Heavy metal content in Danubian fish
Toxic metals are entering the river water with discharges, rainwashes and atmospheric input. In the river system they are mostly bound in suspended matter and fine sediments and settle as a severe risk potential in the depths of dammed-up water. While in Western Europe there are strict statutory requirements for improving the efficiency of water treatment plants, in particular by purification with the best available techniques, the middle and lower reaches of the river Danube continue to be contaminated with untreated effluents from towns and industries. A major role in terms of danger to the ecosystem is played by heavy metals, which stand out by their persistence and accumulability. In this report an overview on recent monitoring of the contamination of Danube fish with heavy metals is presented. The heavy metal enrichment in fish is described on the basis of corresponding investigations over the whole length of the river Danube during the period from 1980 to 1989. Basically, a significant change in the metal content of the fish could be gradually observed downstream. Additionally, increased local concentrations occurred in the middle and lower reaches of the river Danube in the vicinity of conurbations over relatively short stretches, due to the influence of major industrial centres. In the system of contamination classes, elaborated by Wachs (1991-1998), for the grading of the situation of ecosystems in terms of heavy metal content, of all the various compartments, fish constitute the only critical part. The fish organs - in particular the muscles - show a very wide spread of concentrations. Therefore for assessing the danger potential of heavy metals in the ecosystem of streams and rivers according to the author's seven-stage classification system by the contamination of the muscles, which is possible in running waters today, it is also necessary to take account of the average concentration ranges of the liver, kidney and spleen. On this basis, i. e. with the data for several fish organs, it is possible to obtain a more reliable classification for the contamination of a river stretch. Compared to the other compartments of the hydrobionts (zoobenthos, mosses, algae, macrophytes), fish do not play a sensitive role for the indication of metal contamination in rivers. At the end of the eighties, the average levels of metal contamination in fish of the upper reaches of the Danube corresponded predominantly to contamination class II-III or III. The accumulation still present in the sediments of the reservoirs and the continuous remobilization of the metals, in spite of decreasing water contamination levels, also at present, lead now and again to a permanent contamination of fish, which must in addition to the classes II and II-III also be assigned to the class III. Compared with the German and Austrian sections, the middle reaches of the Danube exhibit an increase in metal contamination levels which in part become gradually less the further downstream. The greatest part of the fish analysis data obtained by the author in 1988 was predominantly contamination class III. In the lower reaches the main tributaries carried along high concentrations of heavy metals. There the metal contents of fish corresponded primarily also to class III, and further downstream to class III-IV in the period around 1988. Solely nickel and lead could be allocated downstream to a more favourable class (III). Generally the danger of water contamination grews by looking downstream and dominates in the Ukrainian region of the delta. Therefore heavy metal accumulation in Danube fish is a problem particularly in this part of the river. In the middle of the nineties the Danube contaminations, caused by the greater part of metals, have been reduced, in particular zinc.