The influence of salinity on the solubility of Zn and Cd sulphides in the Scheldt estuary

In the estuary of the river Scheldt, where an oxygen gradient exists in addition to the salinity gradient, redox processes will be of major importance for trace metal mobilisation. In this study, the influence of salinity and pH on the redox processes of dissolved Zn and Cd sulphides is investigated together with the effects on the ratio of the dissolved Zn and Cd concentrations. The speciation of these metals is calculated with the chemical equilibrium programme MINEQL + .Zn sulphides are oxidised at lower oxygen concentrations than Cd sulphides, due to lower stability constants, causing a sudden increase or peak in the dissolved Zn/Cd ratio. The formation of dissolved Cd chloride complexes when oxidation occurs at high salinities (S = 15) increases the mobility of Cd, causing a decrease in the Zn/Cd peak of the total dissolved concentrations. The peak is three to four times smaller at S = 15 than when oxidation occurs at S = 2. The simple model calculations compare very well with field data. The Scheldt estuary is suitable to illustrate these calculations. In the, 1970s, the anoxic part of the estuary reached S = 15-20, but since the early 1980s it has dropped to S = 2-10. Historic data on metals in the estuary from 1978, 1987 and the 1990s were used to compare with the equilibrium calculations. The increase of the dissolved Zn/Cd peak at low salinity as a consequence, of the decreasing anoxic region is confirmed well by the data. The good agreement between model calculations and field data is a proof of the extreme importance of redox processes for the solubility of Zn and Cd sulphides in the estuary

Dissolved aluminium in the ocean conveyor of theWest Atlantic Ocean: Effects of the biological cycle, scavenging, sediment resuspension and hydrography

The concentrations of dissolved aluminium (dissolved Al) were studied along the West Atlantic GEOTRACESGA02 transect from 64°N to 50°S. Concentrations ranged from~0.5 nmol kg-1 in the high latitude surface watersto ~48 nmol kg-1 in surfacewaters around 25°N. Elevated surfacewater concentrations due to atmospheric dustloading have little influence on the deep water distribution. However, just belowthe thermocline, both Northernand Southern Hemisphere Subtropical Mode Waters are elevated in Al, most likely related to atmospheric dustdeposition in the respective source regions.In the deep ocean, high concentrations of up to 35 nmol kg-1 were observed in North Atlantic DeepWater as aresult of Al input via sediment resuspension. Comparatively lowdeepwater concentrationswere associatedwithwater masses of Antarctic origin. During water mass advection, Al loss by scavenging overrules input viaremineralisation and sediment resuspension at the basin wide scale. Nevertheless, sediment resuspension ismore important than previously realised for the deep ocean Al distribution and even more intensive samplingis needed in bottom waters to constrain the spatial heterogeneity in the global deep ocean.This thus far longest (17,500 km) full depth ocean section shows that the distribution of Al can be explained by itsinput sources and the combination of association with particles and release from those particles at depth, thelattermost likelywhen the particles remineralise. The association of Alwith particles can be due to incorporationof Al into biogenic silica or scavenging of Al onto biogenic particles. The interaction between Al and biogenicparticles can lead to the coupled cycling of Al and silicate that is observed in some ocean regions. However, inother regions this coupling is not observed due to (i) advective processes bringing in older water masses thatare depleted in Al, (ii) unfavourable scavenging conditions in the water column, (iii) low surface concentrationsof Al or (iv) additional Al sources, notably sediment resuspension

Neodymium isotopic composition and concentration in the western North Atlantic Ocean: Results from the GEOTRACES GA02 section

The neodymium (Nd) isotopic composition of seawater is commonly used as a proxy to study past changes in the thermohaline circulation. The modern database for such reconstructions is however poor and the understanding of the underlying processes is incomplete. Here we present new observational data for Nd isotopes and concentrations from twelve seawater depth profiles, which follow the flow path of North Atlantic Deep Water (NADW) from its formation region in the North Atlantic to the northern equatorial Atlantic. Samples were collected during two cruises constituting the northern part of the Dutch GEOTRACES transect GA02 in 2010. The results show that the different water masses in the subpolar North Atlantic Ocean, which ultimately constitute NADW, have the following Nd isotope characteristics: Upper Labrador Sea Water (ULSW), eNd = -14.2 ± 0.3; Labrador Sea Water (LSW), eNd = -13.7 ± 0.9; Northeast Atlantic Deep Water (NEADW), eNd = -12.5 ± 0.6; Northwest Atlantic Bottom Water (NWABW), eNd = -11.8 ± 1.4. In the subtropics, where these source water masses have mixed to form NADW, which is exported to the global ocean, upper-NADW is characterised by eNd values of -13.2 ± 1.0 (2sd) and lower-NADW exhibits values of eNd = -12.4 ± 0.4 (2sd). While both signatures overlap within error, the signature for lower-NADW is significantly more radiogenic than the traditionally used value for NADW (eNd = -13.5) due to the dominance of source waters from the Nordic Seas (NWABW and NEADW). Comparison between the concentration profiles and the corresponding Nd isotope profiles with other water mass properties such as salinity, silicate concentrations, neutral densities and chlorofluorocarbon (CFC) concentration provides novel insights into the geochemical cycle of Nd and reveals that different processes are necessary to account for the observed Nd characteristics in the subpolar and subtropical gyres and throughout the vertical water column. While our data set provides additional insights into the contribution of boundary exchange in areas of sediment resuspension, the results for open ocean seawater demonstrate, at an unprecedented level, the suitability of Nd isotopes to trace modern water masses in the strongly advecting western Atlantic Ocean

Interactions of algal ligands, metal complexation and availability, and cell responses of the diatom Ditylum brightwellii with a gradual increase in copper

A continuous culture experiment was conducted to study interactions between copper-binding ligands released by light- limited Ditylum brightwellii, and toxic effects of Cu. on this diatom. Over 6 months, the Cu concentration in the medium has been increased in seven steps (3-173 nM). At each Cu. addition, Cu. speciation, characteristics of Cu sorption to cellular binding sites, and cell characteristics were determined. Physiological effects of Cu. were studied, using indicators for metal detoxification (thiols) and lipid peroxidation (malondialdehyde). Minor amounts of Cu (< 1.4%) were chelated by a minimum amount of EDTA (57 nM), required to maintain a stable long-term continuous culture. The responses of D. brightwellii to Cu were monitored. (1) From 3 to 47 nM added Cu, decreasing pools of glutathione, increasing malondialdehyde contents, an increased release of lipophilic ligands, and cell lysis indicated the enhancement of lipid peroxidation. (2) From 47 to 94 nM Cu, a 16-fold increase in high-affinity (strong) hydrophilic ligands was measured (conditional stability constants K' approximate to 10(12)) that complexed most Cu (maximum 97%); sexual reproduction was stimulated and cell volumes increased. (3) From 126 nM Cu, glutathione pools increased again, whereas cell division rates decreased slightly. (4) At 142 nM Cu, the number of lysed cells reached a maximum, as did the production of lipophilic compounds that complexed similar to2% Cu. As the binding sites of the strong ligands became Cu-saturated above 142 nM Cu, larger amounts of Cu were bound to low-affinity (weak) dissolved ligands (3-30%) and cellular binding sites (0.2-2.5%). Probably due to saturation of organic complexes at 142 nM Cu, the MINEQL- calculated Cu2+ concentrations increased markedly; pCu values decreased from > 11 to similar to 10; division rates were further inhibited; gamma-glutamylcysteine (phytochelatin precursor) was produced. (5) At 157 nM Cu, phytochelatin synthesis started, and Cu-sorption capacities (cell walls and internal binding sites) increased. (6) At 173 nM Cu, the phytochelatin pool sizes and the number of cellular Cu-binding sites increased further. These results suggest that ligands released by a dense bloom of D. brightwellii, either by active excretion or lysis, would have lower affinities for Cu (K' approximate to 10(9)-10(12)) and moderate the availability of Cu less effectively than ligands in natural environments (10(13)-10(14)). In this diatom, the concurring release of ligands, enhanced malondialdehyde production, increasing numbers of presexual cells and cell enlargement may serve as early-warning signals for Cu toxicity, rather than metal- specific phytochelatins that appeared at a stage when cell division was already clearly inhibited. [KEYWORDS: binding site; cell volume; copper; diatom; malondialdehyde; speciation Oxidative stress; nitzschia-closterium; silene-cucubalus; maize seedlings; cadmium; phytoplankton; damage; accumulation;speciation; defense]

Comparison of chemical speciation of copper in the Oosterschelde and Westerschelde estuaries, The Netherlands

The aim of this investigation was to assess the chemical speciation of copper. Although the Cu concentrations in both the dissolved and particulate phases differ strongly between the Oosterschelde (OS) and Westerschelde (WS) estuaries, The Netherlands, the Cu content of the bivalve Macoma balthica from both sea arms is comparable. Therefore, chemical speciation of Cu was examined during 1 year at Mo sites, one in each sea arm. Two dissolved organic ligand groups could be distinguished. For both sea arms, a relatively weak ligand group with logK'=9.27 and a mean ligand concentration of 250 neq Cu l(-1) was determined. Moreover, a relatively strong group was detected with logK'=13.04 and a mean ligand concentration of 180 neq Cu l(-1) in the WS, and with logK'=13.7 and a mean ligand concentration of 48 neq Cu l(-1) in the OS. In both sea arms, the calculated free concentration of CU2+ is extremely low

Comparison of chemical speciation of copper in the Oosterschelde and Westerschelde estuaries, The Netherlands

The aim of this investigation was to assess the chemical speciation of copper. Although the Cu concentrations in both the dissolved and particulate phases differ strongly between the Oosterschelde (OS) and Westerschelde (WS) estuaries, The Netherlands, the Cu content of the bivalve Macoma balthica from both sea arms is comparable. Therefore, chemical speciation of Cu was examined during 1 year at Mo sites, one in each sea arm. Two dissolved organic ligand groups could be distinguished. For both sea arms, a relatively weak ligand group with logK'=9.27 and a mean ligand concentration of 250 neq Cu l(-1) was determined. Moreover, a relatively strong group was detected with logK'=13.04 and a mean ligand concentration of 180 neq Cu l(-1) in the WS, and with logK'=13.7 and a mean ligand concentration of 48 neq Cu l(-1) in the OS. In both sea arms, the calculated free concentration of CU2+ is extremely low (<10(-14) M). The strong ligand group is related to salinity and to dissolved organic carbon (DOG) indicating that the river Scheldt and estuarine/marine DOC supply this material. The weak ligand group is related to salinity and marine chlorophyll a in the WS, and to DOC in the OS. The regulating mechanisms of the distribution of Cu over the chemical species are different for the two sea arms. In the WS, there seems to be no equilibrium between dissolved and particulate Cu. Hence the free Cu concentration is determined by complexation with dissolved organic ligands. In the OS, adsorption on particulate organic matter is the key factor. It is even possible to make a good estimate of free Cu in the OS if only dissolved and particulate Cu and POC are known. It is concluded that Macoma does not accumulate Cu from the dissolved phase since the free Cu concentration is too low; food must be the source. Since desorption of Cu from particulate matter in the polluted WS is slow in contrast to desorption in the relatively clean OS, kinetics of particulate Cu seem to be the reason for the relatively high Cu content of Macoma in the OS. [KEYWORDS: estuaries; copper; speciation; bioavailability; bivalves Cathodic stripping voltammetry; trace-metals; eastern scheldt; organic-ligands; complexation; seawater; sea; phytoplankton; cadmium; pacific]

Relations between free copper and salinity, dissolved and particulate organic carbon in the Oosterschelde and Westerschelde, Netherlands

In a previous investigation, relatively simple relations were found to estimate free Cu, and hence a good approximation of bio-available Cu, at two sites in estuarine waters of the Dutch Oosterschelde and Westerschelde. In the present study, these relations are validated for the remainder of the two sea arms. In the Westerschelde the relation between the concentration of a dissolved organic ligand (K = 10(13)), DOC (dissolved organic carbon) and salinity is valid in a range of salinities at least from 10 up to 25 to 30. This ligand group is primarily riverine and partly estuarine. It is, as far as we know, the strongest Ligand for Cu, and its concentration dominantly affects the free Cu ion concentrations, which are around 50 fM. At high salinities, dilution and mineralisation diminish the riverine member of the ligand group too much and the relation with salinity and DOC is no longer valid. In the Oosterschelde the relations between POC (particulate organic carbon) and particulate Cu and between POC and the ratio of particulate and dissolved Cu are valid for the western, central and eastern compartments. Since equilibrium was assumed to exist between dissolved and particulate Cu, foe Cu can be calculated from POC and particulate Cu concentrations. Due to the large adsorption capacity of POC, the Oosterschelde is with respect to Cu a very well-buffered system with [Cu2+] of 20 fM. [KEYWORDS: estuaries; copper; speciation; bio-availability Cathodic stripping voltammetry; scheldt estuary; eastern scheldt; trace-metals; sea-water; complexation; speciation; zinc; bay; cu]

Vertical gradients for particulate Cu fractions in estuarine water over tidal flats

The speciation of particulate copper was determined at several depths (0, 5 and 15 cm above the sediment surface) in the water column above intertidal flat systems in the polluted estuary Westerschelde (WS) and the relatively un-polluted Oosterschelde sea-arm (OS), in order to assess differences in water quality between the main body of water routinely determined in monitoring programmes and that at the sediment-water interface which is factually interacting with the benthic organisms. The bioavailable fraction was estimated by two sequential extraction's: a pH=5 acetic acid (Ac) and a pH=8 Na- dodecylsulphate (Dod) extraction. Irrespective the different character of the systems, when the differences in salinity were taken into account, dissolved fractions (DOC, pH) differed hardly between seasons and stations. Although occasionally a quantitative differentiation between sampling depths could occur, with higher particulate concentrations (Cu, Ac-Cu, Dod- Cu, C, sediment, POC, chlorophyll a) near the sediment-water interface, the qualitative properties (Cu/POC, Ac-Cu/POC, Dod- Cu/POC, Chl-a/POC, C/N) of the water remained constant all over the tidal cycle and between depths. The measurements carried out during monitoring programmes in main water streams thus result in a strong under-estimation of the quantities of particulate substances (seston, POC, chlorophyll a, Cu) available to the benthos at intertidal flats, whereas the qualitative (relative) properties are correctly estimated. With regard to the interaction with benthic organisms, it is concluded that the unexpected levels of Cu in the clam Macoma balthica (equal or higher in the OS than in the polluted Westerschelde) are partly caused by differences in sediment Cu concentrations (Hummel et al., 1997). Additionally the low concentration of suitable, chlorophyll a related, food in OS force the clam to filtrate at higher rates, whereby a higher volume of water and thus ultimately an equal or higher amount of Cu, will be taken in. [KEYWORDS: copper; speciation; estuary; bioavailability particulate organic carbon; bivalve Macoma-balthica mollusca; dutch wadden sea; scheldt estuary; trace-metals; organic-carbon; food-intake; netherlands; sediment; copper; matter]

A comparison of iron limitation of phytoplankton in natural oceanic waters and laboratory media conditioned with EDTA

The solubility of iron in oxic waters is so low that iron can be a limiting nutrient for phytoplankton growth in the open ocean. In order to mimic low iron concentrations in algal cultures, Ethylenediaminetetraacetate (EDTA) is commonly used. The presence of EDTA enables culture experiments to be performed at a low free metal concentration, while the total metal concentrations are high. Using EDTA provides for a more reproducible medium. In this study Fe speciation, as defined by EDTA in culture media, is compared with complexation by natural organic complexes in ocean water where Fe is thought to be limited. To grow oceanic species into iron limitation, a concentration of at least 10−4 M EDTA is necessary. Only then does the calculated [Fe3+] concentrations resemble those found in natural sea water, where the speciation is governed by natural dissolved organic ligands at nanomolar concentrations. Moreover, EDTA influences the redox speciation of iron, and thus frustrates research on the preferred source of Fe-uptake, Fe(III) or Fe(II), by algae. Nowadays, one can measure the extent of natural organic complexation in sea water, as well as the dissolved Fe(II) state, and can use ultra clean techniques in order to prevent contamination. Therefore, it is advisable to work with more natural conditions and not use EDTA to create iron limitation. This is especially important when the biological availability of the different chemical fractions of iron are the subject of research. Typically, many oceanic algae in the smallest size classes can still grow at very low ambient Fe and are not easily cultivated into limitation under ambient sea water conditions. However, the important class of large oceanic algae responsible for the major blooms and the large scale cycling of carbon, silicon and other elements, commonly has a high Fe requirement and can be grown into Fe limitation in ambient seawater.