Water mixing and biogeochemical cycles which balance input and output of elements help maintain the composition of seawater throughout the planet rather constant. Although the Southern Ocean is not separated from other oceans by continental masses, its waters have distinctive physico-chemical features. The circulation pattern around the continent, which is covered by ice and lacks river drainage systems, and the seasonal distribution of sea ice and primary productivity are among the factors determining the distribution and different concentrations of trace elements in Antarctic waters.
Trace elements have different structural and thermodynamic characteristics which give them a wide range of chemical properties and affect their environmental fate (i.e. uptake by organisms, adsorption and/or co-precipitation with particulate matter). The distribution of different chemical forms in the water column must be known in order to understand the biogeochemical cycling of trace elements. This is a difficult task because seawater, like snow, contains very low trace element concentrations, especially when considering single chemical species. As it is very difficult to avoid contamination during water sampling and analysis, the reliability of many early published data is questionable. Improved techniques for collection, handling and analysis of seawater samples introduced in the 1970s improved the quality of results, and average trace metal concentrations reported for seawater decreased significantly with respect to earlier results (e.g. Patterson 1974).
Very few data on the chemistry of Southern Ocean waters were available before the 1990s. However, earlier studies (e.g. Boyle and Edmond 1975; Boyle et al. 1976; Harris and Fabris 1979; Orren and Monteiro 1985; Bordin et al. 1987) found that, in contrast to oligotrophic surface waters in other oceans, those in the Southern Ocean were often not depleted in nutrients. As discussed in Chapter 3, regions of intense blooms (>3 g C m-3 day1; 5-10 |g chl l-1) in Southern Ocean waters only occur in shallow shelf areas, near receding pack-ice margins and upwelling regions along the Antarctic Polar Front Zone (APFZ; de Baar et al. 1995; Arrigo et al. 1999). Southern Ocean offshore waters, along with North Pacific and equatorial Pacific waters, constitute a major ocean region with high concentrations of macronutrients, low productivity rates and low biomass (0.1 g C m-3 day-1; 0.1-0.2 |g chl l-1). These paradoxical regions (comprising about 25 % of the World Ocean; de Baar et al. 1999) have long been investigated by oceanographers (e.g. Gran 1931). As a rule, besides the usual limiting factors for primary productivity, such as light, grazing rate and water-column stability, the low bioavailability of some essential micronutrients such as Fe was considered a possible explanation for the so-called HNLC waters (Hart 1934). During the last 15 years, HNLC regions have been the focus of large research efforts. After laboratory culture and/or shipboard Fe enrichments (e.g. Martin and Fitzwater 1988;
Morel et al. 1991), several large-scale "in-situ" fertilisation experiments were performed in the equatorial Pacific (e.g. Martin et al. 1994) and Southern Ocean (e.g. Boyd et al. 2000). These experiments essentially show that the lack of Fe and/or co-limitation by Si are responsible for offshore HNLC areas. The addition of Fe induces algal blooms, draw-down in macronutrients (in ratios consistent with the growth of phytoplankton), and shifts in algal community structure. Comparisons between iron-mediated responses in polar and tropical areas (Boyd 2002) show that main differences are essentially due to the temperature-dependence of biological processes.
Studies in different oceans (e.g. Bruland 1980; Danielsson et al. 1985) show that uptake by phytoplankton, assimilation and recycling by zooplankton and scavenging generally determine an increase in concentrations of major nutrients and trace elements such as Cd, Cu, Ni and Zn with depth (i.e. with increasing age of the water). In the Southern Ocean, bioutilised trace metals such as Zn, Cu and Ni do not usually show co-limitation with Fe and, under Fe-depleted conditions, their concentrations in waters are not decreased by algal uptake as expected (Frew et al. 2001). This chapter discusses the distribution and cycling of these trace elements, particularly of metals of environmental and toxicological concern such as Cd, Pb and Hg. The reader interested in general aspects of trace element biogeochemistry in the marine environment can refer to specific books such as those by Riley and Chester (1983), Libes (1992), Bidoglio and Stumm (1994), Salbu and Steiness (1995) and Stumm and Morgan (1996).
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