Trace metals in Baltic Sea waters

	Authors: Christa Pohl and Ursula Hennings Baltic Sea Research Institute, Warnemünde

Key Message

In general concentrations for dissolved and particulate bounded cadmium (Cd), lead (Pb) and zink (Zn) are higher in the western Baltic Sea, while for dissolved copper (Cu) and total mercury (Hg) a slight increase is observed in the Baltic Proper.

Since 1997 decreasing temporal trends were observed for Cd and Cu in the deep water of the Baltic Proper which is due to the stagnation period and the formation of slightly soluble metal sulphides under anoxic conditions. The observation period was perhaps too short for the detection of clear trends, especially because the system was influenced by a major saltwater inflow in 1993 and 1994.

Introduction

Cadmium (Cd), Mercury (Hg), Lead (Pb), Copper (Cu) and Zinc (Zn) are hazardous metals in the Baltic marine environment. Because of their large impact by rivers and the atmosphere (HELCOM 1997; 1998), their toxic effects on the ecosystem and their complex biogeochemical behaviour, it was recommended by HELCOM to monitor these contaminants isochronous.

The enrichment of metals in biota is dependent on their bioavailability. In this context it is useful to differentiate between dissolved and particle bounded metal species as also implemented by OSPAR in the North Sea.

To study the development of trace metal trends in the water column and to establish comparable time series, it was recommended to perform the sampling of these contaminants once a year in wintertime (February), when the biological activities are limited.

Background concentrations

The valuation of background concentrations for trace metals in Baltic waters to centuries, when anthropogenical pollution was reduced to a minimum, is limited. One possibility is the use of comparable data from areas which are less influenced by human activities e.g. the northern Atlantic waters (Table1). The trace metal concentrations in Baltic waters are still higher than in Atlantic waters, although a decreasing trend of 6 % per year for Cd and Cu was revealed between 1980 and 1993 (HELCOM 1996) in Baltic surface waters.

Table 1. Concentrations of dissolved trace metals (ng/kg) from the North Atlantic and the Baltic Sea. (1)Kremling, K. & Streu, P. (submitted); (2)Pohl, C. et al. (1993); (3)Pohl, C. & Hennings, U. (1999); (4)Dalziel, J. A. (1995)

Geographical distribution

Regional differences of cadmium, copper,lead and zinc concentrations in the Baltic surface waters were observed between the western Baltic and the central Baltic Proper for the period 1993-2002, showing higher concentrations in the western Baltic (Schneider and Pohl submitted for the 4th Periodic Assessment). Also the results of the monitoring data from the year 2002 confirm these findings (Figures 1, 2 and 3) for the dissolved and particulate bounded phases and total mercury. One explanation is, that in the 1960^th a dumping area highly contaminated in trace metals was established in the Bay of Lübeck. In combination with wind induced mixing and bioturbation of marine organism this area is still more or less a source for SPM and trace metal enrichment in the watercolumn.

A further explanation is the higher atmospheric input into the western Baltic predicted recently from an atmospheric deposition model which has been based on new Baltic field measurements (B. Schneider, personal communication).

Figure 1. Distribution of cadmium (Cd) and copper (Cu) in dissolved and particulate phases in the Baltic Sea surface waters.

Figure 2. Distribution of lead(Pb) and zink (Zn) in dissolved and particulate phases in the Baltic Sea surface waters.

Figure 3. Distribution of total mercury in Baltic Sea surface waters

Long-term development

Inter annual differences at the sampling stations in the western Baltic for the period 1993-2002 demonstrated that not only salinity but also heavy metal concentrations are subject to fluctuations especially in the transition area between the North Sea and Baltic Sea.

The observation period was perhaps too short for the detection of clear temporal trends in this transition area, especially because the Baltic was influenced by a major saltwater inflow in 1993 and 1994.

A decrease of Cd(dissolved) and Cu(dissolved) concentrations in the last 6 respectively 4 years were observed for the waters below the halocline of the central Baltic, due to the precipitation of sulfidic Cd and Cu species under anoxic conditions in the deep water body (Figures 4 and 5) and their enrichment in surface sediments. This example demonstrates, how variations in the trace metal concentrations are linked to eutrophication.

Figure 4. Trends for cadmium (Cd diss) and bounded to particles (Cd SPM) in the western Baltic, and above and below the halocline in the central Baltic between 1993-2002.

Figure 5. Trends for copper (Cu diss) and bounded to particles (Cu SPM) in the western Baltic, and above and below the halocline in the central Baltic between 1993-2002.

In the last years decreasing lead concentrations were also observed in the Baltic. Reasons for that are the use of unleaded petrol, the fast vertical transport of lead bounded to particles and there enrichment in the sediments. Baltic Sea sediments are still high contaminated in trace metals (Borg & Jonsson1992).

Assuming that the Baltic deep water system changes to oxic conditions it is expected, that remobilisation of metals and their release back into the watercolumn will take place.

Since 1997 / 1999 we observed, that the water body above the halocline is characterised by slightly decreasing trends (not significant) over the passed 2-5 years for the dissolved phases of Cd, Cu, Pb, Zn and Hg (Figures 4-8). Now the question arise, if long-term trace metal trends in surface waters of the central Baltic are a result of reduced input to the Baltic Sea, or if they are related to a “feedback mechanism” including the stabilisation of the anoxic deep water regime and the trace metal export by diffusive exchange across the oxic-anoxic interface.

Figure 6. Trends for lead (Pb diss) and bounded to particles (Pb SPM) in the western Baltic, and above and below the halocline in the central Baltic between 1993-2002.

Figure 7. Trends for zink (Zn diss) and bounded to particles (Zn SPM) in the western Baltic, and above and below the halocline in the central Baltic between 1993-2002.

Figure 8. Trends for total mercury (Hg tot) in the western Baltic, and above and below the halocline in the central Baltic between 1993-2002.

References

Borg, H.; Jonsson, P. (1992): Large scale metal distribution in Baltic sediments. In: P. JONSSON, Doct. Dis. Uppsala University, Sweden; 33 pp.
Dalziel, J. A. (1995): Reactive mercury in the eastern North Atlantic and southeast Atlantic. Mar. Chem., 49, 307-314.
HELCOM (1996): Third periodic assessment of the state of the marine environment of the Baltic Sea, 1989-1993, No. 64B, Background document, pp. 252.
HELCOM (1997): Airborn pollution load to the Baltic Sea 1991-1995. Balt. Sea Environ. Proc. No. 69.
HELCOM (1998): The third Baltic Sea Pollution Load compilation (PLC-3); Balt. Sea Environ. Proc., No. 70; Helsinki Commission; pp.133.
Kremling, K.; Streu, P.: Survey on the behavior of dissolved Cd, Co, Zn and Pb in North Atlantic near-surface waters (30°N/60°W to 60°N/2°W). Deep-Sea Research I submitted.
Pohl, C.; Kattner, G.; Schulz-Baldes, M. (1993): Cadmium, copper, lead and zinc on transects through Arctic and Eastern Atlantic surface and deep waters. J. Mar. Syst.; 4; 17-29.
Pohl, C.; Hennings, U. (1999): Bericht zum Ostsee-Monitoring: Die Schwermetall-Situation in der Ostsee im Jahre 1999. Institut für Ostseeforschung, Warnemünde, Seestr. 15, 18119 Warnemünde, Germany.

Last update 20 November 2003