Temporal distribution of trace metals in Antarctic coastal waters

In the framework of the Italian Research Programme in Antarctica, a nearshore station positioned inside the Gerlache Inlet (Terra Nova Bay, Western Ross Sea) was regularly sampled during the Austral Spring 1997/1998, from November to February. In order to evaluate the physical, chemical and biological variations along the water column, measurements of a large number of parameters were performed at a suitable frequency: temperature, salinity, fluorescence, suspended particulate matter, concentration of dissolved and particulate Fe, Cu and Mn, concentration of nutrients and phytoplankton pigments. Samples of pack ice and marine microlayer were also collected and analysed. Elaboration of data with regards to temporal and vertical components provided a good description of temporal distribution of trace metals in the coastal waters of Terra Nova Bay, in relation to the physical and biological processes. Concerning the surface layer, it was found that concentrations are mainly affected by the dynamic of the pack ice melting and phytoplankton activity. The first process influences both the input of metals from meltwaters and the covering of the seawater surface, allowing atmospheric dust input only when all ice has been melted or removed. Direct release from ice of particulate Fe and Cu were clearly shown by surface concentration peaks and by the high concentrations of suspended particulate matter and particulate metals detected in the ice core section interfaced with the seawater. Moreover, a quite complex dynamic dissolved/particulate partitioning was found, which can account for the temporal uncoupling between the maximum values of particulate and dissolved metals. In absence of pack ice, there is evidence of a further input of Fe and Mn due to atmospheric dust brought into the water column through the marine microlayer. This picture is complicated by phytoplankton activity, which removes micronutrients like Fe and Cu from water, incorporating them into organic particulate matter. The dynamic of metal uptake follows that of primary production which, in the studied area, shows a specific trend characterised by two distinct blooms: the first in the second half of December and the second in late summer. In intermediate and deep waters, the vertical distribution of Fe and Mn was characterised by a substantial constant profile until January; then, an increase of particulate metal concentration with depth was observed along with a corresponding decrease of the dissolved concentrations. This trend is typical of these scavenging-type elements which are removed by particulate matter during the sinking process, in the absence of well-defined water column stratification.