Marine Pollution Bulletin, Volume 24, No. 5, pp. 250-258, 1992. Printed in Great Britain. Marine Pollution Bulletin 0025-326X/92 $5.00+0.00 © 1992 Pergamon Press Ltd Trends in Pollutants in Blue Mussel Mytilus edulis and Flounder Platichthys flesus from two Dutch estuaries, 1985-1990 J. STRONKHORST Ministry of Transport, Public Works and Water Management, Tidal Waters Division, P.O. Box 8039, 4330 EA Middelburg, The Netherlands Temporal trends and length-concentration relation- ships are presented for data from the Joint Monitoring Programme for microcontaminants in the blue mussel Mytilus edulis and flounder Platichthys flesus from the Eros Dollard and Western Scheldt estuaries in the Netherlands over the period 1985 to 1990. Statistical analyses were carried out in conformity with ICES procedures. The analyses show only a decrease in levels of dieldrin in mussels from the Western Scheldt. Cadmium concentrations in flounders from the Ems Dollard have increased. No linear trend has been detected in either estuary for the other contaminants (PCB, PAH, pp'DDE, lindane, Hg, As, Cu, Cr, Zn, Pb, Ni). The importance of taking the fat weight into account in anal- ysing PCB trend and length relationship is illustrated. Concentrations of PCB, PAH, chromium and nickel in mussels and PCB in livers of flounders decrease with increasing size. Mercury concentrations in flounders increase with length. In this paper a statistical analysis of temporal trends is presented for the data of microcontaminants in the blue mussel Mytilus edulis and flounder Platichthys flesus from two Dutch estuaries over the period 1985 to 1990. These data have been collected as the Dutch contri- bution to the Joint Monitoring Programme (JMP) of the Oslo and Paris Commissions (e.g. Akkerman & Van Zwol, 1990). The North Sea Conferences (London, 1987; The Hague, 1990) produced international agreement on reduction of micropollutant loads in the period 1985- 95. In fact the ultimate goal is the reduction of con- taminant levels in the environment, especially in organ- isms. For two fringe areas of the North Sea the trends presented here may serve as one of the interim assess- ment of these goals. General conclusions on environ- mental contaminant levels in the two estuaries can only be drawn with knowledge of chemical processes within the estuarine environment, physiological and biological processes within the organisms and information on trends in other compartments (Misra et al., 1989; ICES, in prep.). To improve the reliability of indentified trends samples were taken in a length-stratified manner because this biological parameter might affect the concentration of microcontaminants (Moriarty et al., 1984). For this reason length-concentration relation- ships are discussed in this paper. A correlation between concentration and length may point to weight- or age- related uptake processes. Materials and Methods Samples of natural mussels and flounders were collected from the Ems Dollard and the Western Scheldt (Fig. 1) each year in October over the period 1985-90. The water quality in the Western Scheldt is mainly determined by the input from the river Scheldt and in the Ems Dollard by industrial activities along the estuary. In the Western Scheldt mussels were sampled in the middle section and flounders in the mouth of the estuary. In the Ems Dollard both species were collected in the mouth of the estuary. At the sampling site for mussels the salinity in the Ems Dollard is higher than in the Western Scheldt (approximately 29%o and 24%0 respectively). Mussels were sampled in five logarithmically equi- distant shell length classes: 25-30 mm, 31-37 mm, 38- 46 mm, 47-56 mm, and 57-70 mm. Each class contains approximately 50 mussels which are pooled prior to analysis. Flounders were sampled in five logarithmically equidistant length classes: 200-225 mm, 225-250 mm, 250-280 mm, 280-315 mm, and 315-350 mm, each containing five organisms. All flounders were analysed individually. In 1985 the programme on flounders in the Ems Dollard was not completed, because all length classes were not rep- 250