Trace metal distributions in shelf waters of the northwestern Black Sea
Introduction
The Black Sea has become a focus of concern regarding the potential effects on its ecosystem of contaminant substances introduced from the industrialized areas along its coastline. Over the last 20 years, the urban and industrial development of the Danube Catchment area—together with the attendant intensification of agriculture—has raised the level of inorganic nitrogen and phosphorus inputs to the adjoining coastal areas so much as to upset the previous dynamics of nutrient, oxygen, pH and organic carbon regulation in these waters (Sur et al., 1996; Cociasu et al., 1996). Moreover, the recent damming of the Danube river has decreased significantly the riverine load of silica to the northwestern Black Sea and seems to be responsible for dramatic shifts in phytoplankton species composition from diatoms (siliceous) to coccolithophores and flagellates (non-siliceous) (Humborg, 1997). Other manifestations of marine eutrophication include an increase in the overall quantities of phytoplankton (Humborg, 1997; Velikova et al., 1999) as well as in the frequency and intensity of blooms of potentially toxic species (Gomoiu, 1985; Jenkinson, 1989), a reduction in zooplankton species diversity (Bochdansky and Herndl, 1996) and changes in the structure of the benthic communities (Gomoiu, 1985).
Although all the stages of eutrophication have been documented in the western Black Sea environment, less attention has been paid to the impact of physical dynamics on eutrophication effects. The surface circulation in the Black sea is considered to be driven by a seasonal thermohaline circulation (Stanev, 1990; Grégoire et al., 1998) acting in concert with the cyclonic wind field (Moskalenko, 1976; Klimok and Makeshov, 1993). The general circulation in the western basin is cyclonic and the main gyre follows approximately the continental slope. However, mesoscale circulation features, including meanders, eddies and filaments develop along the periphery of the Rim Current (Özsoy and Ünlüata, 1997). One frequently observed feature is an anticyclonic eddy located on the shelf, south of the Danube delta. This recurrent eddy is considered responsible for retaining large amounts of fresh water close to the coast and thus enhancing eutrophication effects (Grégoire et al., 1998). In spring, stratification of the water column starts to develop and organic-rich aggregates, which accumulate at or around the pycnocline, are rapidly colonized by bacteria (Alldredge and Crocker, 1995; Decho and Herndl, 1995; Leppard, 1995; Rath et al., 1998). With the breakdown of water column stability occurring in the autumn, the organic-rich aggregates sink down the water column, carrying with them high concentrations of microorganisms. Increased production and sedimentation of organic material has led to a greater incidence of marked oxygen depletion and even anoxia in near bottom waters of the northwestern shelf (Friedl et al., 1998). Fluctuations of the redox conditions in the water column and sediment–water interface can have consequences for the distributions of redox sensitive elements (Mn and Fe). Although Fe and Mn are of little direct concern from an environmental quality point of view, they nonetheless can influence the transport and fate of other metals investigated in the present work (Pb, Cd, Zn, Cu, Ni) which can have deleterious effects on organisms at relatively low concentrations. Cd and Pb are regarded as potentially of greatest concern among these metals, in terms of their toxic effects on the ecosystem.
The work reported in this paper was a component of a multidisciplinary study of the northwestern Black Sea undertaken during the pilot phase of the EC-funded EROS 2000 Project. The objective was to understand the biogeochemical functioning of the northwestern shelf region under summer conditions especially with regard to nutrients and trace metals. The station locations were chosen primarily to provide a good coverage of the Danube influence on the northwestern Black Sea shelf. A deep water station was also occupied to allow comparison with adjacent shelf and with deep water profiles reported for other regions of the Black Sea. An account of the behaviour of trace metals during mixing in the Danube Estuary and plume has been given by Guieu et al. (1998).
Section snippets
Sampling and analytical methods
The EROS 2000 expedition was conducted in July–August 1995 on board R/V Professor Vodyanitskiy. The positions of the stations discussed in this paper are shown in Fig. 1. Samples were taken to obtain detailed coverage of the water column. They were collected using 10-l Teflon-lined Go-Flo bottles, fitted with Teflon taps and deployed on a rosette with a CTD. Water was filtered under nitrogen pressure—to avoid any contamination with oxygen—through Nuclepore filters (0.4 μm pore diameter) using an
Hydrographic properties
The water column at Station 3 in the open Black Sea was characterized by strong density stratification between depths of 20 and 120 m as a result of both temperature and salinity gradients (Fig. 2). A maximum in fluorescence possibly due to accumulation of biological debris was associated with the thermocline. Oxygen was completely depleted below 120 m. Nutrients (nitrate+nitrite, ammonium, dissolved silicon and phosphate) were depleted in surface water due to biological uptake (Fig. 3).
Station 3: a deep water reference station
At Station 3, relatively high dissolved Mn concentrations (10 nM) were found in surface water (Fig. 6), possibly due to atmospheric inputs (Chester et al., 1993). The concentrations decreased to reach a minimum of 1 nM between 60 and 110 m. Below 110 m, which was the upper limit of the redoxcline, dissolved Mn concentrations increased rapidly to reach a maximum at 190 m (8750 nM). Below 200 m, dissolved Mn concentrations decreased again with depth. The shape of the Mn maximum is controlled by a
Conclusions
Measurements of dissolved and reactive particulate metals (Mn, Fe, Co, Pb, Cd, Cu, Ni and Zn) and total particulate metals (Mn and Fe) have provided a systematic picture of the operation of biogeochemical processes in the northwestern Black Sea. They have demonstrated the importance of the magnitude of metal and nutrient inputs from the Danube and from other localized sources, together with the importance of the Mn and Fe cycles in determining trace metal distributions.
Dissolved and particulate
Acknowledgements
This work was supported by the European Commission, Environment and Climate, Project ENV4-CT960286. The authors would like to thank A. Krastev, R. Pentcheva, J. Vervlimmeren and L. Popa for nutrients and oxygen analyses conducted on samples collected during the 1995 EROS 2000 expedition and the chief scientists (V. Egorov and C. Lancelot) for organizing the expedition.
References (72)
- et al.
Why do sinking mucilage aggregates accumulate in the water column
Science of the Total Environment
(1995) Copper, lead and cadmium in coastal waters of the western North Sea
Marine Chemistry
(1985)The control of trace metals distributions in coastal seawater through partition onto suspended particulate matter
Netherlands Journal of Sea Research
(1988)- et al.
Spatial and temporal variability in copper complexation in the North Pacific
Deep-Sea Research
(1990) - et al.
Long-term ecological changes in Romanian coastal waters of the Black Sea
Marine Pollution Bulletin
(1996) - et al.
An improved metal extraction procedure for the determination of trace metals in seawater by atomic absorption spectrometry with electrothermal atomization
Analytica Chimica Acta
(1978) - et al.
Microbial activities and the transformation of organic-matter within mucilaginous material
Science of the Total Environment
(1995) - et al.
Speciation of dissolved copper and nickel in South San Francisco Baya multi-method approach
Analytica Chimica Acta
(1994) - et al.
Benthic fluxes of nutrients in the northwestern Black Sea
Marine Chemistry
(1998) - et al.
Reconnaissance of the main Black Sea's ecohydrodynamics by means of a 3D interdisciplinary model
Journal of Marine Systems
(1998)
On trace metal geochemistry in the western Black SeaDanube and shelf area
Estuarine, Coastal and Shelf Science
Primary productivity regime and nutrient removal in the Danube Estuary
Estuarine, Coastal and Shelf Science
Community metabolism in microbial matsthe occurrence of biologically-mediated iron and manganese reduction
Estuarine, Coastal and Shelf Science
Trends in oxygen content 1911–1984 and occurrence of benthic mortality in the Northern Adriatic Sea
Estuarine, Coastal and Shelf Science
Seasonal coupling between riverborne nutrients, net productivity and hypoxia
Marine Pollution Bulletin
The characterization of algal and microbial mucilages and their aggregates in aquatic ecosystems
Science of the Total Environment
Voltametric estimation of iron(III) thermodynamic stability constants for catecholate siderophores isolated from marine bacteria and cyanobacteria
Marine Chemistry
The biogeochemistry of manganese and iron in the Black Sea
Deep-Sea Research
The distribution and preferential biological uptake of cadmium at 6 degree W in the Southern Ocean
Marine Chemistry
Oxidation of cobalt and manganese in seawater via a common microbially catalyzed pathway
Geochimica et Cosmochimica Acta
Adsorption and coprecipitation of divalent metals with mackinawite (FeS)
Geochimica et Cosmochimica Acta
Isolation and identification of manganese-reducing bacteria and estimates of microbial Mn(IV)-reducing potential in the Black Sea
Deep-Sea Research
Oceanography of the Black Seaa review of some recent results
Earth-Science Reviews
Complexation of iron(III) by natural organic ligands in the Central North Pacific as determined by a new competitive ligand equilibration/adsorptive cathodic stripping voltammetric method
Marine Chemistry
Time series for dissolved cadmium at a coastal station in the western Baltic Sea
Journal of Marine Systems
On the mechanisms of the Black Sea circulation
Earth-Science Reviews
The determination of dissolved manganese and cadmium in seawater at low nmol/l concentrations by chelation and extraction followed by electrothermal atomic absorpion spectrometry
Analytica Chimica Acta
Coastal/deep ocean interactions in the Black Sea and their ecological/environmental impacts
Journal of Marine Systems
Trace metals in the central and southern North Sea
Estuarine, Coastal and Shelf Science
Manganese (II) oxidation in the suboxic zone of the Black Sea
Deep-Sea Research
Changing coastal oceanography of the Black SeaI
Northwestern shelf. Progress in Oceanography
Evidence for organic complexation of iron in seawater
Marine Chemistry
Phytoplankton dynamics and red tides (1987–1997) in the Bulgarian Black Sea
Water Science and Technology
Behaviour of copper in south-eastern United States estuaries
Marine Chemistry
Complexation of Fe(III) by natural organic ligands in the Northwest Atlantic Ocean by a competitive ligand equilibration method and a kinetic approach
Marine Chemistry
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