Stable pollutants in the eastern part of the North Sea

The spatial distributions of most dissolved and particulate trace metals are, at least qualitatively, similar. Their concentrations decrease with distance from the coast and the mouth of the Scheldt estuary (particulates much stronger than dissolved species). The salinity gradient is similar (but opposite) to the trace metals and turbidity gradients. The five trace metals studied can be subdivided into two groups on the basis of their KD's (ratio of particulate metal concentration to dissolved metal concentration). Hg and Pb are strongly associated with particulate matter (KD from 0.5 to 1.5 offshore and from 7.9 to 40 in the coastal area), while Cd, Cu and Zn are predominantly in the dissolved phase (KD from 0.1 to 0.2 offshore - up to 0.5 for Zn - and from 1 to 1.7 in the coastal area). Offshore fluxes of dissolved Cd, Cu, Hg, Pb and Zn are calculated based on diffusive and mixing processes .They can explain the concentration increases in the coastal flow tube (315 km3.y-1), but are insignificant beyond that flow tube. The ratios of the Scheldt output to the offshore flux vary from 38 to 85%, depending on the kind of metal. The ratios of the Scheldt output to the metal flow parallel to the coast is smaller than 1%. The steady-state concentration profiles of particulate metals versus salinity are fairly constant in the coastal-estuarine and marine water masses, but decrease very abruptly from the first to the second water mass. Assuming a conservative behaviour of the particulate metals, offshore fluxes and the resulting concentration increases in the coastal flow tube, were deduced. These calculated concentration increases agree fairly well with the observed values. The contribution of the Scheldt estuary to the flows parallel to the coast ranges from 1.6 to 3.3%. Biological productivity in coastal waters is often very high owing to the high levels of nutrients supplied via coastal inputs or through upwelling. In Belgian coastal waters for example the daily primary production ranges from 10 to 280 mg N.m-2 day-1 corresponding to an annual primary production of about 25 g N.m-2 y-1 (Baeyens et al. 1983, Mommaerts et al. 1984, Baeyens et al. 1984). Shellfish and fish thus find there excellent spawning, nursing and feeding grounds. Coastal ecosystems are very sensitive to pollution, which can perturb severely the biological processes. Heavy metals are amongst the major potential pollutants because of their toxicity. Obtaining reliable, accurate trace metal data in, seawater is still a cumbersome operation. In order to evaluate our analyzing procedures, we participated at several intercalibration exercises: (1) the intercalibration exercises for the analysis of cadmium and mercury in sea water requested by the Joint Monitoring Group of the Oslo and Paris Commissions and conducted by ICES. The results have been reported by Thibaud (1981) for cadmium and 01afsson (1981) for mercury; (2) the intercalibration exercise of trace metals in sediments conducted by the "Centre Oceanologique de Bretagne, CNEXO" .The results have been reported by Joanny et al (1980); (3) for the analysis of mercury in botanical substrates (BCR reference materials), we compared several mineralization-digestion methods. Results have been reported by Dehairs et al. (1982). Participation at these intercalibration exercises offered us the possibility to demonstrate that the analyzing procedures in use in our laboratories for Cd, Cu, Hg, Pb and Zn produce reliable data, at least on samples similar to those that served for the intercalibration exercises. Another encouraging fact is that our trace metal data obtained in the English Channel (Baeyens et al., 1987), the Southern Bight of the North Sea (see Table 1), the northern North Sea (Baeyens et al., 1987) and the Mediterranean Sea (our data are given in the section “Sampling of heavy metals”, this paper; for example Mart et al., 1982 report data for the Western Mediterranean Sea) compare favourably with these obtained by other investigators in the same areas. Analytical techniques and methods are described in detail in Baeyens et al. (1987)