprovincialis) was carried out at four selected sites along the Ligurian coast: Cornigliano, Voltri, Zinola, and Sanremo (Italy). Mussels of a very narrow size range were left in situ for 30 days. Adult specimen of mussels from natural substrates were collected in the same areas. Animals from a mussel farm located in La Spezia were used as controls. Micronucleus frequency and DNA single strand breaks, evaluated by alkaline elution, were used as biomarkers of genotoxicity. Mussels were also analyzed for polycyclic aromatic hydrocarbons (PAH) and heavy metals (Hg and Cd). Different gradients of PAH and metal concentrations were detected in tissues of mussels from different samplings sites. A weak correlation was found between single strand breaks and PAH content while MN frequency correlated with Hg concentration (r = 0.28, P < 0.002). A clear distinction between the sites, allowing classification along a pollution gradient (Sanremo < Zinola < Voltri < Cornigliano) was demonstrated by the analysis of genotoxicity parameters. The obtained results suggested that the micronucleus assay compared with DNA damage determination by alkaline elution allow to better discriminate the selected sites. DNA damage expressed as constant of elution (k ml(-1) x 10(3)) ranges from 30 +/- 9.6 to 89.60 +/- 40.10, and micronuclei frequency from 1.78 +/- 1.04 to 24.4 +/- 12.9, in control animals and in mussels from the most polluted site, respectively. Wild mussels accumulated significant concentrations of chemicals and showed a higher induction of chromosomal damage than caged mussels, expressed as micronuclei frequency. Caged mussels showed higher level of DNA damage than wild mussels, probably as a result of recent exposure. DNA damage was higher in September than in May, as opposed to micronuclei frequency being higher in May than in September. Endogenous and exogenous factors, such as change of pollutant input levels or compositions, could be considered the cause of such variability.     

Oosterschelde estuary (S.W. Netherlands): a self-sustaining ecosystem?

The Oosterschelde estuary has a special position among the Dutch North Sea estuaries. With relatively unpolluted water and high transparency it combines high biotic diversity and high primary and secondary production. Before 1970 a major part of the water of the rivers Rhine and Meuse flowed into the Oosterschelde. The building of a permeable storm-surge barrier (1986) decreased the exchange between Oosterschelde and North Sea. The construction of two additional dams (1987) reduced the very limited fresh water discharge on the estuary even further. The total effect of these changes was a decrease of the nutrient levels in the water column.The estuary is used intensively for culture of mussels and fishery of cockles. Both zoobenthos groups use together up to 60% (in the western part) of the organic food available in the Oosterschelde estuary. The storm-surge barrier resulted in reduced current velocities and increased sedimentation, accompanied by higher transparency, hence accelerating primary production. This phenomenon is counteracted by the lowered nutrient concentrations throughout the year.In the seventies a preliminary carbon budget study indicated that a substantial import of organic carbon was required to sustain the ecosystem. Later studies did not confirm this hypothesis.In the present paper three different methods are presented to answer the question, if internal production suffices to sustain the foodweb. At first a steady state model is applied, secondly, calculations with actually measured process rates are carried out and, finally, a dynamic simulation model is used.The conclusion is drawn that, before the construction of the storm-surge barrier, primary production of organic matter by phytoplankton is much more important than import, although the latter can not be neglected as additional food source for mussels, cockles and zooplankton. The simulation model predicts that this conclusion will not have to be changed in the future. The Oosterschelde will remain largely a self-sustaining ecosystem.     

Biological monitoring of heavy metal pollution in the region of Lake Balaton, (Hungary)

The concentrations of toxic heavy metals (Hg, Cd, Cr, Cu, Zn and Pb) were measured by atomic absorption spectrophotometry in the gills of mussels (Unio pictorum L.) both living in Lake Balaton as well as transferred to various parts of tributaries of the Lake. The measurements were performed separately with two-week intervals during the course of several months. It was found that (1) the concentration of the studied metals varied with time at each location, less variation occurred in the mussels living in the Lake itself. (2) There were both increases and decreases in the heavy metal concentration of the gills, presumably reflecting the changing level of pollution of the water. It is concluded that mussels can be used as biological indicators for detecting temporal variations in the degree of toxic heavy metal contamination in surface waters, and are good objects for signalizing local events of pollution.     

Integrated assessment of heavy metal contamination in sediments from a coastal industrial basin, NE China

The purpose of this study is to investigate the current status of metal pollution of the sediments from urban-stream, estuary and Jinzhou Bay of the coastal industrial city, NE China. Forty surface sediment samples from river, estuary and bay and one sediment core from Jinzhou bay were collected and analyzed for heavy metal concentrations of Cu, Zn, Pb, Cd, Ni and Mn. The data reveals that there was a remarkable change in the contents of heavy metals among the sampling sediments, and all the mean values of heavy metal concentration were higher than the national guideline values of marine sediment quality of China (GB 18668-2002). This is one of the most polluted of the world's impacted coastal systems. Both the correlation analyses and geostatistical analyses showed that Cu, Zn, Pb and Cd have a very similar spatial pattern and come from the industrial activities, and the concentration of Mn mainly caused by natural factors. The estuary is the most polluted area with extremely high potential ecological risk; however the contamination decreased with distance seaward of the river estuary. This study clearly highlights the urgent need to make great efforts to control the industrial emission and the exceptionally severe heavy metal pollution in the coastal area, and the immediate measures should be carried out to minimize the rate of contamination, and extent of future pollution problems.     

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.     

Trace metals in sediments from the humber estuary: A statistical analysis of spatial uniformity

Concentrations of seven heavy metals (Cu, Zn, Ni, Cr, Nb, Pb, As) in sediments of the Humber estuary and its tributary rivers have been examined using a variety of statistical methods. Concentrations of metals are rather uniform once the effect of grain size is adjusted for. This uniformity extends for some distance beyond the turbidity maximum, and in fact well beyond the normal limit of salt penetration. Detailed statistical analysis of the data do however reveal some trend of concentrations with position in the estuary, with concentrations of Zn, Ni, and Pb being slightly lower towards the mouth of the estuary. Nb concentrations are higher near the mouth of the Humber, but this reflects the location of two large discharges of Nb here. Once these anomalous samples are removed, Nb concentrations are also shown to be spatially homogeneous. The spatial trends in Zn, Ni and Pb may be due to inputs of very contaminated particulate matter to the upper estuary, to inward transport of uncontaminated clay particles from the North Sea to the lower estuary or to the exchange of metals between particulate matter and water. There are however also spatial trends in sediment particle diameter, so some caution needs to be exercised in drawing conclusions.