Dissolved and particulate metals (Fe, Zn, Cu, Cd, Pb) in two habitats from an active hydrothermal field on the EPR at 13°N

Le texte intégral est accessible via Archimer: http://archimer.ifremer.fr/doc/2008/publication-3830.pdfInternational audienceThe distribution of Fe, Cu, Zn, Pb, Cd between the dissolved ( 2 μm) fractions was measured after in-situ filtration in two hydrothermal habitats. The total metal concentration ranges exhibit a clear enrichment compared with the seawater concentration, accounting for the hydrothermal input for all the metals considered. Iron is the predominant metal (5-50 μM) followed by Zn and Cu. Cd and Pb are present at the nM level. At the scale studied, the behavior of temperature, pH and dissolved iron is semi-conservative whereas the other dissolved and particulate metals are characterized by non-conservative patterns. The metal enrichment of the > 2 μm fraction results from the settlement and accumulation of particulate matter close to the organisms, acting as a secondary metal source. The enrichment observed in the dissolved fraction can be related to the dissolution or oxidation of particles (mainly polymetallic sulfide) or to the presence of small particles and large colloids not retained on the 2 μm frit. SEM observations indicate that the bulk particulate observed is characteristic of crystalline particles settling rapidly from the high temperature smoker (sphalerite, wurtzite and pyrite), amorphous structures and eroded particles formed in the external zone of the chimney. Precipitation of Zn, Cu, Cd and Pb with Fe as wurtzite, sphalerite and pyrite is the main process taking place within the area studied and is semi-quantitative. The distribution of the dominant observed fauna has been related to the gradient resulting from the dilution process, with the alvinellids worms colonizing the hotter and more variable part of the mixing zone, but also to the metallic load of the mixing zone. Dissolved and particulate metal concentrations are therefore necessary abiotic factors to be studied in a multiparametric approach to understand the faunal distribution in hydrothermal ecosystems

Spéciation des éléments métalliques en milieu estuarien et quantification des flux vers le plateau continental

Le comportement des différentes formes chimiques du fer, du cuivre, du plomb et du cadmium a été étudié dans la zone de mélange eau douce-eau salée le long du gradient salé en liaison avec les variations des caractéristiques hydrologiques et biogéochimiques. Deux systèmes possédant des caractéristiques très différentes ont été retenus la Penzé (Bretagne, Manche Occidentale) qui présente un faible débit et qui est soumis à un brassage vertical intense (système homogène) et la Loire qui présente un débit élevé et dont les eaux s écoulent dans une zone (plateau continental Nord Gascogne) où le brassage vertical est peu intense (système stratifié). Un effort important a aussi été consacré à la mise au point de méthodes d analyse des métaux spécialement adaptées au milieu marin. Des méthodes électrochimiques utilisant la chronopotentiométrie comme technique de redissolution ont été développées. Nos résultats montrent des comportements similaires pour le plomb, le cuivre et le cadmium dans les deux systèmes étudiés. Le plomb subit une coprécipitation importante avec les hydroxydes de fer. Le cuivre désorbe généralement des particules organiques pour former des complexes organiques hydrophobes. Le cadmium lié aux oxydes de fer et de manganèse désorbe pour former des cholorocomplexes ou des complexes organiques. Nos résultats indiquent aussi qu au cours du cycle saisonnier, les apports des différentes formes chimiques métalliques au milieu marin dépendent fortement des variations des caractéristiques hydrologiques et biogéochimiques du système. Il est important de prendre en compte ces modifications de l état du métal pour établir des bilans fiables en métal exporté.The behaviour of various chemical species of iron, copper, lead and cadmium was studied along the whole salinity gradient as a function of hydrological and biogeochemical variations. Our investigations were focused on two systems with very different properties: the Penzé (Britanny, Western Channel) and the Loire-North Biscay Continental Shelf. The former is characterised by a 10w freshwater discharge and well-mixed waters, whereas the latter shows a high freshwater discharge and waters flowing to the North Biscay Continental Shelf where the waters are stratified. A great effort was also devoted to the development of methods based on stripping chronopotentiometric electrochemistry for metal analysis in the marine environment. Our experimental results highlighted similar behaviours for lead, copper and cadmium in both systems : i) Lead was highly coprecipitated by hydrous oxides iron ; ii) Copper was generally desorbed from organic particles and made complexes with non-polar hydrophobic organic ligands ; iii) Cadmium associated with hydrous oxides of iron and manganese desorbed and formed cholorocomplexes or organic complexes. We also showed that fluxes of the various chemical metal species to the marine environment over the seasonal cycle are highly dependent on the hydrological and biogeochemical variations of the system under study. Taking into account these changes in the metal state is paramount for further reliable monitoring of exported metal traces.BREST-BU Droit-Sciences-Sports (290192103) / SudocROSCOFF-Observ.Océanol. (292393008) / SudocSudocFranceF

High-resolution examination of the colloidal speciation of cadmium in estuarine waters (Penzé, NW France)

International audienceWe report here for the first time a high-resolution study of the colloidal cadmium speciation with seasonal variations in estuarine waters. Over this 1-year study, seven colloidal fractions below 0.45 pm were analysed at 6 stations along the salinity gradient of the Penze estuary, with the aim to provide some insights into the mechanisms that control Cd concentrations and behaviour under estuarine mixing. A classic and strong positive Cd anomaly occurred at mid-estuary throughout the seasonal cycle. This anomaly corresponded essentially to an input of low molecular weight (LMW) compounds, i.e. in the size range 5-50 kDa, arguing that besides the usual hypothesis of Cd complexation with chloride, it is also of fundamental importance to consider the role of colloidal organic species in favouring Cd mobilisation. The colloidal repartition of Cd being notably different to that of humic substances, the ligands involved in Cd complexation are more probably of allochthonous origin. Over the annual cycle, total dissolved Cd (<0.45 mu m) concentrations diminished considerably in spring and summer over the whole system, while an increasing of Cd in particles at high salinities was taking place at the same periods. The effects of biological uptake, of the associated pH increase of the waters and of the nature of organic ligands on (1) Cd desorption mechanism and (2) on the resulting Cd speciation are discussed

On the control of copper colloidal distribution by humic substances in the Penzé estuary

In this study, we investigated the variations of colloidal Cu in a temperate macrotidal estuarine system (Penze, NW France). The originality of this work resides on examining seven colloidal/dissolved fractions at seven different periods of the year whereas previous studies on estuaries generally considered two or three fractions and were focused on a unique survey. A high proportion of Cu (similar to 90%) was generally found as colloids (5 kDa-0.45 mu m) throughout the salinity gradient with divergent size distributions being observed over the seasonal cycle. This consisted essentially in two contrasted periods, i.e. winter-spring with a greater association of Cu with high molecular weight (HMW) compounds (50 kDa-0.45 mu m) and summer-autumn with Cu being found mainly as low molecular weight (LMW) forms (5-50 kDa). The comparison of Cu with humic substances (HS) data allowed to us to highlight the importance of the pedogenic refractory organic matter in controlling the concentrations and the size distribution of Cu in the estuary. In the mixing zone, Cu behaved conservative in autumn and winter but important additions of HMW compounds were observed in spring in the lower estuary as the result of particulate organic matter degradation in the sediment. Although HS appears to be the background chelators of Cu in the systems, the strong benthic inputs occurring in spring may be of different (biotic) origin and may be in part responsible for the higher association of Cu with HMW compounds

Seasonal variations of cadmium speciation in the Penzé estuary, NW France

International audienceThe study reported here was carried out in the macrotidal estuary of Penzé (Brittany, Western Channel, France). Ten field stations along the freshwater–seawater mixing zone were sampled each month in order to examine the speciation of dissolved and particulate cadmium. Different biogeochemical parameters (suspended particulate matter, Chlorophyll-a, pH and dissolved organic carbon) were also measured. Levels of total dissolved and total particulate metal ranged from 0.06 to 0.46 nM and from 0.4 to 2.4 μg g−1, respectively. Our data showed a non-conservative behaviour for cadmium as a result of desorption from particles. The rise in total dissolved cadmium in the salinity range 0–20 was about 0.2 nM. Incidentally, a decrease of total particulate concentrations of about 0.5 μg g−1, which corresponds approximately to 0.2 nM, was observed. The study of the various cadmium species led us to identify desorption of cadmium occurred from the iron/manganese oxide-associated fraction. This desorption process leads to the formation of labile complexes, i.e. chlorocomplexes in spring and summer. However, in winter and autumn, a large part of the dissolved fraction was constituted by organic complexes. These complexes can be formed from suspended particles desorption or could be a released from sediment. Analysis of flux indicated that most of the metal inputs to the estuary (usually more than 80%) corresponded to organic cadmium. Spring and summer outputs were mainly constituted of chlorocomplexes whereas cadmium organic complexes were predominant in winter and autumn output fluxes

Seasonal variations of dissolved and particulate copper species in estuarine waters

International audienceThis one-year survey conducted in the macrotidal estuary of Penzé (Brittany, Western part of the Channel, France) was aimed at examining the variations of the various dissolved and particulate copper species. Ten field stations along the whole freshwater–seawater mixing zone were sampled each month. Different biogeochemical parameters (SPM, chl-a, pH and DOC) were also measured. The levels in total dissolved and total particulate copper ranged from 1.8 to 9.5 nM and from 5 to 98 μg g−1, respectively; such amounts are indicative of a pollution-free system. Extractable C18 copper (non-polar hydrophobic organic copper species), in winter and spring, accounted for 30–40% of the total dissolved copper. In summer, this contribution rapidly rose to 60% in the salinity range 20–30; over the same period of time, total particulate copper decreased. The change in dissolved copper speciation and the lowering of particulate copper concentrations were attributed to the release of strong organic ligands by phytoplankton. Our field data evidenced a highly variable behaviour for the various copper species over the seasonal cycle, and then led us to identify the following mechanisms: (i) metal desorption from organic river-flown particles (winter and spring), (ii) metal desorption from resuspended sediment in the upstream section (summer), (iii) competition between particles, non-extractable C18 organic ligands and phytoplankton-released extractable C18 organic ligands to complex copper in the downstream section (summer), and (iv) removal of non-extractable C18 organic copper by adsorption (autumn). Dissolved copper species fluxes were assessed: most of metal inputs to the estuary (60–74%) corresponded to non-extractable C18 organic copper. Winter and spring metal output fluxes were mainly constituted of non-extractable C18 organic complexes; on the other hand, extractable C18 organic complexes were predominant in summer and autumn output fluxes

Distribution and seasonal changes of lead in an estuarine system affected by agricultural practices: The Penzé estuary, NW France

International audienceOver this one-year study, we examined the variations of lead along the salinity gradient of the Penzé estuary (Brittany, Western Channel, France). Concentrations of total dissolved and total particulate lead ranged from 0.04 to 0.62 nM and from 29 to 61 μg g−1, respectively. These values, though being higher than contamination-free systems, remain lower than those found in heavily industrialised systems indicating a limited effect of agricultural practices. Our field data showed, for most seasons (i.e. spring, summer and autumn), a metal removal in the salinity range 0–10, effected mainly by co-precipitation with iron and manganese oxides. On the other hand, after a strong winter ascending flood, an addition of dissolved lead occurred in the estuary following resuspension of particles and desorption mechanisms. This metal addition along with the high water discharge were responsible for the particularly high Pb output flux reported for winter (67 g day−1) compared to the ones estimated for other seasons (1–18 g day−1)

Effects of mixing-induced irradiance fluctuations on nitrogen uptake in size-fractionated coastal phytoplankton communities

International audienceIn coastal waters subjected to strong tidal forcing, phytoplankton populations are exposed to highly variable light regimes. To grow under such fluctuating light environments, phytoplankton adjust their physiological properties. Here, we investigated nitrogen (N) uptake patterns in the western English Channel to determine whether phytoplankton modify their physiological processes involved in N uptake in response to changing irradiance conditions induced by spring-neap tidal cycles. Nitrate (NO3 -) and ammonium (NH4 +) uptake kinetics as a function of irradiance (VN-E curves) were assessed using 15N tracer techniques on two size fractions ( 10μm) of phytoplankton collected at 50% and 1% of surface irradiance during two spring-neap tidal cycles. Overall, the results showed that both small and large phytoplankton, whatever their vertical position in the water column, increased their maximum uptake capacity and their light utilization efficiency for the two N substrates following the decrease in vertical mixing intensity. Moreover, the improvement of irradiance conditions at neap tides was of greater benefit for the larger cells than for the smaller ones and was more favorable for NO3 - uptake than for NH4 + uptake. These findings show that the light regime fluctuation resulting from the relaxation of tidal mixing during spring-neap tidal cycle leads to profound physiological adjustments of N uptake processes in phytoplankton communities. They suggest that the changes in NO3 - uptake by large phytoplankton associated with the fortnightly spring-neap tidal cycle can account for most of the deviation in background productivity in the western English Channel which is based on NH4 + and is dominated by small cells. The dynamic light regime inherent to macrotidal coastal ecosystems could therefore determine, to a large extent, the importance of new vs. regenerated production as well as the size structure of the phytoplankton community

Measurements of trace concentrations of mercury in sea water by stripping chronopotentiometry with gold disk electrode: influence of copper

International audienceA stripping chronopotentiometric method, using a rotating gold disk electrode for mercury measurements in sea water is described. Compared with a same method using a stationary gold film electrode, this method has a eight times higher sensitivity and a detection limit of 5 ng l−1 after 10 min deposition time. Moreover, the time needed for gold plating is eliminated. Compared with other electrochemical methods capable of measuring mercury at low concentrations, the present method is more simplified with no degassing step and no need to use a medium-exchange procedure before the stripping step. These characteristics render the method easily practicable on board oceanographic vessels for ‘in situ' measurement

Determination of ultra-trace Sb(III) in seawater by stripping chronopotentiometry (SCP) with a mercury film electrode in the presence of copper

International audienceThis work reports the determination of ultra-trace of Sb(III) in seawater by using a stripping chronopotentiometric (SCP) method with a mercury film electrode. A sensitivity and detection limit of 360 ms L μg−1 and 8 ng L−1 (70 pM), respectively, were accomplished for a 15-min electrolysis time. Compared to the only two chronopotentiometric methods reported for Sb(III) determination in seawater, our method is more sensitive and does not need to use a medium exchange procedure before the stripping step. Moreover, the use of a double electrolysis potential (−450 mV and −250 mV) allows the analysis of Sb(III) independently from the Cu level in the sample. The method was successfully used to study the behaviour of dissolved Sb(III) in the Penzé estuary, NW France