Bioavailability of heavy metals for the deposit feeder Macoma balthica with special emphasis on copper

In coastal and estuarine regions of Western Europe, concentrations of contaminants in the sediment are often relatively high compared to the waterphase. Consequently, these contaminants might form a serious threat to animals living in and associated with the sediment.As a biological monitor for sediment pollution, the benthic deposit feeding bivalve Macoma balthica could possibly be used as an alternative for the commonly used mussel Mytilus edulis. However, a problem with sediment monitoring is that many sediment associated species (including Macoma balthica) do not accumulate metals relative to the total concentrations measured in the sediment. Macoma balthica lives buried in the sediment. It feeds by picking up particles from the sediment surface with long movable siphons. Consequently, heavy metals can be accumulated from the sediment, but also from overlying and pore water and through food. There is still very little known about the relative importance of these uptake pathways of metals for Macoma balthica. The aim of this research was to assess the contribution of the major uptake routes of heavy metals to the body burdens in Macoma balthica.The importance of the different uptake pathways was assessed with a multi-level approach. Short-term laboratory experiments using the radiotracer 64Cu were carried out at the Interfaculty Reactor Institute in Delft. Long term accumulation studies were carried out with a flow-through system at the field station Jacobahaven of the Tidal Water Division. In addition, an intensive monitoring program in the field was carried out for assessment of the actual situation. With the uptake experiments, emphasis has been laid on the study of copper. In all experiments, environmentally realistic concentrations of metals were used.FoodTo study the role of food in Cu accumulation by bivalves, algae spiked with the radiotracer 64Cu can be used. With spiked algae however, redistribution of Cu between the dissolved and the particulate phase hampers the assessment of the contribution of food. In Chapter Two, a method is described to overcome this problem of redistribution. By adding excess EDTA to the seawater, the biological availability of dissolved Cu was minimized. The effectiveness of complexation by EDTA was controlled through adsorption on Macoma balthica shells and uptake in Macoma balthica tissue.In Chapter Three, this method was used to assess the availability of the radioisotope 64Cu from phytoplankton and water. As far as we know, this isotope has never been used before in marine food chain studies. As a model food source the marine diatom Phaeodactylum tricornutum was allowed to accumulate 64Cu for one day and fed to the clams. Excess EDTA was added to prevent uptake of dissolved 64Cu that could be leaking from the labelled diatoms. In control experiments, unlabelled diatoms were fed to M.balthica in the presence of dissolved 64Cu (with and without EDTA) in order to assure a similar filtration activity. In repeated experiments with varying particulate/dissolved copper ratios, uptake through food always turned out to be at least as efficient as uptake from the water. It was concluded that Cu, associated with food, is well available for uptake by Macoma balthica.Also in a flow-through system, experiments have been carried out with copper-spiked food (Chapter Six). When copper-spiked algae were added to the exposure water, Macoma accumulated significantly more copper. In literature, increased metal uptake with the presence of food is explained by an increased filtration activity. For this reason, filtration rates were measured during the experiment. Filtration rates in the algae-dosage were decreased, compared with the non-feeded situation. These results led again to the conclusion that foodassociated copper contributed significantly to the overall accumulation.Because it was demonstrated that metals in food could contribute to the overall accumulation by Macoma balthica, metal concentrations in benthic diatoms were assessed on several locations along the Westerschelde Estuary (Chapter Seven). By using the lens tissue technique, enabling diatoms to move upwards in plankton gauze sheets, we were able to collect epipelic diatoms without sediment particles. In highly polluted areas, the bioconcentration factor (metals in diatom/metals in sediment) in diatoms was less than 1, whereas in relatively clean areas, the bioconcentration factor was mostly higher than 1. Total metal concentrations were high enough (for copper about 12 microgram per gram dryweight) to contribute significantly to the tissue levels of Macoma, also if the food-associated metals are only partly bioavailable.SedimentIn semi-field experiments, the accumulation and behavioral effect of heavy metals from different sediment types on the benthic bivalve Macoma balthica were assessed (Chapter Five). Sediments with different grain size composition and organic carbon content were spiked with cadmium, copper and zinc and aged for five months, in order to reach equilibrium conditions that would be comparable to the field situation. The maximum metal concentrations in the spiked sediments were comparable with the worst case harbour sludge from Dutch estuarine regions. During the exposure, clean filtered seawater was running continuously over the sediment. The observed effects on burrowing behaviour, mortality and bioaccumulation were to a large extent related to sediment characteristics. The strongest effects and the highest bioaccumulation were observed in sediments with the lowest silt and clay fractions. In sediments with more than 50 % < 20 μm no effects on burrowing behaviour were observed, not even in the highest dosage. In this most polluted sediment tissue body burdens of metals did not reach lethal concentrations. The very low bioavailability of the metals can be explained by the reduced state of the sediment, causing metals to bind with sulphides. In the experiments, spiked aged sediments were much less toxic than freshly spiked sediments. From the results it was concluded that it is very important to pay close attention to experimental setup, so that the achieved data can be extrapolated to the natural situation in the field.WaterIf sediments actually contribute only for a minor part to the metal levels in M. balthica, then the overlying water could be relatively important. In Chapter Four, the radiotracer 64Cu was used to assess the influence of natural organic ligands on the bioavailability of dissolved copper. Biological availability of the 64Cu-complexes was measured by accumulation in the bivalve Macoma balthica. The experiments were carried out with water from the relatively clean Oosterschelde sea arm and the relatively polluted Westerschelde estuary. Adsorption onto shells as well as uptake in tissues was assessed at salinities of 10 per mill and 30 per mill. At a salinity of 10 per mill, uptake of 64Cu was increased, compared with 30 per mill. This increase was only slightly in Westerschelde water, but considerably in Oosterschelde water. Simultaneously with the exposure experiments, ligand characteristics of the natural waters were assessed by anodic and cathodic stripping voltammetry. High ligand concentrations, as occurring in the Westerschelde around February, reduced 64Cu (320 Nm) uptake by more than 50% compared with the Oosterschelde, in spite of the lower salinity. This implied that at high ambient ligand concentrations the influence of salinity on 64Cu uptake was less pronounced.Also in flow-through systems, copper accumulation was measured using low dissolved copper concentrations. At a dissolved copper concentration of 25 microgram per litre (about 10 times the concentration in the Westerschelde Estuary), the accumulated copper caused mortality within a few weeks. However, it was not possible to extrapolate these, or any other toxicity results to the field situation, because toxicity in the field depends largely on the copper complexing capacity of the water. With the copper concentrations, used in laboratory experiments, organic ligands are mostly saturated, whereas in natural situations this is generally not the case. Only if the copper complexing capacity i.e. the interaction of metals with organic matter in natural waters is well defined, statements can be made on copper toxicity in the field.ModelIn order to describe metal accumulation in Macoma balthica, a dynamic simulation model was developed (Chapter eight). The model is composed out of a 'growth' submodel, describing the variation in dry weight of M. balthica during the year, and a 'metal' submodel, describing the accumulation and elimination of cadmium and copper from the water (dissolved), food (particulate) and sediment. The 'growth' submodel was calibrated with data from a field survey. With this survey, animals and sediment were collected during 2 years from 5 locations on an intertidal mudflat, varying in sediment composition. Although Macoma was expected to thrive mainly on diatoms, a good fit for the growth submodel could only be achieved if Macoma balthica in the model was allowed to feed and grow on detritus, also if the nutritional value was low.Tissue metal concentrations depended partly on growth dilution, but more on the dissolved metal concentrations. Sediment composition influenced metal uptake in the sense that in sandier environments, animals spend more time suspension feeding which will increase the relative contribution of the overlying water. Both in the model and in the field situation, sediment metal concentrations contributed only partly (copper) or hardly (cadmium) to the overall metal uptake, except for accidental high metal concentrations on the sandy locations.How harmful are polluted sediments in an estuary like the Westerschelde?It was demonstrated that high concentrations of copper, cadmium and zinc in silty, organic rich sediments with only an oxidized top layer will cause little risk in undisturbed situations. In the Westerschelde estuary, pore water profiles show maxima of Cd, Cu and Zn near the sediment-water interface (Zwolsman, 1993). Macoma balthica hardly ventilates any pore water, so this also does not form any direct risk. However, the sediment and suspended matter act as a source of dissolved trace metals to the water column (Zwolsman, 1993), and in this way, the sediments might form a risk for Macoma and other benthic macrofauna. In spite of the low concentration of biologically available copper, uptake from the overlying water was very efficient. Considering the sensitivity of molluscs for copper, any increase in the bioavailable fraction could have serious consequences for mollusc populations in the Westerschelde. This increase is not to be expected from industrial or agricultural discharge. However, water sanitation measures in Brussels and Antwerp in the near future are expected to decrease the zone of anoxia in the eastern part of the estuary. Oxidation of metal sulphides, followed by complexation of the released metals with e.g. chloride, is an imaginable process to occur. Next to this, the concentration and nature of copper complexing ligands might change due to increased bacterial activity in the oxygenated zone. Both processes can cause a change in the concentration of dissolved bioavailable copper downstream, with possible ecotoxicological consequences.A lot of research on metal accumulation and toxicity has been carried out with commercial species like Mytilus edulis and Crassostrea gigas. From the present research, it has become clear that although deposit feeding bivalves do resemble other bivalves in many aspects, accumulation (and consequently toxicity) of pollutants is rather different, largely due to the difference in feeding behaviour. Macoma balthica has shown to react adequately on variations in metal levels in the environment and is very well able to accumulate and eliminate considerable amounts of trace metals, without showing obvious signs of stress. With respect to biological monitoring, Macoma could very well serve as an alternative for the commonly used biomonitor M. edulis in sandy habitats. Mytilus edulis is less suitable for sediment monitoring because the animals are not in direct contact with the sediment. The large salinity tolerance and wide distribution of Macoma balthica can be regarded as an extra advantage. However, when assessing the harmfulness of sediments using Macoma as a biological monitor, contaminant uptake from the overlying water or from food are factors have to be taken into account. If data on metal concentrations in the various compartments are available, the relative contribution of these can be estimated through a model simulation as presented in Chapter eight. In the near future, the biological availability of trace metals from the overlying water is expected to change as a result of oxidation processes in the eastern part of the estuary. Macoma would be a very suitable organism to measure the sensitivity of bivalve populations to these changing environmental conditions.