Contrasting behaviour of trace metals in the Scheldt estuary in 1978 compared to recent years

Dissolved and particulate trace metals (Cu, Cd, Pb, Zn, Ni, Fe and Mn) measured at six stations along the Scheldt estuary in October/November 1978 are compared with more recent data. Based on Ca content in the suspended matter, three distinct geochemical regions could be distinguished: the upper estuary (salinity 1–7) dominated by fluvial mud, mid-estuary (salinity 7–17) where the composition of the suspended matter remained relatively constant, and the lower estuary where marine mud prevailed. Re-suspension of sediments is the major factor controlling the composition of the particles in the upstream region. Anoxic conditions prevailed in the upper part of the estuary extending to a salinity of 15 in 1978, while at present the seaward boundary of the anoxic water body is located at less saline waters. Furthermore, the present-day metal load is much lower than in 1978. As a consequence of the changed situation, maxima in dissolved concentrations of redox-sensitive metals in the mid/lower estuary have moved as well, which affects the trace metal re-distribution pattern. In the anoxic zone, exchange processes between dissolved and particulate metal fractions were strongly redox regulated, with Fe and Mn as excellent examples. Iron was removed from the dissolved phase in the early stages of mixing resulting in an increase in the suspended particulate matter of the leachable ‘non-residual' Fe fraction from 2 to 3.5%. Due to its slower kinetics, removal of Mn from solution occurred in mid-estuary where oxygen concentrations increased. Cu, Cd and Zn on the contrary were mobilised from the suspended particles during estuarine mixing. External inputs of Pb, and to a lesser extent of Cu, in the lower estuary resulted in the increase of their particulate and the dissolved concentrations. Calculated K_d (distribution coefficient) values were used to assess the redistribution between the dissolved and particulate phase of the investigated metals. Due to the existence of the anoxic water body in the upper estuary, the importance of redox processes in determining the K_d values could be demonstrated. The sequence of K_d values in the upper estuary (Fe, Cd, Zn, Pb > Cu > Ni, Mn) is significantly different from that in the lower estuary (Fe > Mn > Pb, Ni, Zn, Cu, Cd). Thus, in such a dynamic estuary single metal-specific K_d values cannot be used to describe redistribution processes.     

Ten-year observation of the Gironde tributary fluvial system: fluxes of suspended matter, particulate organic carbon and cadmium

A program of long-term observation of suspended solids (TSS), particulate organic carbon (POC) and cadmium transported into the Gironde estuary (France) by its major tributaries has been carried out between 1990 and 1999. This decade included contrasting hydrologic cycles and appears representative of a much longer period (1959–1999). The Garonne and the Dordogne river systems are the main tributaries of the Gironde estuary and derive their waters from drainage basins with different geological, industrial and agricultural features. To better understand their respective contributions, they have been observed separately and compared. Water and TSS fluxes of the Garonne River show greater temporal variations and discharge is more related to the hydrology of the drainage basin (e.g. wet/dry years, local flood events etc.). As POC and particulate Cd concentrations in suspended matter are much less variable than turbidity, their fluxes are mainly controlled by the TSS transport. A major part of annual fluxes of TSS and associated pollutants may occur within few flood days (depending on various parameters, e.g. intensity, duration, season, etc.), and also the succession of dry and wet years has an important influence on annual fluxes. The presented data allow calculating fluvial inputs into the Gironde as the sum of fluxes transported by its major tributaries, the Garonne and the Dordogne river systems. Mean annual fluxes into the Gironde observed in 1990–1999 are about 34×10⁹ m³ year⁻¹ for river water, 3.24×10⁶ t year⁻¹ for suspended solids (TSS) and 9.88×10⁹ mol year⁻¹ for particulate organic carbon (POC). Generally, these fluxes are dominated by the contributions of the Garonne River. However, in dry years, the mean contribution of the Dordogne river system (including Dronne and Isle rivers) to the POC input into the estuary exceeded that of the Garonne. This reflects significant differences in vegetation and soil due to natural properties and land management of the basins. Mean Cd fluxes into the estuary are about 110×10³ mol year⁻¹ of which 19.6×10³ mol year⁻¹ are transported in the dissolved and 90.8×10³ mol year⁻¹ in the particulate phases, respectively. In 1991 (dry year), the net (dissolved) Cd flux towards the ocean exceeded the gross fluvial input of total Cd, suggesting the release of Cd from an important stock in the maximum turbidity zone (MTZ) or the fluid mud of the Gironde estuary.     

Dissolved uranium, vanadium and molybdenum behaviours during contrasting freshwater discharges in the Gironde Estuary (SW France)

Understanding trace metal behaviour in estuarine environments requires sampling strategies and analytical methods adapted to strong physical and geochemical gradients. In this study, we present a specific sampling strategy covering a wide range of hydrological conditions during nine cruises in 2003–2007 to characterise the behaviour of three dissolved metals (uranium, vanadium and molybdenum) in surface and bottom water along the salinity gradient of the highly turbid macrotidal Gironde Estuary using a solid–liquid extraction. Uranium behaved conservatively whatever the water discharges observed. The slight dissolved U depletion compared to the theoretical dilution line between the fluvial and marine end-members occasionally observed in the low salinity range (0–3) was attributed to the mixing of different water bodies of the Gironde tributaries. In contrast, dissolved V behaviour was largely influenced by the hydrological conditions, showing increasingly pronounced addition with decreasing freshwater discharges, (i.e. increasing residence times of water and particles in the estuary). This addition of dissolved V in the low- to mid-salinity range was attributed to desorption processes observed in the Maximum Turbidity Zone (MTZ). The distribution of dissolved Mo concentrations along the salinity gradient was highly variable. Apparent conservative, and non-conservative behaviours were observed and were related to the concomitance of desorption from SPM, inputs from sediments for additive distribution and biological uptake and removal into sediments for subtractive distribution. Based on the whole database (2003–2007), annual net fluxes to the coastal ocean were estimated for dissolved U (15.5–16.6 t yr⁻¹) and V (31.3–36.7 t yr⁻¹).     

Land inputs,behaviour and contamination levels of copper in a ria estuary (NW Spain)

Copper concentrations were measured in the sediments and dissolved phase of the water column in the Lérez Estuary (Pontevedra Ria, Galicia), NW Spain, and in the freshwater-sewage inputs to the estuary. Dissolved copper fluxes in the estuary were quantified. Results show that the freshwater end-member in the Lérez Estuary has minimal copper contamination ([Cu](Dissolved)=7+/-4 nM, Sediment Enrichment Factor=1) and the source of dissolved copper inputs is localized in the saline end-member ([Cu](Dissolved)=20-55 nM, Sediment Enrichment Factor=6-7). A non-conservative behaviour of dissolved copper showing a net addition during estuarine mixing was observed. The budgetary calculations and the copper sediment distribution suggest that the major copper input to the estuary-ria system is located at the Marín-Placeres transect, playing an important role in the net addition of copper within the estuarine mixing. The importance of upwelling in the renewal of the Galician coastal waters and dilution of continental inputs is discussed.     

Trace element diagenesis in pyrite-rich sediments of the Achterwasser lagoon, SW Baltic Sea

The early diagenesis of trace elements (V, Cr, Co, Cu, Zn, As, Cd, Ba, U) in anoxic sediments of the Achterwasser, a shallow lagoon in the non-tidal Oder estuary in the Baltic Sea, was investigated in the context of pyrite formation. The dissolved major redox parameters show a two-tier distribution with transient signals in the occasionally re-suspended fluid mud layer (FM) and a permanently established diagenetic sequence in the sediment below. Intense microbial respiration leads to rapid depletion of O₂ within the uppermost mm of the FM. The reduction zones of Mn, Fe and sulfate overlap in the FM and in the permanently anoxic sediment section which appears to be a typical feature of estuarine sediments, under low-sulfate conditions. Degrees of pyritization (DOP) range from 50% in the FM to remarkably high values > 90% at 50 cm depth. Pyrite formation at the sediment surface is attributed to the reaction of Fe-monosulfides with intermediate sulfur species via the polysulfide pathway. By contrast, intense pyritization in the permanently anoxic sediment below is attributed to mineral growth via adsorption of aqueous Fe-sulfide complexes onto pyrite crystals which had originally formed in the surface layer.The studied trace elements show differential behavior patterns which are closely coupled to the diagenetic processes described above: (i) Zn, Cu and Cd are liberated from organic matter in the thin oxic layer of the sediment and diffuse both upwards across the sediment/water boundary and downwards to be trapped as monosulfides, (ii) V, Cr, Co and As are released during reductive dissolution of Mn- and Fe-oxyhydroxides, (iii) U removal from pore water occurs concomitantly to Fe reduction in the FM and is attributed to reduction of U(VI) to U(IV), (iv) the Ba distribution is controlled by reductive dissolution of authigenic barite in the sulfate reduction zone coupled with upward diffusion and re-precipitation. The incorporation of trace elements into pyrite is most intense for Co, Mn and As, intermediate for Cu and Cr and little to negligible for U, Zn, Cd, V and Ba. The observed trend is largely in agreement with previous studies and may be explained with differing rates for ligand exchange. Slow and fast ligand exchange and thus precipitation kinetics are also displayed by downcore increasing (Mn, Cr, Co and As) or constantly low (Zn, Cu, Cd) pore water concentrations. The downward increasing degrees of trace metal pyritization (DTMP) for Co, Cu, Zn and As are, in analogy to pyrite growth, assigned to adsorption of sulfide complexes or As oxyanions onto preexisting pyrite minerals.     

Speciation of dissolved copper and cadmium in the Loire estuary and over the North Biscay continental shelf in spring

Dissolved cadmium and copper species were measured in the Loire estuary and the North Biscay Bay continental shelf (France) in spring 2002. Metal behaviours along the salinity gradient, metal fluxes from the Loire system and surface distributions on the continental shelf were described and compared with the ones obtained at a period of higher discharge (i.e., winter 2001). Vertical distributions at different stations were also examined with the aim to assess whether the Loire inputs affect the Cu and Cd speciation over the continental shelf. In the Loire estuary, different metal behaviours were observed from winter. Cu and its various species mostly followed a theoretical dilution line whereas a Cd addition composed of chloro- and organic complexes was observed. According to our estimations, the Cu flux was composed, as in winter, of 40% hydrophobic organic complexes. On the other hand, Cd organic complexes accounted for less in the total dissolved metal flux than they did in winter (55 and 78% for spring and winter, respectively). On the continental shelf, a C₁₈Cu-to-TDCu ratio of 40% was observed in the stratified area under Loire influence. For Cd, the organic fraction was relatively important in the surface waters near the mouth of the estuary and decreased deeper and seaward. Thus, the Cu and Cd speciation on the North Biscay Bay continental shelf are firstly determined by the extent of the estuarine plume. Beyond the estuarine influence, biological processes appeared to be the origin of the observed variations.     

Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf

Current velocity and suspended sediment concentration measurements at anchor stations in the downstream extremity of the Gironde estuary indicate that during periods of high river discharge, a significant amount of suspended sediment is transported out of the estuary onto the adjacent continental shelf. The vertical profile of the residual (non-tidal) suspended sediment flux is similar to that of the residual current velocity, with a net upstream flux near the bottom and an overlying seaward-directed transport. The overall, depth-integrated result is a net seaward transport of suspended sediment out of the estuary. It appears that this net seaward transport varies directly with tidal amplitude.Aerial photography and water sampling indicate that during high river inflow, the downstream extremity of the turbidity maximum extends onto the continental shelf at ebb tide. The tidal and coastal current patterns of the inlet and inner shelf induce a northward transport of the turbid estuarine water, and at each tidal cycle, a certain amount of suspended sediment leaves the estuary; part of this sediment is deposited in a silt and clay zone on the continental shelf.     

Improving estuarine net flux estimates for dissolved cadmium export at the annual timescale: Application to the Gironde Estuary

Dissolved Cd (Cd_D) concentrations along the salinity gradient were measured in surface water of the Gironde Estuary during 15 cruises (2001–2007), covering a wide range of contrasting situations in terms of hydrology, turbidity and season. During all situations dissolved Cd concentrations displayed maximum values in the mid-salinity range, reflecting Cd addition by chloride-induced desorption and complexation. The daily net Cd_D fluxes from the Gironde Estuary to the coastal ocean were estimated using Boyle's method. Extrapolating Cd_D concentrations in the high salinity range to the freshwater end member using a theoretical dilution line produced 15 theoretical Cd concentrations (Cd_D⁰), each representative of one distinct situation. The obtained Cd_D⁰ concentrations were relatively similar (201 ± 28 ng L⁻¹) when freshwater discharge Q was >500 m³ s⁻¹ (508 ≤ Q ≤ 2600 m³ s⁻¹), but were highly variable (340 ± 80 ng L⁻¹; 247–490 ng L⁻¹) for low discharge situations (169 ≤ Q ≤ 368 m³ s⁻¹). The respective daily Cd_D net fluxes were 5–39 kg day⁻¹, mainly depending on freshwater discharge. As this observation invalidates the existing method of estimating annual Cd_D net fluxes, we proposed an empirical model, using representative Cd_D⁰ values and daily freshwater discharges for the 2001–2007 period. Subsequent integration produced reliable Cd_D net flux estimates for the Gironde Estuary at the annual timescale that ranged between 3.8–5.0 t a⁻¹ in 2005 and 6.0–7.2 t a⁻¹ in 2004, depending on freshwater discharge. Comparing Cd_D net fluxes with the incoming Cd_D fluxes suggested that the annual net Cd_D addition in the Gironde Estuary ranged from 3.5 to 6.7 t a⁻¹, without any clear temporal trend during the past seven years. The annual Cd_D net fluxes did not show a clearly decreasing trend in spite of an overall decrease by a factor 6 in Cd gross fluxes during the past decade. Furthermore, in six years out of seven (except 2003), the annual Cd_D net fluxes even exceeded river borne total (dissolved + particulate) gross Cd fluxes into the estuary. These observations were attributed to progressive Cd desorption from both suspended particles and bottom sediment during various sedimentation–resuspension cycles induced by tidal currents and/or continuous dredging (navigation channel) and diverse intra-estuarine sources (wet deposition, urban sources, and agriculture). Provided that gross fluxes remain stable over time, dissolved Cd exportation from the Gironde Estuary to the coastal ocean may remain at the present level for the coming decade and the estuarine sedimentary Cd stock is forecast to decrease slowly.     

Historical changes in the Columbia River Estuary

Historical changes in the hydrology, sedimentology, and physical oceanography of the Columbia River Estuary have been evaluated with a combination of statistical, cartographic, and numerical-modelling techniques. Comparison of data digitized from US Coast and Geodetic Survey bathymetric surveys conducted in the periods 1867–1875, 1926–1937, and 1949–1958 reveals that large changes in the morphology of the estuary have been caused by navigational improvements (jetties, dredged channels, and pile dikes) and by the diking and filling of much of the wetland area. Lesser changes are attributable to natural shoaling and erosion. There has been roughly a 15% decrease in tidal prism and a net accumulation of about 68 × 10⁶m³ of sediment in the estuary. Large volumes of sediment have been eroded from the entrance region and deposited on the continental shelf and in the balance of the estuary, contributing to formation of new land. The bathymetric data indicate that, ignoring erosion at the entrance, 370 to 485 × 10⁶m³ of sediment has been deposited in the estuary since 1868 at an average rate of about 0.5 cm y⁻¹, roughly 5 times the rate at which sea level has fallen locally since the turn of the century.Riverflow data indicate that the seasonal flow cycle of the Columbia River has been significantly altered by regulation and diversion of water for irrigation. The greatest changes have occurred in the last thirty years. Flow variability over periods greater than a month has been significantly damped and the net discharge has been slightly reduced. These changes in riverflow are too recent to be reflected in the available in the available bathymetric data.Results from a laterally averaged, multiple-channel, two-dimensional numerical flow model (described in Hamilton, 1990) suggest that the changes in morphology and riverflow have reduced mixing, increased stratification, altered the response to fortnightly (neap-spring) changes in tidal forcing, and decreased the salinity intrusion length and the transport of salt into the estuary.The overall effects of human intervention in the physical processes of the Columbia River Estuary (i.e. decrease in freshwater inflow, tidal prism, and mixing; increase in flushing time and fine sediment deposition, and net accumulation of sediment) are qualitatively similar to those observed in less energetic and more obviously altered estuarine systems. A concurrent reduction in wetland habitats has resulted in an estimated 82% reduction in emergent plant production and a 15% reduction in benthic macroalgae production, a combined production loss of 51,675 metric tons of organic carbon per year. This has been at least partially compensated by a large increase in the supply of riverine detritus derived from freshwater phytoplankton primary production. Comparison of modern and estimated preregulation organic carbon budgets for the estuary indicates a shift from a food web based on comparatively refractory macrodetritus derived from emergent vegetation to one involving more labile microdetritus derived from allochthonous phytoplankton. The shift has been driven by human-induced changes to the physical environment of the estuary.While this is a relatively comprehensive study of historical physical changes, it is incomplete in that the sediment budget is still uncertain. More precise quantification of the modern estuarine sediment budget will require both a better understanding of the fluvial input and dredging export terms and a sediment tranport model designed to explain historical changes in the sediment budget. Oceanographic studies to better determine the mechanisms leading to the formation of the turbidity maximum are also needed. The combination of cartography and modelling used in this study should be applicable in other systems where large changes in morphology have occurred in historical time.     

夏季珠江口沉积物?水界面重金属分布特征及其影响因子研究

珠江口受复杂径?潮动力耦合作用的影响,河口重金属迁移转化机制复杂多变。本文基于2018年夏季珠江口及其邻近海域海水和沉积物的调查资料,研究了珠江口多动力因子驱动下7种重金属元素汞、砷、锌、镉、铅、铜和铬的沉积、迁移和积累机制。结果表明:相比于溶解态重金属（水体中）的赋存状况,吸附态重金属（沉积物中）更稳定,污染也更严重;采用皮尔逊相关分析和主成分分析计算出重金属元素与环境因子之间的响应关系,溶解态重金属主要以稀释混合过程为主,吸附态重金属受有机碳和氧化还原作用的影响较大;沉积物?水界面重金属的分配系数显示出铅和铬易被吸附在颗粒物上,而镉和汞易溶解在水体中,揭示了河口复杂动力影响下元素在不同介质的形态转换特征;除了镉?铬、镉?铜和铬?砷这3组元素间不显著相关,其他元素间的显著相关性表明了重金属元素具有相似的来源,并采用主成分分析探讨了重金属元素的潜在来源,主要来源为工业废水,农业和大气沉降次之。研究结果可为有效控制重金属的排放和河口污染治理提供重要支撑。     

Impact of trace metals on denitrification in estuarine sediments of the Douro River estuary,Portugal

Denitrification influences the nitrogen budget in estuaries by removing fixed nitrogen from the inorganic pool; rates are dependent on both geological and geographic conditions as well as increasing anthropogenic impacts. In this study the effects of copper (Cu), chromium (Cr), zinc (Zn), cadmium (Cd) and lead (Pb), on the denitrification pathway were evaluated in subtidal and intertidal sediments of the Douro River estuary. Dinitrogen, N₂O and NO₂⁻ production rates were measured in triplicate slurries of field samples under different treatments of metal concentrations. Results demonstrated that similar metal amendments led to different site responses for denitrification, suggesting that variations in sediment properties (metal concentrations, grain size, organic matter content, etc.) and/or differences in denitrifying community tolerance modulate the level of metal toxicity. Denitrifying communities in subtidal muddy sediments were not affected by increasing concentrations of metals. In contrast, intertidal sandy sites revealed high sensitivity to almost all trace metals tested; almost complete inhibition by Cr (95%) and Cu (85%) was observed for 98 and 79 μg per gram of wet sediment respectively, and by Zn (92%) at the highest concentration added (490 μg per gram of wet sediment). Moreover, the addition of trace metals stimulated N₂O and NO₂⁻ accumulation in intertidal sandy (Zn, Cu, Cr and Cd) and muddy sediments (Cu and Zn), demonstrating a pronounced inhibitory effect on specific steps within the denitrification enzymatic system. In summary, the results obtained suggest that, according to the type of estuarine sediment, trace metals cannot only reduce total N removal from an estuary via denitrification but also can enhance the release of N₂O, a powerful greenhouse gas.     

Metal distributions and their fluxes at the coastal boundary of a semi-enclosed ria

Water column samples have been collected in the outer channel of the Ferrol Ria (NW Spain) during four occasions over a tidal cycle. The objective was to study the exchange of dissolved and particulate Cd, Cu, Pb and Zn and particulate Al, Fe and Si between the ria and the adjacent coastal waters. This study provides the first extensive dataset on dissolved and particulate metal concentrations in the water column of a Galician ria. Typical concentrations of dissolved Cd (96 ± 31 pM), Cu (8 ± 4 nM), Pb (270 ± 170 pM) and Zn (21 ± 10 nM) were similar than in other European Atlantic shelf and coastal waters. The fraction of metals in the particulate phase followed the trend: Pb > Cu Zn > Cd. The outgoing water from the ria was enriched in dissolved and particulate Cu, Pb and Zn compared with incoming waters, whereas Cd concentrations were similar for both waters. The suspended particulate matter was composed of a mixture of marine and continental material. The latter end-member was found to arise from the metal-rich ria bed sediments, which is diluted by the dominant metal-poor marine end-member. The net output flux of Cu from the channel is balanced by the freshwater inputs to the ria, and the net Zn flux gave a positive output to coastal waters. For Pb, the net flux to the coastal waters is less than that input from the rivers, as a result of its particle reactivity and deposition in sediments. On the contrary, a net input flux of dissolved Cd from coastal waters was observed, highlighting the oceanic source of this metal in the Galician rias. Results from the budget calculations are in agreement with the differential geochemical behavior of these elements in coastal waters.     

Vertical distribution of the macrobenthic community and its relationships to trace metals and natural sediment characteristics in the lower Douro estuary, Portugal

Vertical distribution (0–15 cm) of the macrobenthic community and its relationships to natural sediment characteristics and trace metal contents and bioavailability were studied at five locations in the lower Douro estuary, Portugal. An analysis of vertical metal distribution, for the interpretation of anthropogenic impact on the estuarine sediments, was also investigated. Sediment characterisation included organic matter, grain size, metals (Al, Fe, Cu, Pb, Cr, Ni, Cd, Zn and Mn), acid volatile sulphide (AVS) and simultaneously extracted metals (SEM). The macrobenthic community had low diversity (14 species), was dominated by small size opportunists and seemed to be controlled mainly by natural factors such as grain size distribution, Al and Fe contents and sediment depth. The vertically heterogeneous distribution of macrobenthic community appears to affect redox status of the sediments and consequently metal bioavailability. Despite anthropogenic contamination in terms of Zn, Cu, Pb, Cr and Ni having already been detected in the north bank, the analysis of vertical distribution was essential for the identification of current anthropogenic contamination in terms of Zn, Pb and Cd in the south bank.     

Trace metals in the Göta river estuary

The concentrations of the trace metals Cd, Cu, Fe, Ni, Pb and Zn in the Göta River estuary have been investigated. The following metal fractions have been determined: acid-leachable, dissolved, labile and particulate.The estuary represents a salt wedge type estuary and is situated in a densely populated region of Sweden. The metal concentrations found for the dissolved fraction is in the range of what can be considered as background levels for freshwater. It is difficult to evaluate any estuarine processes other than conservative mixing for Cd, Cu, Ni and Zn. The dissolved levels in the freshwater end member are Cd, 9–25 ngl^?1; Cu, 1·1–1·4 μgl^?1; Fe, 20–75 μg l^?1: Ni, 0·7–0·9 μg l^?1: Pb 0·09–0·2 μg l^?1; and Zn, 6–7 μg l^?1:The results from the acid-leachable fraction show that at high suspended load the particles sediment in the river mouth. The trace metal levels in this fraction are subject to large variations.     

Modelling and mass balance assessments of nutrient retention in a seasonally-flowing estuary (Swan River Estuary,Western Australia)

An integrated mass balance and modelling approach for analysis of estuarine nutrient fluxes is demonstrated in the Swan River Estuary, a microtidal system with strong hydrological dependence on seasonal river inflows. Mass balance components included estimation of gauged and ungauged inputs to the estuary and losses to the ocean (outflow and tidal exchange). Modelling components included estimation of atmospheric (N fixation, denitrification) and sediment–water column nutrient exchanges. Gross and net denitrification derived using two independent methods were significantly correlated (r² = 0.49, p < 0.01) with net rates averaging 40% of gross. Annual nitrogen (N) and phosphorus (P) loads from major tributaries were linearly correlated with annual freshwater discharge and were 3-fold higher in wet years than in dry years. Urban drains and groundwater contributed, on average, 26% of N inputs and 19% of P inputs, with higher relative contributions in years of low river discharge. Overall, ungauged inputs accounted for almost 35% of total nitrogen loads. For N, elevated loading in wet years was accompanied by large increases in outflow (7x) and tidal flushing (2x) losses and resulted in overall lower retention efficiency (31%) relative to dry years (70%). For P, tidal flushing losses were similar in wet and dry years, while outflow losses (4-fold higher) were comparable in magnitude to increases in loading. As a result, P retention within the estuary was not substantially affected by inter-annual variation in water and P loading (ca. 50% in all years). Sediment nutrient stores increased in most years (remineralisation efficiency ca. 50%), but sediment nutrient releases were significant and in some circumstances were a net source of nutrients to the water column.     

Molecular weight and chemical reactivity of dissolved trace metals (Cd,Cu, Ni) in surface waters from the Mississippi River to Gulf of Mexico

It is generally assumed that estuarine mixing is continuous for metals from terrestrial sources, gradually decreasing towards the open ocean endmember. Here we show that, chemical reactivity, determined by ion exchange method, and molecular weight distributions, obtained using cross-flow ultrafiltration, of dissolved Cd, Cu, and Ni in the surface waters of the Gulf of Mexico varied systematically across the estuarine mixing zone of the Mississippi River. Most size or chemical affinity fractions of dissolved metals (<0.4 μm) were linearly related to salinity (10.8–36.6), suggesting that the distribution of these elements was mainly controlled by continuous mixing processes. Dissolved concentrations across the salinity gradient ranged for Cd: 87–187 pM; Cu: 1.4–18.3 nM; and Ni: 2.6–18.8 nM, with highest values near the Mississippi river mouth, and lowest concentrations in a warm core ring in the Gulf of Mexico. Dissolved Cd was mostly present as a truly dissolved (<10 kDa, 97 ± 1%) and cationic fraction (Chelex-100 extractable, 94 ± 4%). A novel observation across the estuarine mixing zone was that colloidal metal concentrations were identical to either inert (for Cu, Ni) or AMPG-labile anionic (Cu, Cd) fractions. The difference in behavior between Cu and the other two metals might indicate differences in the biopolymeric nature of the metal–organic chelates. In particular, the anionic-organic Cd fractions accounted for just 3 ± 1%, on average. However, for Cu, it was 24 ± 4%, and for Ni, it was 9 ± 6%. The fractions of the total dissolved metal fractions that were “inert” averaged 31 ± 10% for Cu and 29 ± 12% for Ni. Small but noticeable amounts (6 ± 3%) of dissolved inert Cd fractions were also present. Apparent non-local transport processes, likely associated with cross-shelf sediment resuspension processes, could have been responsible for the relatively high concentrations of ‘inert’ and ‘anionic’ metal fractions in high salinity coastal waters, and accounting for the persistence of metals bound to humic substances in the Gulf of Mexico.     

Spatial and temporal distribution of cadmium and copper in water and zooplankton in the Bahía Blanca estuary,Argentina

Cadmium and copper in the dissolved and particulate phase and in zooplankton were determined in the Bahía Blanca estuary during six surveys from March to December 2005. Temperature, pH, salinity, dissolved oxygen, suspended particulate matter, particulate organic matter and chlorophyll-a were also considered. Dissolved Cd was below the detection limit (0.2 μg L⁻¹) for almost the entire study period whereas Cu concentrations (0.5–2.4 μg L⁻¹) indicated a continuous dissolved Cu input. Particulate Cd concentrations ranged from below the detection limit (<0.01) to 28.6 μg g⁻¹ d.w. while particulate Cu ranged from below the detection limit (<0.04) to 53.5 μg g⁻¹ d.w. Cd in mesozooplankton ranged from below the detection limit (<0.01) to 37.4 μg g⁻¹ d.w. Some of the Cd levels were higher than those reported for other aquatic ecosystems. Cu in the mesozooplankton ranged from 1.3 to 89.3 μg g⁻¹ d.w., values which were within the reported values or higher than other studies. The log of the partition coefficients (log (K_d)) of Cd was 0.04, while log (K_d) for Cu ranged from −0.39 to 2.79. These values were lower than both those calculated for other estuaries and the typical coefficients for marine environments. The log of the bioconcentration factor (log BCF) of Cd was 1.78, indicating that Cd concentration was higher in the zooplankton than in the dissolved phase. Log BCF of Cu ranged from 1.15 to 3. The logs of the biomagnification factors (log BMF) of Cd were low, with a range between −3.45 and 2.21 and those for Cu ranged from −0.1 to 3.35. Positive values indicate biomagnification while negative values indicate biodiminution. In general, no significant dissolved Cd concentration appeared to be present in the Bahía Blanca estuary and Cu values did not indicate a critical environmental status. The particulate phase seemed to be the major carrier for Cd and Cu and TPCu values were within the normal values for an anthropogenically stressed estuary but not for a strongly polluted system. This fraction was the most important metal source for the mesozooplankton. Moreover, the highest metal concentrations were in the mesozooplankton since most of the bioconcentration and biomagnification factors were positive, especially for Cu.     

An evaluation of metal bioaccessibility in estuarine sediments using the commercially available protein, bovine serum albumin

The bioaccessibility of metals (Al, Ca, Fe, Mn, Ag, Cd, Co, Cu, Ni, Pb, Sn, Zn) in oxic estuarine sediments has been evaluated using solutions of a commercially available protein (bovine serum albumin; BSA) that mimic the chemical conditions encountered in the gut environment of many deposit-feeding organisms. Over a 20 h incubation period with 5 g L^− 1 BSA, metal mobilisation was generally biphasic in that a relatively short period of rapid release was succeeded by more gradual release or approach to equilibrium, although in some cases metal readsorption was evident during the time-courses. Availability to BSA, defined as metal released after 20 h relative to metal extracted by boiling aqua regia, was greatest for Cd, Ni, Ca and Zn and lowest for Fe and Mn, and correlated well with, but was an order of magnitude lower than, metal digested by acidified hydrogen peroxide. Time-courses conducted in the absence of the protein revealed that significant quantities of Ca and Mn were water-soluble, reflecting the partial dissolution of carbonates and hydrous Mn oxides, respectively. Additional experiments indicated a net increase in metal release with increasing BSA concentration and, with the exception of Ca and Mn, a substantial increase in metal mobilisation after sediment organic matter had been digested by peroxidation. These observations suggest that, apart from Ca and Mn, metal release proceeds via complexation with component amino acids of the protein, denudation of organic host phases, and exposure of inorganic, metal-bearing minerals. Accordingly, the bioaccessibility of a metal is predicted to be dependent on its (i) affinity for proteinaceous ligands, (ii) association with components of digestible sediment organic matter, and (iii) degree of binding at relatively weak sites on sediment phases that are exposed or modified by the action of proteins and other chemical constituents of the gut environment.     

Morpho-sedimentary evidence of Holocene coastal changes near the mouth of the Gironde and on the Medoc Peninsula,SW France

The Medoc Peninsula is a triangular area of land between the Atlantic Ocean on the west and the Gironde estuary on the east. The Gironde, orientated SE-NW, is the largest estuary in France. On the Medoc Peninsula Holocene sediments cover a substratum of Tertiary limestones and Plio-Pleistocene fluvial terraces. The Gironde originated as an incised valley during the Weichselian glacio-eustatic fall (100 000–18 000 B.P.) and has acted as a sink for fine sediment throughout the Holocene sea level rise (SLR) (18 000 B.P. onwards). Conversely, the Atlantic littoral zone, characterised by sandy beaches and dunes, has been subjected to erosion throughout the Holocene transgression.The incised valley of the present Gironde was inundated by the sea approximately 10 000 B.P. At this time, the rate of SLR exceeded that of sediment supply, producing a large accommodation space in which transgressive tidal-estuarine muds and sands were deposited. As the rate of SLR decreased around 6000-4000 B.P., sedimentation became more pronounced and the available accommodation space began to decrease significantly. Landward-derived fluvial sediments began to prograde over the tidal muds and sands, and a first generation of salt marshes formed in the lateral valleys. Around 2575-1420 B.P., a sandy chenier ridge formed at the edge of a first generation of salt marshes and wholly or partly separated them from the Gironde. On the eastern side of the ridge a second generation of marshes began to form after 1200 B.P. Reclamation of the first and second generations of marshes occurred during the seventeenth and eighteenth centuries, respectively. On the estuarine side of the eighteenth century dyke the modern intertidal flats and salt marshes began to form after the eighteenth century. Sedimentation within the estuary decreased the accommodation space and led to the increased transport of sediment to the shelf after 2 000 B.P. This process was also aided by climatic and anthropogenic factors. Future evolution of the Gironde estuary is likely to consist of further marsh growth and chenier development. However, future increases in the rate of SLR, and the degree of storminess, may cause a shift to an erosional regime in parts of the lower estuary.According to previous work [25, 66], three different dune fields migrated landwards across the Atlantic littoral zone over the Holocene period. A field of isolated barchan dunes moved landwards over the Plio-Pleistocene fluvial terraces before 5100 B.P. From some time after this, until around 3000 B.P., a field of parabolic dunes was active. Finally, a barchan dune field was active from around 3000-2000 B.P. to the end of the eighteenth century/beginning of the nineteenth century, when the dunes were stabilised by pine plantations. It is tentatively suggested, using these dates and dune morphology, that dune formation was controlled by sand supply governed by the rate of SLR. However, the role of climatic changes such as aridity, storminess, windiness and the associated effects of vegetation cover, is also likely to be important. In the future the Atlantic coast is likely to continue to erode, although the extensive plantation of the aeolian dunes with pine forests is likely to prevent large-scale transgressive activity.     

Dissolved Cd behaviour in some selected french and chinese estuaries. Consequences on Cd supply to the ocean

Few data on dissolved trace metals in rivers and estuaries are presently available. This paper is an attempt to provide additional data on dissolved concentrations obtained in polluted and so-called unpolluted river estuarine systems. Data for two major French rivers (Gironde, Rhône) have been compared with the Chinese Yellow River (Huanghe) and Yangtze River (Changjiang). Dissolved Cd concentrations have been measured by differential pulse anodic stripping voltammetry. Average concentrations range from 15 ng kg⁻¹ to 50 ng kg⁻¹ (0.13–0.45 nmol kg⁻¹) in the French rivers and are below 4 ng kg⁻¹ (0.040 nmol kg⁻¹) in the Chinese rivers.In all the estuaries studied the dissolved Cd concentrations depict a systematic bulge in the mixing zone which is attributed largely to remobilization processes from particulate matter when the chlorinity increases. Other parameters that may also play a significant role for remobilization are discussed. The processes concerned lead us to reassess the net Cd river input to the oceans, and this should be taken into account for a more precise evaluation of the residence time of oceanic Cd.