HEAVY METAL TRANSFER IN TERRESTRIAL ECOSYSTEMS

Heavy metal transfer in terrestrial ecosystems

Nicolas Bogaert, Gijs du Laing, Filip Tack & Marc Verloo (Laboratory of Analytical Chemistry and Applied Ecochemistry, Gent University, Coupure Links 653, B-9000 Gent, Belgium)

Frederik Hendrickx, Jean-Pierre Maelfait & Johan Mertens (Laboratory of Animal Ecology, Gent University, K.L. Ledeganckstraat 35, B-9000 Gent, Belgium)

Corresponding author: Nicolas.Bogaert@rug.ac.be

Abstract

To explain the bioavailability and uptake of metals by organisms in wetlands, physico-chemical characteristics, total metal contents and fractionation patterns of surface sediments of tidal wetlands along the Scheldt estuary (Belgium) were related to metal contents of inhabiting wolfspiders (Pirata piraticus). The BCR sequential extraction procedure was applied to differentiate between reactive metal pools. No important differences were apparent in metal fractionation between salt- and freshwater sediments, nor between seasons. Metal contents in wolfspiders seemed unrelated to metal fractions as determined by the BCR sequential extraction procedure. Strong correlations however were established between spider metal concentrations on one hand and the exchangeable cations to total exchange capacity ratio and sediment chloride contents on the other hand.

Introduction

Wetlands can be seriously destabilised by deposition and accumulation of heavy metals in sediments. Metals may bioaccumulate in living organisms and be transferred into the foodchain. Predators of contaminated organisms can be chronically or acutely intoxicated (Crommentuijn et al., 1995), and this may lead to drastic changes in species composition and biodiversity in contaminated areas. Based on the knowledge about metal transfer from the soil through the food chain, new and more relevant tolerance limits can be drafted for contaminated ecosystems and vulnerable and tolerant links of the food chain can be identified (Butovsky and Van Straalen, 1995).

Total soil metal concentrations indicate the degree of pollution, but do not provide information about bioavailability and toxicity with respect to specific biotic components. These are determined among other factors by the specific physicochemical form of the metals. Often sequential extraction procedures are applied to differentiate between reactive metal pools in an attempt to explain the bioavailability and the uptake of metals by organisms living in the affected soils or sediments (Ma and Rao, 1997).

Many Flemish rivers and their sediments are more or less contaminated with heavy metals. Due to tidal flow, natural systems of salt, fresh and brackish water are found along the shores of the river Scheldt. Total metal contents and fractionation patterns in the sediments of these ecosystems will be determined and related to metal transfer in wolfspider. Wolfspider is a frequently occurring soil dwelling predator on saltings and in turn is a source of food for many vertebrates such as birds, mammals and amfibians. As bio-accumulators they can play an important role in the transfer of heavy metals into the foodchain.

methods

Eight surface sediments (0-1 cm) of reed dominated tidal wetlands were sampled at different intervals along the Sea-Scheldt estuary over a distance of 78 km. A reference site was chosen outside the estuary. On the same sites, wolfspider (Pirata piraticus) was sampled by hand picking. Sampling occurred during winter, spring and summer.

After digestion with aqua regia, Cd, Cu, Pb and Zn were analysed by atomic absorption spectrometry. Metal fractionation was performed on air-dried and ground samples according to the three step sequential extraction procedure of BCR (Quevauviller et al., 1994). The material remaining after extraction was further digested with aqua regia to obtain a pseudototal metal content.

Results and discussion

Zn

Pb

Cu

Cd

Sediment total metal contents

Fig. 1. Mean surficial total metal contents (mg kg^-1 DM) and standard deviations in saltings along the Scheldt estuary (Distance to mouth = 0 represents the reference site)

Metal levels were all elevated, compared to the reference site (Fig. 1). Reference site concentrations were consistent with background levels, except for Pb, due to the presence of a highway.

Levels of all metals significantly varied with sampling location and increased by a factor 2-3 over a distance of 78 km inland. This is in accordance with the water quality of the river Scheldt, which improves in seaward direction due to dilution with relatively uncontaminated seawater and marine sediment.

According to a variance analysis, seasonal differences were dependent of sampling location.

BCR-fractionation

The bioavailability of metals in sediments is thought to decrease approximately in the order of the extraction sequence, from readily available to unavailable, because the strength of used extraction reagents increases with the sequence. In agreement with other observations (Tack and Verloo, 1996), Cd and Zn followed a similar distribution. About 80-90% was isolated in the exchangeable and acid-extractable fractions. Copper and Pb are known to be more immobile metals in sediments and this is reflected in their fractionation. Less than 5% of the total Cu and Pb contents was associated with the exchangeable and acid-extractable fractions. Recoveries for all metals fairly well approximated 100%.

No important differences were apparent in metal fractionation between brackish and freshwater sediments.

Metal concentrations in spiders

Fig. 2 shows metal concentrations in wolfspider sampled during summer on 3 sediments along the Scheldt and on the reference site.

Fig. 2. Mean spider total metal concentrations (mg kg^-1 DM) and standard deviations (Distance to mouth = 0 represents the reference site)

While total sediment metal contents decreased in seaward direction, spider metal concentrations followed the opposite trend and decrease inland.

According to correlation analysis total sediment metal contents were not related to bioavailability. Neither could a relationship be established between concentrations of different fractions, as accomplished by BCR sequential extraction, and spider metal concentrations. Even EDTA-extractable contents seemed unrelated to metal contents in wolfspiders. So far no sediment metal fraction could be separated that would indicate bioavailability towards the wolfspider P. piraticus.

Strong correlations however were established between spider metal concentrations and the exchangeable cations to total exchange capacity ratio. These correlations were strongest for Cd and Zn, both metals that were predominantly present in the exchangeable fraction. Even so between sediment chloride contents and spider metal concentrations strong correlations were found. Formation of chloride complexes could play an important role in the bioavailability of these metals.

Based on these two factors a simple model was set up, describing the relation between metal contents in Pirata piraticus on one hand and chloride contents and the exchangeable cations to total exchange capacity ratio on the other hand.

References

Butovsky, R.O. and Van Straalen, N.M. (1995). Copper and zinc contents in trophic chains of terrestrial arthropods in the Moscow region. Pedobiologia 39, 481-487.

Crommentuijn, T., Doodeman, C.J.A.M., Van Der Pol, J.J.C., Doornekamp, A., Rademaker, M.C.J. and Van Gestel, C.A.M. (1995). Sublethal sensitivity index as an ecotoxicity parameter measuring energy allocation under toxicant stress: application to cadmium in soil arthropods. Ecotoxicology and Environmental Safety 31, 192-200.

Ma, L.Q. and Rao, G.N. (1997). Chemical fractionation of cadmium, copper, nickel and zinc in contaminated soils. Journal of Environmental Quality 26, 259-264.

Quevauviller, P., Rauret, G., Muntau, H., Ure, A.M., Rubio, R., Lopez-Sanchez, J.F., Fiedler, H.D. and Griepink, B. (1994). Evaluation of a sequential extraction procedure for the determination of extractable trace metal contents in sediments. Fresenius Journal of Analytical Chemistry 349, 808-814.