Twentieth century overview of heavy metals in the Galician Rias (NW Iberian Peninsula)
Introduction
Knowledge of trace metal distribution in marine media—water and sediments—has been traditionally poor mainly due to difficulty in obtaining accurate results (Olafsson, 1983). However, thanks to the perfection of sampling techniques, storage, handling of samples and analytical techniques—avoiding sample contamination (Moody and Beary, 1982)—and also the availability of highly sensitive analytical instruments (Howard and Statham, 1993, Bruland, 1983), the amount and quality of work published in this field has increased globally. In the Galician Rias, the emergence of research papers on trace metals in the coastal environment reached continuity from 1993, with an increase in the number of researchers dedicated to this field. However, in contrast to other areas this research is uncoordinated both in study areas and methodology mainly due to the lack of review papers like the ones available for other coastal areas (Bryan and Langston, 1992, Cutter, 1991).
Metals are natural constituents of the environment. Copper and zinc, for example, are oligoelements whose presence in small amounts in nature is essential for living organisms. Other metals, in contrast, do not have any known biological role, as is the case for mercury and cadmium. At high concentrations trace metals can become toxic for living organisms and behave as conservative pollutants. Therefore, human beings located at the top of the trophic chain are especially sensitive to these contaminants due to bioaccumulation. This possibility provides a further reason for compiling the available information in the heavy metal field.
Metals enter the environment and oceans by two means: (1) natural processes (including erosion of ore-bearing rocks, wind-blown dust, volcanic activity and forest fires); and (2) processes derived form human activities by means of atmospheric deposition, rivers, direct discharges or dumping (Clark, 2001). For some metals, natural and anthropogenic inputs are of the same order (for example Hg and Cd), whereas for others (for example Pb) inputs due to human activities dwarf natural inputs (Clark, 2001). Human activity is mainly located in the fluvial watersheds and in the rias and estuarine margins. These areas are therefore important for the fate of contaminants due to coastal industrial activity and human settlement. The Galician Rias are sensitive to anthropogenic influence due to the socioeconomic importance generated in these areas by industry, aquaculture, port activities, fishing, tourism, and recently designated natural parks. Aquaculture in the Galician Rias is a growth industry and in 2000 the rias supported 3386 mussel rafts producing 2.5×108 kg year−1, i.e. 40% of European Union total seafood production. Moreover, 107 kg year−1 of other bivalves and 3×106 kg year−1 turbot are also obtained from mariculture.
As observed in other estuaries (Helland, 2001, Balci, 1999, Millward and Glegg, 1997) the Galician Rias are assumed to behave as metallic deposition frontiers. This is another reason to compile all the existing knowledge of heavy metals in the 18 rias (Fig. 1) which penetrate the Galician coastline (1720 km).
The rias receive the main freshwater input at their head, being classified as partially stratified estuaries with positive residual circulation (Prego, 1990). They are mesotidal estuaries with a tidal range between 2 and 4 m. The river contributions vary from the north to the south of Cape Finisterre. The first presents smaller basins and are subjected to an oceanic climate, with flows between 10 and 30 m3 s−1 during the wet season and below 5 m3 s−1 in the dry season. The second group, with a Mediterranean climate and the largest basin surface areas, vary between 40 and 180 m3 s−1 and lower than 20 m3 s−1 in the wet and dry seasons, respectively. Thus, the annual freshwater contribution to the rias has been estimated as 245 and 780 m3 s−1 (Vergara and Prego, 1997). Another physical influence on the coastal zone are upwelling events. From May to September, a sub-surface upwelling of Eastern North-Atlantic Central Water (ENACW) is normal and most intense near Finisterre Cape (Fraga, 1981). South of Finisterre the upwelling is more intense and closer to the coast than in the north, where upwelling is discontinuous and nearer to the edge of the continental shelf (Prego and Bao, 1997). The upwelling affects the ria residual circulation (Prego and Fraga, 1992), with a reinforcement during the dry season. Thus the ria–ocean exchange may be half of the wet season estimation.
Section snippets
Heavy metals in the water column
The studies carried out in the Galician rias on heavy metals in the water column are resumed in Table 1 (Cd, Cu, Fe, Ni, Pb and Zn) and Table 2 (Sn). Also indicated are the sampling area, samplings and stations, analytical technique employed and reported trace metal concentration range. There is a need to highlight the lack of data available in this compartment, as observed from the few papers addressing the water column and almost no data published in the mainstream literature. Most of the
Heavy metals in ria sediments
The studies carried out in the Galician Rias on heavy metals in the sediment are focused on the surface sediment and resumed in Table 4a and b. Also indicated are the sampling areas, number of stations, sediment fraction, analytical technique and metal concentration range found. This compartment has been more vigorously studied than the water column. The published works are recent, from 1988 onward, although there was previously a contamination control study in some of the rias and
Heavy metals in biota
Together with the analysis of water and sediments, one of the basic approaches to evaluate the marine pollution is the biomonitoring, which allows the status of metal contamination to be identified and quantified by means of the analysis of biota (Widdows et al., 1995). An organism must have several characteristics in order to be used as a bioindicator (Philips, 1980), and these are: (1) wide geographical distribution; (2) reasonably abundant and available throughout the year; (3) tolerant of
Cadmium
Its average concentration in the earth's crust is 0.1 μg g−1, and erosion has led to a 2–3 fold sediment enrichment (Förstner, 1980). The average Cd concentration in unpolluted sediments ranges from 0.04 to 0.8 μg g−1, whereas in contaminated zones values can reach 30–400 μg g−1 (Förstner, 1980, Förstner, 1986, Breder, 1988). Dissolved cadmium concentrations in unpolluted river and coastal waters are below 1 nM (see Table 3 and references cited therein).
In the sediments of the rias, cadmium is
Conclusions and future research needs
The presence and impact of heavy metals in the environment, especially in marine ecosystems from anthropogenic contamination, essentially requires (1) descriptive understanding and monitoring, (2) research on transport fluxes and biogeochemical processes and (3) the impact of those metals on organisms. With regards to the myriad of different coastal systems, there is little available information for systems characterized by head river flows lower than 100 m3 s−1 and also for ria-type estuaries
Acknowledgements
A Cobelo-Garcı́a would like to thank the Spanish Ministry of Science and Technology for financial support (FPI grant). This work was supported by FEDER and CICYT under the projects Biogeochemical Processes in the Ria de Ferrol, reference 1FD97-0479-C03-02, and Hydrodinamic and Hydrochemistry of River Anllóns—Rı́a de Laxe System, reference HID99–0699. We would like to thank AW Dale for assistance with the English translation.
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