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.     

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.     

The benthic ecology of river estuaries entering the Severn Estuary

The fauna of the Rivers Avon, Usk and Wye was similar to that for soft sediment in the Severn Estuary. It was dominated by Nereis diversicolor, Corophium volutator, Aiacoma balthica and oligochaetes. Maximum densities of Corophium and Macoma were higher than in the main estuary, while that of Nereis was similar. Distributions of dominant species showed gradients along river length and across river profile. These are discussed.     

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.     

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.     

Geochemical composition of sediments in the Scheldt estuary with emphasis on trace metals

Superficial sediments of the Scheldt estuary were collected with a Van Veen grab at 57 stations between Temse and Vlissingen. They were analysed for major elements (Si, Al, Fe, Ca, Mg, Na, K, Cinorg and Corg) and trace metals (Cd, Pb, Cu, Zn, Cr, Ni, Co, Mn and Li). Factor analysis indicates that 44% of the variance can be explained by one factor which exhibits a high saturation for trace metals, organic matter, Al and Fe, all variables typical of fine mud. The high scores of this first factor are almost exclusively present in the upper estuary except for one area in front of Terneuzen. The second factor, which explains 23% of the variance, is typical of the carbonates and the third one (19% of the variance) is representative of the clay minerals. These two factors are more evenly distributed over the estuary. As usual, a strong influence of granulometry on the distribution of trace elements in the sediments was observed. Intercomparison of their composition within the Scheldt or with those of other aquatic systems requires thus a normalization procedure. This problem has been studied in detail by analysing various size fractions (63–16, 16–8, 8–4, <4 m) obtained by elutriation of the sample or by using a parameter characteristic of the fine fraction such as the concentration of a typical element (Al, Fe, Li, Corg). The normalization of trace metals allowed us to evaluate an enrichment factor of the trace elements in the estuarine deposits due to mans activities. In addition, it demonstrates the decrease of the anthropogenic impact on the composition of sediments by comparing the composition of sediments collected in 1976 and in 1994.     

Temporal and spatial trends in heavy metal concentrations in the marine mussel Mytilus edulis from the Western Scheldt estuary (The Netherlands)

Since the first North Sea Conference (1984, Bremen), all countries bordering the North Sea made commitments to reduce discharge of hazardous substances into the North Sea. From Belgium and The Netherlands, large reductions (upto 90) in heavy metal emissions from land-based sources have been reported between 1985 and 2000. Recently, some studies in the Western Scheldt estuary (WS) have shown that total metal concentration in the water, sediments and suspended particles have decreased compared to levels in the 70s. However, data on aquatic organisms is still very limited and it is therefore difficult to confirm whether the reductions in pollution input and generally improving water quality in the WS have a corresponding impact on the levels of heavy metals in aquatic organisms. The current study measured metal concentrations in the soft tissues of mussels, Mytilus edulis (known to be good indicators of environmental metal pollution) during the period 1996–2002. Spatial (salinity and pollution gradients), temporal and seasonal variations were also studied. Results showed a down-stream decreasing trend for the metals studied (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) during all sampling campaigns. There was also a significant seasonal effect on tissue metal concentrations, with a peak observed around spring in both WS and the nearby less polluted Eastern Scheldt (ES). On temporal trends, a clear drop of metals in mussels was observed in the early 80s coinciding with the start of the efforts to reduce chemical pollution input into the North Sea. Since those early reductions, metal concentrations in mussels generally remained unchanged upto mid 90s. However, in recent times metal concentration in mussels have increased significantly, for example Cd in 2002 was almost 10 times the values in 1983 and similar to levels seen during the peak in the 70s. Other metals also increased in the 90s also reaching levels seen in the 70s. As there is no indication of recent increase in metal input into the estuary, we suggest that increased metal concentrations observed in mussels in recent years especially in the upper estuary are most likely a result of changes in physical and chemical speciation and metal bioavailability. Such changes may be caused by changes in some water quality parameters in the estuary (i.e. increased dissolved oxygen, concentration of organic matter), resulting in conditions that favour releases of sediment-bound metals into the water column. The relationship between metal content and season showed very similar annual profiles in the polluted WS and less polluted ES. Thus, seasonal variations in metal concentrations appear to be largely controlled by biological processes, while total body burdens are dependent on environmental levels and bioavailability.     

Mercury in benthic invertebrates of the Elbe estuary

Hg concentrations in benthic invertebrates of the Elbe estuary were analyzed by atomic absorption spectrophotometry and instrumental neutron activation analysis. In general Hg levels in organisms decreased from the limnic region to the marine environment. Highest Hg levels were found inAsellus aquaticus andRadix balthica taken from the Elbe upstream of Hamburg (0.35 and 0.34 ppm wet weight). The concentrations in gammarid species decreased from 0.20 ppm (limnic region) to 0.02–0.05 ppm (brackish and marine environment). Hg levels in organisms from the brackish region proved to be 0.08–0.16 ppm(Littorina littorea), 0.04–0.09(Crangon crangon) 0.05–0.10(Corophium volutator) and 0.04–0.08 ppm (wet weight)(Nereis diversicolor). Some factors which may influence the heavy metal concentrations in aquatic organisms are discussed, such as: food chain, weight of organisms, and elimination via moulting products in the case of crustaceans.     

Atmospheric emission and cycling of carbonmonoxide in the Scheldt Estuary

Axial profiles of dissolved carbon monoxide(CO) from four surveys of the Scheldt estuaryconfirmed that the estuary is a source ofatmospheric CO, with an emission range of 4–404nmol m^–2 h^–2. Surface water COconcentration and atmospheric emission werespatially variable, with an order of magnitudedifference between the upper and lower estuaryin spring, and seasonally variable with highestlevels in spring and lowest in winter. AnnualCO emission was estimated to be 700 (396–1032)× 10³ mol, equivalent to 0.02–0.05% ofdissolved organic carbon (DOC) input to theestuary. CO photoproduction rates were an orderof magnitude greater in the upper estuary inspring and correlated with DOC concentration.Total CO production from DOC photodegradationwas estimated to be 8.5–18 × 10³ mol COd^–1, equivalent to 0.21–0.44% of riverineDOC input in spring. The deficit betweenproduction and emission suggests that microbialCO oxidation accounts for 68% of photoproducedCO, with highest oxidation rates at lowsalinities. The results indicate that suspendedparticulate material indirectly influencesestuarine CO distribution and emission.Assuming that the Scheldt is representative, estuaries do notcontribute significantly to the oceanic or global CO budgets.     

Metal exchanges in the fauna-sediment system. The case of Nereis diversicolor and Scrobicularia plana in the Bou Regreg estuary (Morocco)

This study identifies the types and quantities of metals retained by Nereis diversicolor and Scrobicularia plana in a predominantly arid estuary environment.Certain abiotic parameters and a number of physiological processes linked to metabolic and reproductive functions play a decisive role in the seasonal influence of metal levels on the tissues of these organisms.Given the deposivoric diet of the organisms, the sedimentary bed would appear to be the principal source of metal accumulation.The following classification illustrates the bioaccumulation of metals: 209-1The possibility that these metals may be carried into the estuary by sewage water from the cities of Rabat and Sále should certainly not be ignored.     

Metals in molluscs and associated bottom sediments of the southern baltic

The concentration of Zn, Cu, Pb, Cd, Ni, Co, Ag, Mn, Fe, Ca, Mg, K and Na in molluscsMacoma balthica, Mya arenaria, Cardium glaucum, Mytilus edulis andAstarte borealis from the southern Baltic was determined. The surface sediments and ferromanganese concretions associated with the molluscs were also analysed for concentration of these metals. Species- and regiondependent differences in the metal levels of the organisms were observed. The properties of molluscs analysed which have a tendency toward elevated biological tolerance of selected trace metals were specified. The interelement relationship between metal concentrations in the soft tissue and the shell was estimated and was discussed.     

A review of assessment and remediation strategies for hat spot sediments

Sediments at the extreme end of the scale of contamination, designated as hot spot sediments, are considered. The characterization of such hot spots, and an approach for the quantitative assessments of the behaviour and fate of pollutants in such sediments are covered. Experiments with sediments containing extreme levels of heavy metals showed release rates of 56 mg m^–2 d^–1 of dissolved zinc and 0.004 mg m^–2 d^–1 of dissolved mercury. When these sediments were resuspended, the dissolved fluxes were increased by a factor of 2.2 and 128 for zinc and mercury, respectively. The biological implications of hot spot sediments are dealt with, since sediments are an important habitat for many organisms. Various alternatives for clean-up operations including dredging and capping are discussed.     

The influence of the spectral composition of irradiance on primary production in the Eastern Scheldt (The Netherlands)

The error inin vivo ¹⁴C incubator measurements of primary production in the Eastern Scheldt when neutral density filters were used and the error obtained when no account was taken of the spectral changes in submarine irradiance that occur with increasing depth, were evaluated theoretically. By multiplying the photosynthetic action spectra of two marine algae by calculated irradiance in the euphotic layer using Kd and Kd() respectively, the gross primary production P[Ed(400–700)] and P[Ed()] was computed. In the green-brown waters of the Eastern Scheldt estuary the use of neutral density filters was sufficient to simulate the underwater light conditions. In clear waters it can cause an overestimation of the gross production.     

Is bacterioplankton production in the Ria de Aveiro influenced by salt marshes and bed sediment?

The contribution of potential export of materials from bottom sedimentsand salt marshes into the water column of a shallow estuarine system of Ria deAveiro to the observed high bacterial productivity in the mid section of thisestuary was evaluated. Vertical profiles of physical, chemical and bacterialvariables were studied in the marine and brackish water zones, and oftransversal profiles in the brackish zone only. Although the concentrations ofseston (17–241 mg l^–1), particulate organiccarbon (3–15.5 mg l^–1) and chlorophyll(1.2–7.0 g l^–1) varied widely, thevertical and transversal profiles were without much variation. Total bacterialnumber (0.2–8.5 × 10⁹ cellsl^–1) and the number of particle-attached bacteria(0.02–2.50 × 10⁹ cellsl^–1)along vertical and transversal profiles did not differ much, but the rate ofbacterial production (0.05–14.2 g C l^–1h^–1) and dissolved organic carbon concentration(6.0–69.2 mg l^–1) were frequently highernear the salt marsh margin at the brackish water transect. The increase inproductivity could not be associated with runoff of particulate matter butcoincided with the inputs of dissolved organic carbon. The results of verticaland transversal profiles point to a minor role of particulate matter additionsfrom the salt marsh area or from bed sediments.     

Transport of particulate organic material between salt marsh and estuary

Summary The macrophyte production and the transport of particulate organic matter between march and adjacent estuary have been investigated for a 30 ha salt marsh along the Oosterschelde estuary, The Netherlands.The primary production of macrophytes, measured with Smalley's method, was 837–1030 g dry organic matter.m^–2.year^–1. Measurements of amounts of particulate organic matter transported through one of the main tidal creeks in the salt march resulted in a net import. On average 31% of the material brought in by the flood settled in the marsh. The majority of this material is smaller than 63 m. On the other hand large floating material is exported during storm tides, although the quantity seems to be smaller than that of the suspended material imported.The differences between various marshes with regard to export and import of organic matter are explained in terms of marsh level, primary production, turbulent diffusion, sinking and resuspension of particulate matter and biotic transformations.     

Seasonal variability in mercury inputs into the Ria de Aveiro,Portugal

Water, suspended particulate matter (SPM) and sediments were collected from the Esteiro de Estarreja (Ria de Aveiro, Portugal), which receives considerable quantities of waste mercury from a chlor-alkali plant. Dissolved and particulate Hg concentrations in the effluent ranged between 4 –167 g I^–1 and 141–3144 g g^–1, respectively, at pH values of >10. The effluent plume undergoes significant chemical changes during advection downestuary. The evidence suggested that adsorption of dissolved Hg onto organic-rich SPM was an important process. A maximum sediment Hg concentration of 500 g g^–1 was found about 1.5 km from the discharge, as a result of the settling of Hg-rich SPM. Downestuary Hg concentrations in sediments decline to about 100 g g^–1 at the mouth of the Esteiro. The particle-water interactions are discussed in terms of the transport of dissolved and particulate Hg into the Ria de Aveiro.     

The impact of the Scheldt input on the trace metal distribution in the Belgian coastal area (results of 1981–1983 and 1995–1996)

Offshore fluxes of Cu, Zn, Cd, Pb and Hg were calculated based onresidual flow patterns and salinity gradients along the Belgian coast. Theresidual flow lines along the Belgian coast are more or less parallel to thecoast except in the area where the north-easterly flowing watermass comingfrom the Channel encounters the south-westerly-oriented Scheldt outflow,forming a residual hydrodynamical front. From the steady-state salinitypattern, diffusion coefficients perpendicular to the residual flow werededuced; they ranged from 21 to 108 m² s^-1. Offshore fluxes of dissolved and particulate trace metals based on diffusiveand mixing processes are calculated. The steady state profiles of dissolvedmetals show a dilution effect in the coastal waters, reaching an almostconstant concentration in the marine watermass in the 1981–1983dataset. The ratios of the Scheldt input of trace metals to the totaldissolved offshore flux vary from 38 to 55% (1981–1983),depending on the kind of metal, and from 55 to 91% (1995–1996).The ratio of the Scheldt input to the dissolved metal flow parallel to thecoast, is in both periods (1981–1983 and 1995–1996), smallerthan 1%. The steady-state concentration profiles of particular metalsversus salinity are fairly constant in the coastal-estuarine and marinewatermasses, but decrease very abruptly from the first to the secondwatermass. Assuming a conservative behaviour of the particular metals,offshore fluxes and the resulting concentration increases agree fairly wellwith the observed values. The ratios of the Scheldt input to the particulatetrace metal offshore flux vary between 30 to 46% (1981–1983)and 13 to 37% (1995–1996). The contribution of the Scheldtestuary to the flows parallel to the coast ranges from 1.6 to 2.9%(1981–1983) and from 0.6 to 1.6% (1995). This revised version was published online in July 2006 with corrections to the Cover Date.     

Benthic variability in intertidal soft-sediments in the mesohaline part of the Schelde estuary

The benthic (zoobenthos and microphytobenthos) and physical characteristics of intertidal sediments were studied in April and September 1997 on 10 locations, differing in elevation and exposure to tidal currents, situated on three mudflats in the mesohaline part of the Schelde estuary. Sediment characteristics were spatially and temporally relatively homogeneous among the sampling locations, all characterized by a high proportion of mud. It emerged that the nature of the sediment (in terms of granulometric characterization, organic matter) did not fully explain the substantial spatial and temporal (seasonal) variability in the benthos (pigment contents and benthic fauna), but that other environmental conditions explained the observed variability. Chlorophyll a and fucoxanthin contents in the top layer (0–0.5 cm) of the sediment were about 7 times higher in April than in September, but a large spatial variation was observed, especially in April, which was mainly attributed to a difference in elevation of the mudflats. The high pigment contents in April were accompanied by a relative low abundance of the benthic fauna. In April sub-surface deposit feeders, mainly Oligochaeta, were numerically most important in the 1000 and 500 m fraction of the zoobenthos, and Nematodes in the 250 m fraction. The surface deposit feeders Manayunkia aestuarina and Copepoda were locally very abundant, showing a strong positive correlation with elevation and pigment contents. The critical shear stress for erosion, measured in situ with the SedErode device, varied between 0.26 and 0.43 N m^–2 in April. The critical shear stress for erosion was lower in September when pigment contents were significantly lower and benthic fauna abundance was higher. In September Corophium volutator, Heteromastus filiformis, Nereis diversicolor and Macoma balthica dominated the 1000 m fraction, Oligochaeta and C. volutator the 500 m fraction, and Nematodes, M. aestuarina, Oligochaeta and Copepoda the 250 m fraction. It is argued that in estuarine systems with high seasonal variability in river flow rate and therefore salinity, the upper-middle estuarine fauna may switch each year between an oligohaline and a mesohaline fauna, which result in communities that seldom progress beyond early benthic-community succession. The majority of the benthic infauna was found in the upper 3 cm in both seasons, with surface deposit feeders (e.g. C. volutator, M. aestuarina) mainly occurring in the top 1 cm of the sediment, showing a strong correlation with both elevation and pigment contents and sub-surface deposit feeders (e.g. H. filiformis, Oligochaeta) occurring deeper in the sediment, showing a positive correlation with the mud content of the sediment.     

The use of Quetol 651 for the post-embedding immunohistochemical demonstration of gamma-aminobutyric acid on semithin sections

Summary Quetol 651 was used as an embedding medium for the demonstration of aminobutyric acid (GABA) in semithin sections by the peroxidase—anti-peroxidase method. In order to demonstrate the immunoreactivity, the embedding medium was partially dissolved using absolute ethanol containing 0.8–1m NaOH or KOH for 5–7 min. The experimental procedure was elaborated by testing the GABAergic sites in the endings surrounding the small neurones of the anterior exterolateral nucleus of a mormyrid fish and in the pyramidal cells of the electrosensory lateral line lobe of gymnotoid fish by applying anti-GAD (glutamic acid decarboxylase) antiserum. To test the general validity of the use of Quetol 651, GABAergic sites were also identified in the central nervous system of an insect, the honey bee, with anti-GABA and anti-GAD antisera. The intensity of labelling revealed by immunoperoxidase applied to Quetol 651-embedded semithin sections, demonstrated high precision and gave good resolution for light microscopical observations.     

Response to stress of mussels (Mytilus edulis) exposed in dutch tidal waters

• 1.1.Mussels were exposed in the Dutch coastal zone and the Western Scheldt estuary. After six weeks of exposure, trace metals, PCBs and PAHs were measured in the soft tissue.
• 2.2. Tissue concentrations of contaminants are high in the Western Scheldt and intermediate near the Rhine outflow, except for the lower PCB congeners.
• 3.3. Results show that the survival in air is significantly lower at higher tissue concentrations, in particular of the lower PCB congeners.
• 4.4. The clearance rate is reduced at the highest tissue concentrations.