Hydrodynamics of suspended particulate matter in the tidal freshwater zone of a macrotidal estuary (the Seine Estuary, France)

The effects of fortnightly, semidiurnal, and quaterdiurnal lunar tidal cycles on suspended particle concentrations in the tidal freshwater zone of the Seine macrotidal estuary were studied during periods of medium to low freshwater flow. Long-term records of turbidity show semidiurnal and spring-neap erosion-sedimentation cycles. During spring tide, the rise in low tide levels in the upper estuary leads to storage of water in the upper estuary. This increases residence time of water and suspended particulate matter (SPM). During spring tide periods, significant tidal pumping, measured by flux calculations, prevents SPM transit to the middle estuary which is characterized by the turbidity maximum zone. On a long-term basis, this tidal pumping allows marine particles to move upstream for several tens of kilometers into the upper estuary. At the end of the spring tide period, when the concentrations of suspended particulate matter are at their peak values and the low-tide level drops, the transport of suspended particulate matter to the middle estuary reaches its highest point. This period of maximum turbidity is of short duration because a significant amount of the SPM settles during neap tide. The particles, which settle under these conditions, are trapped in the upper estuary and cannot be moved to the zone of maximum turbidity until the next spring tide. From the upper estuary to the zone of maximum turbidity, particulate transport is generated by pulses at the start of the spring-neap tide transition period.     

Mercury Dynamics in a San Francisco Estuary Tidal Wetland: Assessing Dynamics Using In Situ Measurements

We used high-resolution in situ measurements of turbidity and fluorescent dissolved organic matter (FDOM) to quantitatively estimate the tidally driven exchange of mercury (Hg) between the waters of the San Francisco estuary and Browns Island, a tidal wetland. Turbidity and FDOM??representative of particle-associated and filter-passing Hg, respectively??together predicted 94?% of the observed variability in measured total mercury concentration in unfiltered water samples (UTHg) collected during a single tidal cycle in spring, fall, and winter, 2005?C2006. Continuous in situ turbidity and FDOM data spanning at least a full spring-neap period were used to generate UTHg concentration time series using this relationship, and then combined with water discharge measurements to calculate Hg fluxes in each season. Wetlands are generally considered to be sinks for sediment and associated mercury. However, during the three periods of monitoring, Browns Island wetland did not appreciably accumulate Hg. Instead, gradual tidally driven export of UTHg from the wetland offset the large episodic on-island fluxes associated with high wind events. Exports were highest during large spring tides, when ebbing waters relatively enriched in FDOM, dissolved organic carbon (DOC), and filter-passing mercury drained from the marsh into the open waters of the estuary. On-island flux of UTHg, which was largely particle-associated, was highest during strong winds coincident with flood tides. Our results demonstrate that processes driving UTHg fluxes in tidal wetlands encompass both the dissolved and particulate phases and multiple timescales, necessitating longer term monitoring to adequately quantify fluxes.     

Particulate matter grain-size characteristics and flocculation in a partially mixed estuary

The concentration and grain size of the natural and deflocculated inorganic suspended particulate matter were measured along the length of the Miramichi Estuary and interpreted with respect to flocculation and transport properties. Changes in particulate matter concentration are associated with regular changes in grain-size characteristics. In the turbidity maximum region of the estuary the suspended matter occurs mostly as large flocculated particles whereas, in the waters with lower particle concentrations, a larger proportion of the material occurs as fine material. At higher concentrations natural floc modes and inorganic grain modes vary simultaneously but at low concentrations the two modes vary inversely. This modal relationship and the variation in organic matter within the estuary is proposed to result from variation in inorganic—organic composition of flocs. Increase in settling rates due to flocculation is believed to increase the trapping effect of the estuarine circulation that produces the turbidity maximum.     

Distribution and reactivity of oxyanions (Sb, As, V, Mo) in the surface freshwater reaches of the Gironde Estuary (France)

The present work reports on the temporal variations of Sb, As, V and Mo concentrations in the surface freshwater reaches of the Gironde Estuary (SW France). Dissolved (<0.2 and <0.02 μm) and particulate Sb, As, V and Mo concentrations were measured in samples collected with high temporal resolution (every 30 min) during two consecutive tides at a fixed station located upstream of the city of Bordeaux and the maximum turbidity zone (MTZ). In addition, measurements of suspended particulate matter concentration, physico-chemical parameters, Cl⁻ concentration, water depth, current velocity and particulate Th concentration were performed either at the same time resolution or continuously. The data obtained suggested that variations in particulate As, V and Mo concentrations were most probably related to tidal cycling near the sampling site of fine grained particles from the MTZ located downstream. Significant differences in the behavior of the dissolved target oxyanions, mostly present in the <0.02 μm fraction, occurred. The behavior of Sb was conservative. Variations of the dissolved As and V concentrations showed similar cycling trends, strongly related to tidal cycles. These As and V cycles were interpreted as a mixing between upstream freshwater and downstream water enriched in dissolved As and V by desorption from the MTZ particles. The observed trend in the variation of the dissolved Mo was more complex than that of As and Sb and attributed to the mixing of the water bodies mentioned above coupled to a point source input of dissolved Mo from an intra-estuarine source. The contribution of this suspected Mo source to the dissolved Mo concentrations measured at the sampling site was estimated from the ratio of dissolved Mo to dissolved As concentrations. The additional dissolved Mo signal, coupled to the monitored ebb and flood length and associated current velocities, suggested an anthropogenic input which may derive from industrial activity near the city of Bordeaux. Extrapolating these results to the annual scale suggested that this dissolved Mo may be equivalent to 45-90% of the annual dissolved Mo flux into the Garonne Branch, highlighting the importance to further investigate the origin and behavior of Mo in the fluvial estuary near Bordeaux.     

The regulation of trace element concentrations in river and estuarine waters contaminated with acid mine drainage: The adsorption of Cu and Zn on amorphous Fe oxyhydroxides

The partition of Cu and Zn between the particulate and dissolved phases has been investigated in the Carnon River and Restronguet Creek, England; a system contaminated by acid mine drainage. It is shown that binding to the surfaces of amorphous Fe oxyhydroxides regulates Cu and Zn concentrations in solution, in both fresh and saline waters. The sorptive process is pH dependent and is in general agreement with laboratory studies of trace metal adsorption on amorphous Fe oxyhydroxides.     

Mercury in the Tinto-Odiel Estuarine System (Gulf of Cádiz, Spain): Sources and Dispersion

The Tinto and Odiel are small rivers draining one of the largest sulphide deposits in the world. As a result of these deposits and a large industrial complex, the adjacent marine area receives a high amount of metal input. Mercury distribution in the Tinto-Odiel estuary, the Huelva Ría and the Gulf of Cádiz was assessed in water and suspended particulatematter (SPM) and sediments. In the rivers and estuaries, dissolved (HgD)and particulate (HgP) mercury showed wide variations (13 to 200 pM and0.3 to 330 nmol g^-1 respectively) depending on the presence or notof sulphidic waters, phosphogypsum deposits, detrital pyrite and oxyhydroxides.In the Ría, concentrations were lower than 10 pM and 4.5 nmol g^-1 for HgD and HgP respectively. In surface waters of the Gulf of Cádiz, the average HgD concentration (pm standard deviation) was 2.9 pm 0.9 pM, which is similar to that of North Atlantic Central Waters. The surface sediments collected in the rivers, the Ría and the Gulf showed systematically enriched mercury compared to pre-industrial levels. Vertical mercury profiles in dated sediment cores were typical of anthropogenically influenced environments starting in the early Roman age. These distribution features suggest that most of the Hg discharged by the Huelva Ría is trapped in the sediments of the Gulf of Cádiz.     

Manganese and suspended matter in the Yaquina Estuary,Oregon

The longitudinal distribution of total suspended matter and total, dissolved, and particulate manganese in a small coastal plain estuary is described. The distribution of manganese is a consequence of estuarine circulation; a within-estuary maximum is inversely correlated with river flow, and is a function of residence time in the estuary, resuspension in the upper estuary, and desorption from particles introduced from within the estuary or from the river. The turbidity maximum is similarly most pronounced during low river flows. The upper estuary (salinity <15‰), comprising a small percentage of the total estuary volume during low flow, receives material from the river and along the bottom from the lower estuary; this material is returned to the water column by resuspension and desorption from estuarine and riverine particles. The lower estuary tends to damp out these processes because of the greater volume and (residence) time available for mixing.     

The pathway of mercury in contaminated waters determined by association with organic carbon (Tagus Estuary,Portugal)

Reactive dissolved Hg (Hg_R), non-reactive dissolved Hg (Hg_NR), particulate Hg (Hg_P), dissolved organic C (DOC), particulate organic C (POC), salinity and other interpretative parameters were determined in water samples collected in the North Channel and in adjacent areas of the Tagus estuary (Portugal). Higher concentrations of both dissolved and particulate Hg in the North Channel indicate a pollution source and raise the possibility of Hg escaping to adjacent areas by tidal action. This transport was confirmed by the increase of Hg_R with salinity and Hg_NR with DOC, along a longitudinal axis paralleling the North Channel. Apparently, Hg leaving this channel is progressively complexed by inorganic and organic ligands. Near the mouth of the estuary, values decreased reflecting dilution with seawater. Moreover the Hg_P:POC ratio also increased seaward, suggesting mixing with Hg enriched particles that escaped the North Channel, or incorporation of dissolved Hg species in river-derived particles. These results suggest that the pathway of anthropogenic Hg in contaminated waters may be identified by their enrichment in organic matter, both in the dissolved and particulate fraction.     

Distribution of colloidal trace metals in the San Francisco Bay estuary

The size distribution of trace metals (Al, Ag, Cd, Cu, Fe, Mn, Ni, Sr, and Zn) was examined in surface waters of the San Francisco Bay estuary. Water samples were collected in January 1994 across the whole salinity gradient and fractionated into total dissolved (<0.2 μm colloidal (10 KDa–0.2 μm) and < 10 kDa molecular weight phases. In the low salinity region of the estuary, concentrations of colloidal A1, Ag, and Fe accounted for ≥84% of the total dissolved fraction, and colloidal Cu and Mn accounted for 16–20% of the total. At high salinities, while colloidal Fe was still relatively high (40% of the dissolved), very little colloidal Al, Mn, and Cu (<10%) and no colloidal Ag was detectable. Colloidal Zn accounted for <3% of the total dissolved along the estuary, and colloidal Ni was only detectable (<2%) at the river endmember. All of the total dissolved Cd and Sr throughout the estuary consisted of relatively low molecular weight (<10 kDa) species. The relative affinity of metals for humic substances and their reactivity with particle surfaces appear to determine the amounts of metal associated with colloids. The mixing behavior of metals along the estuary appears to be determined by the relative contribution of the colloidal phase to the total dissolved pool. Metals with a small or undetectable colloidal fraction showed a nonconservative excess (Cd, Cu, Ni, and Mn) or conservative mixing (Sr) in the total dissolved fraction, relative to ideal dilution of river water and seawater along the estuary.

The salt-induced coagulation of colloidal A1, Fe, and Cu is indicated by their highly nonconservative removal along the salinity gradient. However, colloidal metals with low affinity for humic substances (Mn and Zn) showed conservative mixing behavior, indicating that some riverine colloids are not effectively aggregated during their transport to the sea. While colloidal metal concentrations correlated with dissolved organic carbon, they also covaried with colloidal Al, suggesting that colloids are a mixture of organic and inorganic components. Furthermore, the similarity between the colloidal metal:A1 ratios with the crustal ratios indicated that colloids could be the product of weathering processes or particle resuspension. Distribution coefficients for colloidal particles (K_c) and for large, filter-retained particles (K_d) were of the same magnitude, suggesting similar binding strength for the two types of particles. Also, the dependence of the distribution coefficients on the amount of suspended particulate matter (the so-called particle concentration effect) was still evident for the colloids-corrected distribution coefficient (K_p+c) and for metals (e.g., Ni) without affinity for colloidal particles.     

Biogeochemical behavior of arsenic species in the seine estuary in relation to successive high-amplitude primary production, anoxia, turbidity, and salinity events

Sixty samples were collected in June 1996 at regular intervals within the Seine estuary in France in order to study the arsenic (As) behavior in response to climatic and hydrologic conditions leading to major events in chlorophyll activity, anoxia, turbidity, and salinity. It was determined that arsenate (As5, 15–23 nM) is still the dominant chemical form of dissolved As in the upper, estuary. A concentration of up to 37 nM was observed in the high turbidity zone at the freshwater-seawater interface, presumably a result of pumping and remineralization of plankton particles that develop at salinities >10. The formation of arsenite (As3) was related to two processes: anoxia in the upper estuary and primary production in the superoxygenated, lower estuary. Dimethyl arsenic (DMA) concentrations (0 to 2.2 nM) were not directly correlated with chlorophyll content, but rather with oxygenation rates. Monomethyl arsenic (MMA) concentrations were low. Dissolved organoarsenic concentrations refractory to the formation of volatile hydrides showed high concentrations in the river estuary (1 to 6 nM), and their degradation was probably the cause of DMA and MMA and ultimately of the recycling of dissolved inorganic As. The remineralization of particulate arsenic (AsP) in the high turbidity zone at the freshwater-seawater interface, was apparent once AsP concentrations were normalized with respect to aluminium. This remineralization process, even though it does not exclude the possibility of intra-estuarine As inputs, could account for a major part of the high dissolved total As concentration observed in this specific zone, both in the Seine and other estuaries.     

Transport and retention of suspended particulate matter and bacteria in the Humber-Ouse Estuary, United Kingdom, and their relationship to hypoxia and anoxia

Data are presented on dissolved oxygen (DO) concentrations and their relationship to salinity, suspended particulate matter (SPM), concentrations, and the turbidity maximum in the Humber-Ouse Estuary, United Kingdom, during summer 1995. Measurements in the upper Humber during March 1995 showed DO in the range 82% to 87% of saturation. Suspended particulate matter concentrations were <5000 mg l^?1 and salinity was in the range 0.5 to 12. In contrast, a pronounced DO sag occurred in the upper reaches of the Ouse during medium and spring tide, summer conditions. The DO minimum was essentially an anoxic level and was associated with the location of the turbidity maximum, at salinities between about 0.4 and 1.5. SPM concentrations at 1 m beneath the surface reached 25,000 mg l^?1 in the turbidity maximum, between about 20 km and 40 km from the tidal limit. Suspended particulate matter concentrations were much lower at neap tides, although dense suspensions of SPM (>60,000 mg l^?1) occurred within 1 m of the bed in the turbidity maximum region. A spring-neap record showed a dramatic and tidally controlled decrease in DO at very low salinities as the tides progressed from neaps to springs. An anchor station located down-channel of the turbidity maximum showed that about 95% of the variance in DO, which varied from 28% at low-water slack to 67% at high-water slack, could be explained in terms of salinity variation. At the up-channel margins of the turbidity maximum, DO increased from zero (anoxia) near high water to 60% near low water slack, in contrast to the behavior down-channel of the turbidity maximum. About 82% of the variance in DO could be explained in terms of salinity variations alone. Only 43% of the DO variance could be explained in terms of SPM alone. Up-channel of the turbidity maximum, SPM concentrations were relatively low (<3000 mg l^?1) and DO levels varied from 48% of saturation near high water to 83% near low water slack. About 76% of the variance in DO could be explained in terms of salinity variations alone. Within the turbidity maximum region, DO varied from <2% saturation on the early flood and late ebb and maximized around 7% at high water slack. About 63% of the variance in DO could be explained in terms of salinity variation alone. This increased to 70% when suspended particulate matter was taken into account. Only 29% of the DO variance could be explained in terms of suspended particulate matter alone. Because bacteria were likely to have been the cause of the observed reduction in DO, the numbers of bacteria, both free-living and attached to particles, were measured in the turbidity maximum region. Numbers of free-living bacteria were low and most of the bacteria were attached to sediment particles. There was a linear correlation between total bacterial number and suspended particulate matter concentration, suggesting that the strong DO demand was exerted locally as a result of bacterial activity associated with increased suspended particulate matter concentrations. An order of magnitude analysis of DO consumption within the Ouse’s turbidity maximum, based on the premise that DO depletion was directly related to suspended particulate matter concentrations and that DO addition was due to reaeration, indicates that complete deoxygenation could have occurred with an oxygen depletion rate of ～0.01 mg DO h^?1/g suspended particulate matter during the residence time of waters within the turbidity maximum (～7 d). This rate was sufficiently fast that anoxic to aerobic conditions were able to develop a spring-neap periodicity within the turbidity maximum, but too slow to generate substantial intratidal fluctuations in DO. This is in accordance with the observations, which show that relatively little of the intratidal variance in DO could be explained in terms of suspended particulate matter fluctuations, whereas most of the variance could be explained in terms of salinity, which behaved as a surrogate measure for the proximity of the turbidity maximum.     

Aluminum in the macrotidal Yalujiang estuary: Partitioning of Al along the estuarine gradients and flux

Water samples were collected from the Yalujiang estuary in both flood periods (August 1992 and August 1994) and dry season (May 1996) and were analyzed for aluminum (Al). Al behaves non-conservatively in the Yalujiang estuary, and a significant loss (70–80%) in dissolved concentration is observed in the upper estuary, in spite of seasonal variation in water discharge and sediment load. About 0.4×10⁶, tons of Al is annually transported from Yalujiang to the estuary, of which the particulate pool clearly dominates. The particulate Al flux through the Yalujiang contributes 35% of the total Al input from Chinese rivers to the Yellow Sea. The data sets from size fractionation and C-18 SPE separation demonstrate that a large fraction of dissolved Al is in colloidal (≈50%) and organically complex (≈60%) forms in the Yalujiang. The observed scavenging from solution to particulate pools in the estuary is most likely through the flocculation of colloidal and organic-complexed Al, which results in a considerable change in dissolved-particulate partitioning, shown by laboratory mixing experiments. Exchange between dissolved and particulate phases is examined by analysis of K_d, the distribution coefficient. The empirical relationship of K_d with chlorinity and suspended matter concentrations was investigated with field observations and model simulations. The model indicated that K_d values of Al are inversely related to the amount of total suspended matter, but K_d-chlorinity plots show different features between dry and flood seasons.     

Partitioning of trace metals between particulate,colloidal and truly dissolved fractions in a polluted river: the Upper Vistula River (Poland)

Metal partitioning depends on the physical–chemical conditions of a system and can be affected by anthropogenic inputs. In this study, the authors report the results of trace metal partitioning between particulate (>1.2 μm), colloidal (1.2 μm–1 kDa) and truly dissolved (<1 kDa) fractions in the polluted section of the Upper Vistula River compared with the non-polluted headwaters. It was found that the salt input in the Vistula River induced a decrease of colloid concentration and the increase of suspended particulate matter. Compared with upstream from the polluted section, the metal concentrations (Co, Cu, Cr, Mn and Zn) in the colloidal fraction were lower. It was mainly due to the rapid colloid coagulation at increased salinity, the competition with ligands and major ions (Ca and Mg) and the weak mobility of metals associated with particles at the pollution sources.     

Mobilization of trace metals in a tropical turbid estuary influenced by a monsoon season

The selective removal of trace metals by suspended matter in high turbidity zones plays a major role in the fluvial transport of terrigenous metals to the marine environment. The seasonal longitudinal variability of trace elements (Cu, Zn, Cd, Ni, Pb, Fe, and Mn) in Cochin estuary, a tropical positive estuary, was studied and the results were compared with the prevailing situation in other subtropical waterways. The hydrodynamical features showed increasing turbidity downstream with increasing salinities during both the seasons. In contrast with the temperate estuaries where the development of turbidity maxima causes the removal of metals, the estuaries of tropics modify the fluvial transport of metals by the way of redistribution between the dissolved and particulate fractions in the intermediate salinities. In Cochin estuary, the distributional features of trace metals are primarily influenced by the variations in salinities and river discharges. Consequently, this gives rise to two different types of distributional patterns: (1) during premonsoon, the estuarine reactivity is more pronounced and hence, mid-estuarine solubilization of the particulate metal appears to play a prominent role in controlling the fluxes of trace metals studied and (2) but during monsoon, the hydrological conditions influence the downstream transport of the metals more by physical dilution than chemical reactivity.     

Fluxes of dissolved combined neutral sugars (polysaccharides) in the Delaware estuary

This project examined concentrations, composition and turnover neutral sugars in the Delaware estuary to gain insights into dissolved inorganic nitrogen (DIN) use by heterotrophic bacteria and into the lability and diagenetic state of dissolved organic material (DOM) during passage through the estuary. Dissolved free monosaccharides were not measurable (<5 nM) in the estuary whereas concentrations of dissolved combined neutral sugars (DCNS) were high, much higher than observed in oceanic waters. DCNS made up a similar fraction of dissolved organic carbon (DOC) as in the oceanic waters examined to date, and the monosaccharide composition of the DCNS pool was similar to that of oceanic waters. The composition did not vary substantially within the estuary or seasonally, but it did vary among three size fractions of the organic matter pool. Glucose was enriched in the low molecular weight fraction of DOC and in particulate material, whereas the high molecular weight DOC fraction was slightly depleted in glucose. Depletion experiments indicated that DCNS is not used extensively on the day time scale in the Delaware estuary, although freshly-produced polysaccharides may still be important carbon sources for heterotrophic bacteria. The very low concentrations of free monosaccharides in the Delaware estuary help to explain why DIN use by heterotrophic bacteria is relatively low in this estuary. Although DOC-DIN interactions in the Delaware apparently differ from oceanic waters, the portion of DOM traced by DCNS, which is thought to be the labile fraction, appears to be similar to that of oceanic DOM, suggesting that organic material in the estuary is degraded extensively before being exported to the coastal ocean.     

Methylmercury Bioaccumulation in an Urban Estuary: Delaware River,USA

Spatial variation in mercury (Hg) and methylmercury (MeHg) bioaccumulation in urban coastal watersheds reflects complex interactions between Hg sources, land use, and environmental gradients. We examined MeHg concentrations in fauna from the Delaware River estuary, and related these measurements to environmental parameters and human impacts on the waterway. The sampling sites followed a north to south gradient of increasing salinity, decreasing urban influence, and increasing marsh cover. Although mean total Hg in surface sediments (top 4 cm) peaked in the urban estuarine turbidity maximum and generally decreased downstream, surface sediment MeHg concentrations showed no spatial patterns consistent with the examined environmental gradients, indicating urban influence on Hg loading to the sediment but not subsequent methylation. Surface water particulate MeHg concentration showed a positive correlation with marsh cover whereas dissolved MeHg concentrations were slightly elevated in the estuarine turbidity maximum region. Spatial patterns of MeHg bioaccumulation in resident fauna varied across taxa. Small fish showed increased MeHg concentrations in the more urban/industrial sites upstream, with concentrations generally decreasing farther downstream. Invertebrates either showed no clear spatial patterns in MeHg concentrations (blue crabs, fiddler crabs) or increasing concentrations further downstream (grass shrimp). Best-supported linear mixed models relating tissue concentration to environmental variables reflected these complex patterns, with species specific model results dominated by random site effects with a combination of particulate MeHg and landscape variables influencing bioaccumulation in some species. The data strengthen accumulating evidence that bioaccumulation in estuaries can be decoupled from sediment MeHg concentration, and that drivers of MeHg production and fate may vary within a small region.     

Rate of mercury loss from contaminated estuarine sediments

The concentration of mercury in contaminated estuarine sediments of Bellingham Bay, Washington was found to decrease with a half-time of about 1.3 yr after the primary anthropogenic source of mercury was removed. In situ measurements of the mercury flux from sediments, in both dissolved and volatile forms, could not account for this decrease. This result suggests that the removal of mercury is associated with sediment particles transported out of the study area. This decrease was modeled using a steady-state mixing model.Mercury concentrations in anoxic interstitial waters reached 3.5 μg/l, 126 times higher than observed in the overlying seawater. Mercury fluxes from these sediments ranged from 1.2 to 2.8 × 10^?5 ng/cm²/sec, all in a soluble form. In general, higher Hg fluxes were associated with low oxygen or reducing conditions in the overlying seawater. In contrast, no flux was measurable from oxidizing interstitial water having mercury concentrations of 0.01-0.06 μ/l.     

Modeling the transport and distribution of lead in tidal Keelung River estuary

A vertical, two-dimensional heavy metal (lead) transport model incorporated into the hydrodynamic, salt, and sediment transport modules was developed to simulate the lead concentration in the tidal Keelung River estuary of northern Taiwan. We validated the developed model with measured data, including longitudinal velocity, salinity, suspended sediment, and heavy metal (lead) concentration, obtained in 1998. An exponential relationship relating the salinity and suspended-sediment concentrations was established to calculate the partition coefficient of lead in the estuary. The simulated results of dissolved, particulate, and total lead concentrations agreed well with the measured data. A model sensitivity analysis indicated that the partition coefficient plays an important role in the distribution of dissolved and particulate lead concentrations along the tidal Keelung River estuary.     

Transformation of phosphorus in the Elbe estuary

The distribution of dissolved and particle-bound phosphorus (P) was investigated in the Elbe estuary during March 1995. The forms of particulate P were studied with a sequential extraction technique. Organic P dominated particle-bound P in the outer reaches of the estuary (52%), decreased to a minimum of 21% in the turbidity zone, and increased to 33% further upstream. Fe-bound P was the second most important P species in the outer reaches (27%) and dominated in the turbidity zone (up to 57%) and upstream of the turbidity zone (up to 48%). The P:Fe ratio increased with decreasing salinity, from 0.11 in the outer reaches to about 0.22 at zero salinity. Dissolved inorganic P release from reverine suspended matter was about two to three times larger than release, from marine suspended matter and was dominated by release of Fe-bound P. Dissolved inorganic P release from marine and from riverine organic matter were of equal importance. Because marine suspended matter dominates in the estuary, this suggests riverine organic matter is remineralized much faster than marine organic matter. This is in line with the refractory nature of marine organic matter (no phytoplankton bloom) and the easily degradable character of the riverine suspended matter (phytoplankton bloom) in the Elbe estuary during March 1995.     

Spatiotemporal changes in cadmium contamination in the Seine estuary (France)

Cadmium (Cd) is among the major contaminants in the Seine estuary. In the biota, the RNO (Réseau National d’Observation, the French Mussel-Watch) has shown that Cd concentrations in mussels living at the mouth of the estuary are related to changes in inputs to this area of phosphogypsum, a calcium sulphate that is a by-product of the phosphoric acid naturally enriched with Cd. In the water column, Cd concentrations at several key estuary sites show a very marked trend toward decreased contamination in the particles as well as in the dissolved phase. The behavior of Cd in the estuary has been studied between 1991 and 1998 in the framework of the scientific program Seine-Aval. This program has highlighted punctual Cd inputs in the estuarine water column. The partition of Cd between the dissolved and the particulate phase, previously described in various estuaries, is characterized by an intense phenomenon of solubilization in the mixing zone freshwater-seawater, but the colloidal Cd fraction remains low along the whole salinity gradient, about 5% to 10% of the apparent dissolved fraction. Although the decrease of inputs induced a fall of Cd concentrations in the water column, laboratory experiments show that the estuarine particles are far from being exhausted in Cd. Despite continuous efforts to reduce the urban and industrial inputs into the estuarine and coastal waters, the Seine estuary still remains very contaminated by Cd.