Trace metals in Baltic sea water

	Author: Christa Pohl Baltic Sea Research Institute, Warnemünde

Summary

Since 1993 the trace metal concentrations in Baltic Sea surface waters are still in the same order of magnitude.

Key message

In general concentrations for dissolved and particulate bounded Cd, Pb and Zn are higher in the western Baltic Sea, while for dissolved Cu and total 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 sulfides 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 (Tab.1).
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, lead and zinc concentrations in the Baltic surface waters were observed between the western Baltic and the central Baltic Proper for the period 1993-2000, 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 2001 confirm these findings (Figures 1 & 2) for the dissolved and particulate bounded phases. 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).
In contrast to the elements cadmium, lead and zinc, the surface concentrations for total mercury (Hgtot) and dissolved copper are slightly higher in the central Baltic Proper.

Figure 1.	Figure 2.

 Long term development

Inter annual differences at the sampling stations in the western Baltic for the period 1993-2001 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 3 & 4) and their enrichment in surface sediments. This example demonstrates, how variations in the trace metal concentrations are linked to eutrophication.
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.

Figure 3.	Figure 4.

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