The biogeochemistry and distribution of dissolved trace metals in the Aegean Sea

Abstract

Samples for the analysis of dissolved Mn, Co, Fe, Pb, Cu, Ni, Cd, Zn, nitrate plus nitrite, silicon and phosphate were collected at 7 stations in the northern and 6stations in the southern Aegean Sea, eastern Mediterranean, in both March and September 1997. This represents the first extensive dissolved trace metal data set for this region. Additional samples were collected near hydrothermal vents in Milos, and across the Thermaikos Gulf. Trace metal distributions and concentrations were interpreted with respect to the particularly complex hydrography and circulation patterns of the Aegean Sea. The results show elevated surface concentrations of Mn, Co, Fe, Pb, Cu, Ni and Zn which can be attributed to both atmospheric deposition in the southern Aegean and a combination of atmospheric deposition, the outflow of the Black Sea Water and potential coastal inputs within the northern Aegean. Spatial and temporal metal variability in surface waters was observed, with higher concentrations for all metals in the northern Aegean due to additional surface sources. In September 1997 the overall higher concentrations of Mn, Co, Pb, Cu, Ni and Zn are attributed to the shallowing of the thermocline located at approximately 100 m in March to around 50 m in September, although the possibility of enhanced aolian deposition is not excluded. Elevated signatures of dissolved Mn, Co, Fe and Pb are associated with Black Sea Water and, to a lesser extent with Levantine Intermediate Water in the northern Aegean for March 1997. In the more complex hydrography of the south Aegean the elevated dissolved nitrate plus nitrite and silicon are associated with an intrusion of Transitional Mediterranean Water. In addition, dissolved trace metal results from Thermaikos Gulf and Milos do not suggest a significant input of metals from these sources into the Aegean Sea during the period of sampling. Atmospheric metal fluxes for the central and north-western Mediterranean Sea were used to calculate the residence times in the Aegean surface waters. These fluxes were compared with metal fluxes from sediment traps and particulate metal data in the northern and southern Aegean during the period of sampling. Linear regression analysis was applied to Black Sea influenced surface water waters in the northern Aegean in order to estimate the concentrations of Mn, Co, Fe, Cu and Ni in the Straits of Dardanelles.For the metals Mn, Fe and Pb the calculated short residence times in southern Aegean surface waters of 0.3 to 6.0 years reflect their high particle reactivity and scavenging onto particles and transport to depth. Co exhibits a similar cycling pathway to Mn but with longer surface residence times of 4 to 40 years. The vertical distributions of Ni, Cu, Cd and Zn do not resemble those of nutrients. Ni, Cu, and Zn exhibit surface elevation in the northern Aegean, and to a lesser extent in the southern Aegean, whereas Cd depth profiles were found to be homogenous. These profiles are the result of surface metal sources combined with the oligotrophic nature of the Aegean Sea. The present work suggests that the Aegean Sea, as well as the Mediterranean is not in a steady state with respect to Ni, Cu, Zn and Cd. Concentrations in the southern Aegean are overall comparable to recent high quality observations in the open Mediterranean. However, there appears to be an increase in dissolved Cu and Ni from the western to the eastern basins of the Mediterranean Sea due to continuous inputs to surface waters.

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