Trace metal distributions in shelf waters of the northwestern Black Sea

Abstract

Measurements of dissolved and leachable particulate trace metals (Mn, Fe, Co, Pb, Cd, Zn, Cu and Ni) and total particulate Mn and Fe were made on seawater samples collected from the northwestern Black Sea during the EROS 2000 expedition conducted in July–August 1995. The investigation concentrated on waters of the shelf and shelf edge, but included one deeper water (1440 m) station. In the oxic layer of the deep station, the suspended particulate fractions of Mn and Fe were a major part of the total metal mass, consistent with the presence of the “Fine Particle Layer” which forms on the shelf and spreads all over the Black Sea with intensities decreasing from the coast. Dissolved and total particulate concentrations were, respectively, Mn, 0.69–9.6, 1.2–29; Fe, 0.79–3.03, 2.3–7.4 nM. Dissolved Cu and Ni concentrations were relatively high (1–8 and 8–12 nM, respectively), and did not show any depletion in surface oxic waters, possibly as a result of strong organic complexation. Dissolved Pb concentrations (100–200 pM) were higher than were generally found on the shelf. This was attributed to atmospheric inputs combined with less efficient scavenging of metals in these low SPM waters. The distribution of dissolved Co closely resembled that of dissolved Mn reflecting coupling through oxidation of Mn. Concentrations of dissolved Cd and Zn were low in surface water (0.07–0.09 and 0.9–2.0 nM, respectively), and increases in concentrations with depth were sharply reversed around the top of the redoxcline. For most metals (Mn, Fe, Co, Pb, Cu, Cd, Zn) dissolved concentrations were low in the anoxic layers as a result of solubility by formation of, or association with, solid sulphide phases. Dissolved Ni was not affected by sulphide precipitation. At most of the shelf stations there were clear enhancements of dissolved Mn and Fe in the deepest waters, consistent with other evidence that significant benthic fluxes of these metals arise through the redox conditions in the region of the sediment-water interface. In the shelf water column, dissolved Mn and Fe concentrations ranged between 1.2 and 1350 and between 0.4 and 181 nM, respectively; the highest concentrations were found near the bottom. Particulate concentrations of Mn and Fe were high, implying high oxidation rates of Mn(II) and Fe(II) and/or high supply rates from rivers. Total particulate concentrations of Mn and Fe were 0.7–1050 and 2.3–2650 nM, respectively; the highest concentrations were found in surface and bottom waters. The distributions of particulate Mn and Fe were consistent with the isopycnal transport of Mn and Fe oxyhydroxides from the shelf by the coastal circulation. Distributions of other trace metals (Co, Pb, Cu, Ni, Cd, Zn) were considerably influenced by riverine inputs. Relatively high dissolved and available particulate metal concentrations were generally found in surface waters at stations directly influenced by the Danube River. Some trace metals (Co, Ni, Cd and Zn) were influenced by Mn and Fe cycling and increases in their dissolved concentrations occurred at a number of stations near the sediment–water interface. Dissolved and available particulate metal concentrations (nM) at stations on the shelf were, respectively: 0.171–1.80, 0.003–0.437 (Co); 0.014–0.614, 0.010–1.48 (Pb); 7.6–28.8, 0.048–3.75 (Cu); 11.0–17.5, 0.018–2.10 (Ni); 0.033–0.161, 0.003–0.063 (Cd); 1.01–8.33, 0.135–7.58 (Zn).

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).