Trace contaminants in water-column and food-web of the Scheldt estuary

Abstract

Action plans for a better protection of the North Sea were presented at the successive North Sea Conferences. These plans included far-reaching undertakings to reduce aqueous discharges and atmospheric emissions of substances such as trace metals that are toxic, persistent and liable to bioaccumulate. For cadmium, lead and mercury the reduction targets were achieved, but an additional effort is necessary for copper and zinc. It is interesting to compare, for the various trace metals, the above mentioned emission reduction percentages with the immission reduction levels observed in the Scheldt estuary over the same period. The Scheldt estuary is not only a receiving water body: evacuation of pollutants to the sea, storage in the bottom sediments and depuration via efflux to the atmosphere all contribute to a reduction of the metal burden in the water column. All in- and outgoing flows have been put together in a box-model, and a mass balance for each box has been calculated. Only in 5 boxes on 30, the imbalance was larger than 20% with a highest value of 29%. For mercury, also the food-chain transfer in the Scheldt, including bioconcentration and bioaccumulation factors, has been studied. Despite the fact that Hg levels in the Scheldt estuary are much higher than in the North Sea, it appears that Hg concentrations in fish are even slightly lower in the Scheldt. A more striking observation is the very low MMHg fraction in Scheldt fish (average 57%), compared to that in North Sea fish (average 95%)