de Vries, MB

van der Meer, J

van Haren, RJF

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Hydrobiologia 195: 105-118, 1990.D. S. McLusky, V. N. de Jonge and J. Pomfret (eds), North Sea-Estuaries Interactions. 105© 1990 Kluwer Academic Publishers. Printed in Belgium.Cadmium and copper accumulation in the common mussel Mytilus edulisin the Western Scheldt estuary: a model approachR. J. F. van Haren'' 3, J. van der Meer"4 & M. B. de Vries2'Rijkswaterstaat, Tidal Waters Division, P. O. Box 20907, 2500 EX Den Haag, The Netherlands; 2DelftHydraulics, P. O. Box 177, 2600 MH Delft, The Netherlands; 3Present adress: Theoretical Biology, FreeUniversity, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands; 4Present adress: NetherlandsInstitute of Sea Research, P. O. Box 59, 1790 AB Den Burg, Texel, The NetherlandsKey words: Mytilus edulis, accumulation, mathematical model cadmium, copper, speciationAbstractThe Western Scheldt of the Dutch Delta area is severely contaminated with trace metals. Accumulationmodels of trace metals in the mussel Mytilus edulis are required to predict the biological efficiency ofreductions in the metal and organic matter load. Two models are constructed: a black-box model anda physiologically structured model. The black-box model predicts metal accumulation in mussels fromuptake and elimination parameters. The physiological model attempts to improve predictions by takinginto account the kinetics of individual uptake and elimination routes. These in turn, are taken asdepending upon two more general physiological processes, the ventilation rate and the metabolic rate.Metal uptake via food and water are expressed as relative fractions. Metal input is differentiated intoparticulate adsorbed, and dissolved species.The reliability of the two models is evaluated by comparing predicted concentrations for mussels withmeasurements. Model predictions for copper deviate less than 100% from measured concentrations, butneither model appears to predict cadmium concentration with sufficient accuracy since deviations of morethan 100% occured. The introduction of physiological refinements did not improve performance. Foodmediated contributions for cadmium and copper to total body burden had been overestimated in themodel by a factor of 100 when compared to literature values. The physiological model did predict thatthe ratio of food mediated contribution to total body burden is probably different for cadmium and copperand decreases with increasing salinity for both. As yet there are no measurements available to confirmsuch predictions.We conclude that additional laboratory experiments should be done for a better understanding ofwhy there is poor agreement between the few field observations and the simulations. In these experimentsmussels grown under different environmental condition can be tested for their accumulation capacityof trace metals. More field observations are needed.Introduction cadmium load at the Dutch-Belgian border was10 tons/year in 1981 and 6 tons/year in 1987. TheThe Western Scheldt estuary of the Dutch Delta copper load was 62 to 73 tons/year for 1981 andarea has severe contamination problems both for 1987. Consequently, cadmium concentrations intrace metals and organic micropollutants. The the common mussel Mytilus edulis in Western106Scheldt reached values of 874 nmol g adw-'(ashfree dry weight) in 1981 and 22 nmol g adw -in 1987. Copper concentrations obtained valuesof 346 nmol g ADW - ' in 1981 and 197 nmol gadw- ' in 1987. These concentrations are con-siderably higher than the background levels of5.78 nmol g adw- for cadmium and 122 nmol gadw- ' for copper derived from measurements in1981 at Killary Harbour near Leenaun in WesternIreland (DeKock & Marquenie, 1981). Thesevalues should be compared with effect concentra-tions of cadmium for mussels which approximate44 nmol g adw- ' (Marquenie & Vethaak, inpress) and to lethal concentrations of copper formussels which approximate 790-940 nmol gadw- ' (DeKock, 1986). It is evident that metals,and in particular cadmium, are cause for greatconcern.In 1987 the Dutch government started a projectto investigate the distribution and behaviour ofcontaminating substances in the estuary and pro-duce a Water Quality model. This model willderive results from a series of modules related towater and particle transport, to chemical distribu-tion and to bio-accumulation of metals. Thepresent paper concerns the development andevaluation of two bio-accumulaten models, whichdiffer according in the degree to which biologicalprocesses have been taken into account asvariables influencing uptake and elimination ofmetals. Both are evaluated, using timeseries fielddata for dissolved and adsorbed metal concen-tration as the input for the simulation of cadmiumand copper concentrations in mussels. The resultsare then compared to field measurements.Model assumptionsIn designing these models a number of as-sumptions were made about the routes of metaluptake in mussels and the nature of bio-availablemetal species in relation to specific uptake route.Uptake routeIn the mussel, the main metal species are thewater dissolved species, taken up via the gills andparticle adsorbed metals ingested with food.Views on the relative importance of these twouptake routes have changed in the literature withtime. Initially metal uptake with food was con-sidered the major source for body burden (Pen-treath, 1973; George, 1980), or of equal impor-tance to that of via the gills (Schulz-Baldes, 1974).Recent work however has pointed to metal uptakevia the gills as the major contributor to bodyburden (Kohler & Riisgard, 1982; Borchardt,1983; King & Davies, 1987 and Riisgard et al.,1987).Bio-available trace metal speciesThe nature of the bio-available species will de-pend on the mechanism of uptake (George, 1980;Baudo, 1982; Luoma, 1983). Available data differaccording to, whether metal uptake via the gills isconsidered to occur through passive diffusion(George, 1980; Carpene & George, 1981;Simkiss, 1983; Langston & Bryan, 1984;Viarengo etal., 1987) or to active, carriermediated transport (Janssen & Scholz, 1979).Since ATP-inhibitors appear unable to affect cad-mium uptake via the gills (Carpene & George,1981), the assumption of passive diffusion seemsthe more likely one. In this case, the main metaluptake species to consider, should be the smallionized froms, the chlorides and the hydroxides,since in studies on solubility of metals in artificialbilayers solubility is associated with the above(Simkiss, 1983). This fits with the finding thatincreasing salinity (chlorinity), which is asso-ciated with decreasing ionization of cadmium,does decrease its toxicity (Engel & Fowler, 1979).The outcome of experiments in which NTA (nitri-lotriacetic acid) was used to replace chloride asthe metal complexing agent suggested the presentsurvival of the grass shrimp Palaemonetespugio isa function of free cadmium ion and not of totalcadmium concentration (Sunda et al., 1978;Salomons & Frstner, 1984. The toxicity andbioavailability of copper has also been associatedwith the ionized species (Engel et al., 1981).Particulate matterAll previous arguments support the assumptionthat for the gills, the bio-available fraction of

Cadmium and copper accumulation in the common mussel Mytilus edulis in the Western Scheldt estuary: a model approach

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Cadmium and copper accumulation in the common mussel Mytilus edulis in the Western Scheldt estuary: a model approach

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