Dissolved nutrient distributions in the Central English Channel

Samples for analysis of nitrate plus nitrite, dissolved reactive phosphate and silicate were collected in the English Channel along a transect between Cherbourg and the Isle of Wight, within the framework of the FLUXMANCHE II programme. A total of 130 samples were obtained during 15 cruises between December 1994 and December 1995, thus giving a full seasonal coverage. Nutrient concentrations were generally highest on the UK side of the Channel, where riverine inputs were greatest. Nutrient fluxes across this transect from west to east for the winter period, when the nutrients exhibited quasi-conservative behaviour, were calculated using the N, P and Si data, and estimates of water fluxes derived from a two-dimensional hydrodynamic numerical model. A gyre system off the Isle of Wight led to localised east to west fluxes of nutrients. Although nutrient concentrations are lower in the central Channel than in the coastal zones, the nutrient fluxes are greatest through this central zone because of the higher fluxes of water through this part of the transect. The fluxes obtained through this mid-Channel transect are greater than fluxes estimated through the Straits of Dover for a similar winter period as part of the earlier FLUXMANCHE I (December 1990– March 1991) programme, suggesting significant variability from year to year, and/or removal of nutrients in the eastern Channel.     

Fluxes and seasonal changes in composition of organic matter in the English Channel

Dissolved and particulate organic matter (DOM and POM) have been investigated along a transect between Cherbourg and the Isle of Wight. In addition, the relative contribution of different sources of POM have been assessed by the use of lipid biomarkers (e.g. fatty acids). Seawater samples were collected at two depths (subsurface and above the bottom) at five stations located on the transect during five cruises (from September 1994 to July 1995). Particulate organic carbon (POC) and dissolved organic carbon (DOC) concentrations vary between 30–530 μg l⁻¹ and 0.5–2.7 mg l⁻¹, respectively, for all the cruises. Fluxes of POM and DOM have been estimated at 0.6×10¹² g yr⁻¹ and 6.5×10¹² g yr⁻¹ of carbon, respectively. General fluxes of water and therefore of DOC and POC are oriented eastward. However, around the Isle of Wight a westward oriented flux exists due to a gyre located in the area. The major DOC and POC fluxes occur in the central part of the Channel where the water column is deepest. Seasonal variations of different sources of POM (algal, bacterial and terrigenous) have been examined for the five cruises. The fresh algal organic fraction is relatively important in September in coastal waters with a predominance of diatom species on the English side, whereas it has a low or undetectable contribution during winter months. The bacterial fraction generally varies in concert with the algal component. It is low during the winter period and more important in bloom or post-bloom conditions, as for example in May. Terrestrial organic matter is restricted to coastal areas in September, and is present at low levels in May and July. Nevertheless, in November and February, terrigenous inputs have been clearly identified for the whole transect even in central waters.     

Sources, sinks and resuspension of suspended particulate matter in the eastern English Channel

Seasonal observations on the nature and concentration of suspended particulate matter (SPM) are presented for a cross-section of the English Channel, between the Isle of Wight (UK) and Cotentin peninsula (France) i.e. the western boundary of the eastern English Channel. The highest concentrations of suspended material are found adjacent to the English coastline, whereas the offshore waters are associated with low concentrations. Seasonal variations in the concentration and nature of suspended material are identified, with highest concentrations in winter. At this time, the suspended particles are characterised generally by peaked grain size spectra and an enrichment in coarse silt particles; in summer, the distributions are generally flat. The diatom communities found within the suspended matter indicate that material resuspended in the coastal zone and the estuarine environments is transported offshore. SPM fluxes (based upon the observed SPM concentrations and the output from a 2-D hydrodynamic model) from the western Channel through the Wight–Cotentin Section, ranged between 2 and 71×10⁶ t a⁻¹ with a mean of around 20×10⁶ t a⁻¹ over the period of the observations (1994–1995). These fluxes are comparable to the order of magnitude and mean value reported as output through the Dover Strait. Therefore, it is possible that the eastern English Channel may be characterised as an area of fine-grained sediment ‘bypass'. This interpretation is corroborated by: (a) the absence of fine-grained sediment deposits over the area; and (b) correlation between the potential resuspension time of the fine particles and the seabed sediment distribution.     

Some aspects of vanadium and chromium chemistry in the English Channel

Dissolved and particulate vanadium and chromium concentrations along a transect between Cherbourg and the Isle of Wight were investigated in the English Channel. Seawater samples were collected at two different depths (surface and bottom) at five sampling stations during five cruises carried out between September 1994 and September 1995. Calculated mass flows through this Channel section were 7600 T yr⁻¹ for vanadium (about 66% was in the dissolved phase) and 1650 T yr⁻¹ for chromium (about 50% was in the dissolved phase). Dissolved chromium concentrations do not vary significantly along the transect. Seasonal variations in chromium distribution linked to biotic parameters were noted during the September 1994 cruise, when a significant relationship between particulate chromium and algal organic carbon was found. In addition, dissolved Cr (III) and Cr (VI) were well correlated with both algal and terrestrial organic matter. In the May cruise, during phytoplanktonic decay, particulate chromium was correlated with the detritic and bacterial organic fractions. These observations suggest interactions between chromium and biotic material. During the winter period, no relationship between chromium and POC was found. Dissolved and particulate vanadium concentrations varied, respectively, between 15 and 28 nmol l⁻¹ and 2–32 nmol l⁻¹. Values of dissolved vanadium showed depletion in the Channel with respect to oceanic waters. This loss in dissolved vanadium was higher near the English coast, but was compensated for by the increase in the particulate vanadium. There was no clear relationship between dissolved vanadium and algal organic carbon and it is inferred that the vanadium transfer cannot be explained by trapping with biotic material. On the other hand, the dissolved vanadium depletion could be attributed to the presence of ferric oxyhydroxide phases in particles, which have strong adsorbing properties for a range of dissolved metal ions.     

Copper,zinc, cadmium,nickel, iron and manganese in the Southern Bight of the North Sea

Ni, Cd, Cu, Zn, and Fe were determined in water and suspended matter samples and Mn in suspended matter samples from the Southern Bight region of the North Sea. Four distinct areas, comprising the Rhine estuary, the Dutch coastal waters, the centre of the Bight, and the English coastal water can be distinguished on the basis of the distribution of the trace metals. The total concentrations of all metals were high in the Rhine estuary, decreased in a seaward direction to reach very low values, comparable to continental shelf water, in the centre of the Bight, and increased again towards the English coast. Elevated concentrations of dissolved Cu, Ni, Cd and Zn near the English coast are not only due to river inputs. Over the entire region, the fraction of Cd, Ni and Zn in solution was consistently more important than the fraction in the suspended particulate phase. In contast, the fraction of Fe and Mn in the solid phase was consistently greater than the fraction in solution. Total concentrations of Fe, Mn and Cu are more dictated by the suspended particulate matter concentration than Cd, Ni and Zn.     

Dissolved and particulate trace metals in a Scottish sea loch: an example of a pristine environment?

In the near pristine environment of a silled fjord on the west coast of Scotland samples were taken for the determination of dissolved and particulate trace metals (Fe, Mn, Cu, Ni, Cd, Zn and Pb), together with nutrient and hydrographic data, during 19 surveys carried out over a year. An indication of the pristine nature of the environment are the low concentrations of dissolved silicon, phosphate and nitrate which are considerably lower than those of coastal waters which are subject to larger anthropogenic burdens. Distributions of dissolved Cu, Ni and Cd were found to broadly reflect conservative mixing of freshwater and seawater with both end members having similar concentrations. The concentration of dissolved Cu and Ni in seawater entering upper Loch Linnhe (Cu 0.28 μg l⁻¹; Ni 0.26 μg l⁻¹) was consistent with the 1:1 conservative mix of Irish Sea water and North Atlantic surface water predicted from radio-caesium tracer experiments (Mackay & Baxter, 1985). Atmospheric input of trace metals to upper Loch Linnhe appeared to be a relatively minor term in the mass balance relative to fluvial inputs. Values of distribution coefficients Kd were similar to those previously reported for the coastal environment. Iron showed the strongest affinity for the suspended sediments; with particulate percentages of the total load usually greater than 80%. Lead and Mn showed a similar strong affinity to the particle phase. For Cu, Ni and Zn the mass of the element in the dissolved phase was generally greater than that in the particulate fraction. Cadmium, was least associated with the particles, with typically greater than 90% existing in the dissolved phase.     

Distribution of dissolved and labile particulate trace metals in the overlying bottom water in the vistula river plume (southern Baltic Sea).

Overlying bottom water samples were collected in the Vistula River plume, southern Baltic Sea, (Poland) and analysed for dissolved and labile particulate (1 M HCl extractable) Cu, Pb, Zn, Mn, Fe and Ni, hydrological parameters being measured simultaneously. Particulate organic matter (POM), chlorophyll a and dissolved oxygen are key factors governing the chemical behaviour of the measured metal fractions. For the dissolved Cu, Pb, Zn, Fe and Ni two maxima, in the shallow and in the deeper part of the river plume, were found. In the shallow zone desorption from seaward fluxing metal-rich riverine particles account for markedly increased metal concentrations, as confirmed also by high particulate metal contents. For Pb, atmospheric inputs were also considered to have contributed to the elevated concentrations of dissolved Pb adjacent to the river mouth. In the deep zone desorption from detrital and/or resuspended particles by aerobic decomposition of organic material may be the main mechanism responsible for enrichment of particle-reactive metals (Cu, Pb, Zn) in the overyling bottom waters. The increased concentrations of dissolved Fe may have been due to reductive dissolution of Fe oxyhydroxides within the deep sediments by which dissolved Ni was released to the water. The distribution of Mn was related to dissolved oxygen concentrations, indicating that Mn is released to the water column under oxygen reduced conditions. However, Mn transfer to the dissolved phase from anoxic sediments in deeper part of the Vistula plume was hardly evidenced suggesting that benthic flux of Mn occurs under more severe reductive regime than is consistent with mobilization of Fe. Behaviour of Mn in a shallower part has been presumably affected by release from porewaters and by oxidization into less soluble species resulting in seasonal removal of this metal (e.g. in April) from the dissolved phase. The particulate fractions represented from about 6% (Ni) and 33% (Mn, Zn, Cu) to 80% (Fe) and 89% (Pb) of the total (labile particulate plus dissolved) concentrations. The affinity of the metals for particulate matter decreased in the following order: Pb > Fe > Zn > or = > Cu > Mn > Ni. Significant relationships between particulate Pb-Zn-Cu reflected the affinity of these metals for organic matter, and the significant relationship between Ni-Fe reflected the adsorption of Ni onto Fe-Mn oxyhydroxides. A comparison of metal concentrations with data from other similar areas revealed that the river plume is somewhat contaminated with Cu, Pb and Zn which is in agreement with previous findings on anthropogenic origin of these metals in the Polish zone of southern Baltic Sea.     

Atmosphere-coastal ocean fluxes of particulate arsenic and antimony

Aerosol concentrations of arsenic determined on samples from the English Channel were in the range<0.05 to 11.0 ng As (SCM)⁻¹ and from the North Atlantic<0.03 to 0.36 ng As (SCM)⁻¹. Antimony concentrations were in the range<0.06 to 2.7 ng Sb (SCM)⁻¹ in the English Channel and<0.02 to 0.4 ng Sb (SCM)⁻¹ for the North Atlantic. The bulk aerosol in each area was predominantly of marine origin, although an anthropogenic component was evident in some samples. Aerosol deposition to coastal waters may be a more important source of dissolved arsenic and antimony than the riverine input. The results suggest that the flux of soluble arsenic from the aerosol to the English Channel is five times higher, and that of antimony 15 times higher, than the corresponding dissolved fluxes from the River Tamar. The concentrations and fluxes of the metals in the open ocean are an order of magnitude lower than in the coastal environment. The potential for the interhemispheric transfer of the metals is also examined.     

Concentrations and transport of trace metals in the St. Lawrence River

Samples of raw water were collected in the St. Lawrence River during six sampling trips from August 1990 to April 1992. Water samples were analyzed for both dissolved and particulate phases for five trace metals. Partition coefficients (Kd) and metal fluxes were calculated in order to determine metal transport. A mass balance equation was used for the determination of the major metal sources to the St. Lawrence River and an estimation of metal loadings to the estuary was made. Average dissolved metal concentrations were found to be Cd 10 ng/L, Co 74 ng/L, Cu 64 ng/L, Fe 69 µg/L and Mn 700 ng/L. Particulate concentrations were (in µg/g) 1.68 for Cd, 31 for Co, 73 for Cu, 25 mg/g for Fe and 1.69 mg/g for Mn. Co, Fe and Mn were transported essentially in the particulate phase while Cd and Cu were predominantly found in the dissolved phase at 56% and 48% of the total metal concentration respectively. Log Kd values varied from 5.1 (for Cu) to 6.8 (for Mn). In the dissolved phase the major sources were found to be the Great Lakes and the Ottawa River whereas in the particulate phase Québec tributaries appear to be the most important. Industrial inputs are quite important in both the dissolved and the particulate phases for Cd, whereas other sources are very variable, especially for the dissolved phase.     

A reassessment of trace metal budgets in the Western Mediterranean Sea

This paper presents inputs and output fluxes of dissolved metals (As, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn) into and out the Western Mediterranean. These flux estimates are based on the most recently published concentrations and fluxes for the atmosphere, the rivers and the straits. Comparison of the different sources shows the predominance of the inputs through the straits over other sources. The river input is smaller than the atmospheric input except for As. For all elements except Fe, output flux and input flux are balanced; iron budget indicates transfer from the dissolved to the particulate phase.     

Oceanographic distributions of cadmium,zinc, nickel,and copper in the North Pacific

Vertical profiles of Cd, Zn, Ni, and Cu have been determined at three stations in the North Pacific and in the surface waters on a transect from Hawaii to Monterey, California. The distributions found are oceanographically consistent and provide a needed confirmation and extension of several recent studies on the marine geochemistries of these metals. Cadmium concentrations average 1.4 pmol/kg in surface waters of the central North Pacific and show a strong correlation with the labile nutrients, phosphate and nitrate, increasing to values of 1.1 nmol/kg at depths corresponding to the phosphate maximum. Zinc is depleted in surface waters of the central gyre to an average value of 0.07 nmol/kg and increases to a deep maximum of 9 nmol/kg exhibiting a strong correlation with the nutrient silicate. Nickel concentrations average 2.1 nmol/kg in surface central gyre waters and increase to a deep maximum of 11 nmol/kg. Nickel is best correlated with a combination of phosphate and silicate. Copper averages less than 0.5 nmol/kg in surface waters of the central North Pacific and increases gradually to values of 5 nmol/kg in bottom waters. The Cu profiles show evidence of intermediate and deep water scavenging. The involvement of these metals in the internal biogeochemical cycles of the sea is responsible for their distributions which are predictable on the basis of oceanographic parameters.     

Concentrations of trace elements in marine fish and its risk assessment in Malaysia

Concentrations of trace elements (V, Cr, Mn, Co, Cu, Zn, Ga, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, Hg, Tl, Pb and Bi) were determined in muscle and liver of 12 species of marine fish collected from coastal areas in Malaysia. Levels of V, Cr, Mn, Co, Cu, Zn, Ga, Sr, Mo, Ag, Cd, Sn, Ba and Pb in liver were higher than those in muscle, whereas Rb and Cs concentrations showed the opposite trend. Positive correlations between concentrations in liver and muscle were observed for all the trace elements except Cu and Sn. Copper, Zn, Se, Ag, Cd, Cs and Hg concentrations in bigeye scads from the east coast of the Peninsular Malaysia were higher than those from the west, whereas V showed the opposite trend. The high concentration of V in the west coast might indicate oil contamination in the Strait of Malacca. To evaluate the health risk to Malaysian population through consumption of fish, intake rates of trace elements were estimated on the basis of the concentrations of trace elements in muscle of fish and daily fish consumption. Some specimens of the marine fish had Hg levels higher than the guideline value by US Environmental Protection Agency (EPA), indicating that consumption of these fish at the present rate may be hazardous to Malaysian people. To our knowledge, this is the first study on multielemental accumulation in marine fish from the Malaysian coast.     

Removal fluxes of Mn and Fe from the nearshore waters along the west coast of Taiwan

Dissolved and particulate Mn and Fe in the nearshore waters were determined at 27 stations along the west coast of Taiwan during 19-23 November 2004. The latitudinal distributions of Mn and Fe were very similar; however, the concentration in the dissolved phase was lower, whereas that in the particulate phase was higher in the northern regime. The higher percentage of Fe than Mn that was associated with particles resulted in a two-order of magnitude higher particle-water partition coefficient for Fe, K_d(Fe), than that for Mn, K_d(Mn). The removal fluxes of Mn and Fe could be estimated by multiplying the particulate ²³⁴Th removal flux with the Mn/²³⁴Th and Fe/²³⁴Th ratios in suspended particles, which ranged from 0.1 to 3.8 mmol m⁻² d⁻¹ for Mn, and from 3.4 to 194.5 mmol m⁻² d⁻¹ for Fe. Residence times ranged from 1 to 20 days for Mn and Fe were estimated in the nearshore.     

Trace metals associated with deep-sea tailings placement at the Batu Hijau copper–gold mine,Sumbawa, Indonesia

The Batu Hijau copper–gold mine on the island of Sumbawa, Indonesia operates a deep-sea tailings placement (DSTP) facility to dispose of the tailings within the offshore Senunu Canyon. The concentrations of trace metals in tailings, waters, and sediments from locations in the vicinity of the DSTP were determined during surveys in 2004 and 2009. In coastal and deep seawater samples from Alas Strait and the South Coast of Sumbawa, the dissolved concentrations of Ag, As, Cd, Cr, Hg, Pb and Zn were in the sub μg/L range. Dissolved copper concentrations ranged from 0.05 to 0.65 μg/L for all depths at these sites. Dissolved copper concentrations were the highest in the bottom-water from within the tailings plume inside Senunu Canyon, with up to 6.5 μg Cu/L measured in close proximity to the tailings discharge. In general, the concentrations of dissolved and particulate metals were similar in 2004 and 2009.     

A laboratory study of the biogeochemical cycling of Fe,Mn, Zn and Cu across the sediment-water interface of a productive lake

Laboratory incubation experiments were carried out on sediment cores collected from Esthwaite Water, U.K., during April 1987, when the sediments displayed a characteristic surface (1.5 to 2 cm) oxide floc. The experiments were undertaken at 10°C, in the dark, under variable redox and pH conditions for periods of ~ 720 h (30 d). In some cases, realistic amounts of decomposing lake algae were added to simulate the deposition of an algal bloom. Pore waters and overlying waters were obtained from the incubated sediment cores at various time intervals and the samples analysed for pH and dissolved Fe, Mn, Zn and Cu by AAS. The results demonstrated that trace metal concentrations at the sediment-water interface can show rapid, pulsed responses to episodic events associated with controlling factors such as algal deposition and mixing conditions. The variations in dissolved Fe and Mn concentrations could generally be explained by their well known redox behaviour. Appreciable loss of Mn from solution under conditions of well-developed anoxia was consistent with adsorption of Mn²⁺ by FeS. Cu and Zn were both rapidly (24 h) released into solution during incubation of sediment cores prior to the development of anoxia in the overlying waters. Their most likely sources were the reductive remobilization of Mn oxides and the decomposition of organic matter. The addition of decomposing algae to a series of cores resulted in even higher interfacial dissolved concentrations of Cu and Zn, probably through acting as a supplementary source of the metals and through increased oxide dissolution. Switching from anoxic to oxic conditions also rapidly increased dissolved Cu and Zn concentrations, possibly due to their release during the oxidation of metal sulphides. The enhanced releases of dissolved Cu and Zn were generally short-lived with removal being attributed to the formation of sulphides during anoxia and to adsorption by Fe and Mn oxides under oxic conditions.     

Speciation of organic carbon,Cu and Mn in the River Marne (France): the role of colloids

The distribution of organic carbon (OC) and of some metals (Cu, Mn) amongst the particulate (>0·2 μm), colloidal (10 kDa–0·2 μm) and the truly dissolved (<10 kDa) fractions of the River Marne was investigated during the phytoplankton spring bloom. A tangential ultrafiltration (UF) device was utilized to separate the colloidal fraction. On average, 22% of the OC, 31% of the Cu and 53% of the Mn, usually assigned to the so‐called dissolved fraction, were found in the colloidal fraction. The colloidal fraction exhibited a behaviour different from that of the particulate and truly dissolved fractions. Autochthonous production led to enrichment in the colloidal and particulate OC pools: up to 47% of the total dissolved OC was in the colloidal fraction. An increase in the colloidal metal fraction, concomitant with a fall in the truly dissolved fraction, coincided with peaks in phytoplankton during the bloom. These phenomena might be related either to an increase in pH associated with photosynthetic activity, resulting in the precipitation of truly dissolved forms into the colloidal fraction, or to scavenging of the truly dissolved metals by the algal species of colloidal size. The interaction between the colloidal and the truly dissolved phases was very important. The partition coefficients of the Cu and Mn between the colloidal and truly dissolved fractions were higher than between the particulate and the truly dissolved fractions. This pattern is consistent with a greater specific surface area of colloids than macroparticles. Consequently, the adsorption and complexation capacities are enhanced in the colloidal fraction of the particulate matter. The extraction of hydrophobic complexes with Cu using C₁₈ Sep‐Pak columns, showed that the Cu occurring in colloidal, total dissolved or truly dissolved forms was significantly complexed by the organic matter. The truly dissolved fraction might be complexed up to 100% during a phytoplankton bloom. Copyright © 1999 John Wiley & Sons, Ltd.     

Dissolved trace metal distributions in Port Jackson estuary (Sydney Harbour), Australia

Concentrations of dissolved metals (Cd, Cu, Ni, Mn and Zn) were determined for summer and winter, under low-flow conditions in Port Jackson, a microtidal, well-mixed estuary in south-east Australia. Mean concentrations of Cd (0.04+/-0.02 microg/l), Ni (0.86+/-0.40 microg/l), Mn (20.0+/-25 microg/l) and Zn (6.47+/-2.0 microg/l) were below water quality guidelines. Concentrations of Cu (1.68+/-0.37 microg/l), however, slightly exceeded recommended values. Dissolved Ni and Mn behaved mostly conservatively, whereas Cd, Cu and Zn showed mid-estuarine maxima. Peaks in Cd, Cu and Zn concentrations were located in the upper estuary, independent of the salinity and suspended particulate matter loading, and were consistent with anthropogenic inputs of metals in the estuary. Concentrations of dissolved Cu were highest in summer, whereas concentrations of Cd, Ni and Mn were significantly lower in summer than winter (P< or =0.05). The increase in temperature and biological activity during summer explained the seasonal variation. The sequence of log K(d) values (20-30 salinity) was Mn>Zn>Cu>Ni. These results give unique information concerning the contemporaneous distribution of dissolved trace metals in the Port Jackson estuary and they provide a data set against which the long-term contamination may be assessed.     

Space-time variations of river water chemistry in RF southern Far East

Concentrations and seasonal variations of water chemistry, including dissolved and particulate forms of Fe, Mn, Zn, Cu, Pb, Cd, and Ni in rivers of Primorskii Krai are determined. It is shown that, unlike the macrocomposition, the effect of hydrological regime on the concentration of dissolved metal forms is controversial and depends on anthropogenic load, watershed landscapes, and pH variations. Elevated concentrations of dissolved metal forms are recorded in the beginning of spring flood and during low-water period. Beyond the limits of local impact of wastewater, the concentrations of dissolved forms of Cu, Zn, Ni, Pb, and Cd in river waters of the region insignificantly differ from the clearest rivers of the World.     

Fluxes of selected trace metals from the Seine estuary to the eastern English Channel during the period August 1994 to July 1995

Two sampling cruises conducted in the Seine estuary (France) under low-water and flood conditions produced high resolution profiles for dissolved cadmium, lead, copper, zinc and nickel concentrations versus salinity. The distribution of dissolved trace metals differed depending on hydrologic conditions, partly because of the dilution of upstream inputs during flood periods. Daily fluxes of these dissolved trace metals were estimated for the two sampling periods (September 1994 and February 1995) by extrapolating the dilution lines observed in higher salinity waters to salinity=0 and then multiplying the effective freshwater concentrations thus obtained by the corresponding freshwater flow. Several procedures were subsequently applied to deduce each daily flux for the year studied from data for these two periods. A consensus was found among these procedures, allowing the determination of net fluxes of dissolved trace metals with a precision of 20–35%. The net fluxes thus estimated were 4 T yr⁻¹ for Cd, 4 T yr⁻¹ for Pb, 40 T yr⁻¹ for Cu, 130 T yr⁻¹ for Zn and 50 T yr⁻¹ for Ni.     

Cadmium and manganese distributions in the Hudson River estuary: interannual and seasonal variability

Surface waters collected along the salinity gradient of the Hudson River estuary in four cruises between 1995 and 1997 were size-fractionated into particulate (>0.45 μm), ‘dissolved' (<0.45 μm), colloidal (10 kDa, 0.45 μm) and low molecular weight (<10 kDa) phases. Dissolved Cd concentrations (range: 0.11–1.19 nM) in surface waters of the estuary appear to have decreased fourfold (from an average of 2.36 to 0.61 nM) over a 23-year period, since the initial analysis of samples collected in the 1970s by Klinkhammer and Bender [Estuar. Coastal Shelf Sci. 12 (1981) 629–643]. This interannual decline reflects improvement in sewage treatment and the elimination of industrial Cd sources to the Hudson River estuary. In contrast, dissolved Mn levels (range: 0.033–1.46 μM) have remained relatively constant over the same period of time, suggesting that anthropogenic sources have very limited impact on Mn concentrations in the estuary. The concentrations of both Cd and Mn appeared to strongly depend on the season and/or river discharge. The highest concentrations were detected under low freshwater discharge, implying that limited hydraulic flushing allows a build-up of metals in the water column. Although the decline in Cd levels within the estuary reflects a reduction in the magnitude of anthropogenic inputs, mass balance estimates indicated that current sources of Cd to the estuary include sewage discharges (in the lower estuary around Manhattan) and diagenetic remobilization from industrial Cd deposited in sediments nearly 2 decades ago (in the upper estuary near Foundry Cove). Moreover, under low river discharge, the sources considered in our model (sewage, riverine input, atmospheric deposition, and benthic fluxes) could account for no more than 60% of the Cd exported from the lower estuary to the ocean. This suggests that undefined sources such as ground water and inputs from other watersheds (e.g., Long Island Sound and Newark Bay) may potentially influence the water quality of the New York Harbor. The size-fractionated metal concentrations indicated that most of the traditionally defined ‘dissolved' Cd and Mn consisted of <10 kDa molecular weight species. High molecular weight colloidal species of Mn accounted for about 50% of the dissolved fraction at the riverine end-member and <5% at intermediate and high salinities. Colloidal Cd accounted for <6% of the dissolved phase throughout the estuary. Unlike the non-conservative excess (relative to ideal dilution of river water and seawater) of dissolved Mn observed along the estuary, high molecular weight colloidal Mn appeared to be removed at the head of the estuary. The small contribution of colloidal Cd and Mn to the ‘dissolved' phase suggests that remobilization from suspended particulate phases and/or from sediments occurs through the formation of small molecular weight species.