Copper and Zinc Content of Periphyton from Two Rivers As a Function of Dissolved Metal Concentration
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Aquat. Sci. 64 (2002) 300-306 1015-1621/02/030300-07 © EAWAG, Diibendorf, 2002 IAquatic Sciences Research Article Copper and zinc content of periphyton from two rivers as a function of dissolved metal concentration Renata Behra *, Rolf Landwehrjoha nn, Katrin Vogel, Bettina Wagner and Laura Sigg Swiss Federal Institute of Environmental Science and Technology (EAWAG), Oberlandstrasse 133, CH-8600 Diibendorf, Switzerland Received: 26 November 2001; revised manuscript accepted: 13 July 2002 Abstract. Measuring the metal content of periphyton is a from two sites of the Thur River did not reflect the dis- means of evaluating the state of metal contamination in solved metal concentrations in the water, probably due to the environment with respect to levels of metals and their speciation effects. Nonexchangeable (intracellular) metal bioavailability. The aim of this study was to identify rela- determined after washing of periphyton samples with tionships between the dissolved metal concentrations and EDTA was variable and unpredictable when considering the total or intracellular metal concentrations in the peri- total metal content only. The different relationships in the phyton from two rivers. Metal levels at non-contaminated two rivers between the metal contents in periphyton and sites were comparable in both rivers (0.07-0. 71 µmol Cu the dissolved concentrations indicate the influence of g-1 dw and 0.22-4.36 µmol Zn g-1 dw). The metal content speciation on adsorption and bioavailability. Results are of periphyton from three sites of the Birs River was gen- discussed in respect to the significance of metal content erally reflective of the dissolved Cu and Zn concentra- as a tool for evaluating metal bioavailability. tions in the water. In contrast, metal content in periphyton Key words. Copper; zinc; bioavailability; periphyton; EDTA non-exchangeable metal. Introduction metal available for uptake by organisms, and hence their effects upon them. Contamination of the aquatic environment by heavy met- Studies on the metal content of periphyton (benthic als occurs as the result of various human activities. Water algal assemblages) also are being used to evaluate aquatic quality criteria are used for regulating environmental metal contamination and metal bioavailability. This ap- metal concentrations in order to protect living systems proach is based on the rationale that metal bioavailability from adverse effects of elevated ambient metal concen- affects the metal concentration adsorbed to and accumu- trations. Appropriate management tools are needed to lated in the organisms making up the community. Labo- provide information on metal concentrations, and their ratory studies have demonstrated that the bioavailability changes, in the environment. Usually chemical analyses of metals to algae is determined by the free metal ion ac- ofwater or sediment are employed to evaluate the state of tivity (Sunda and Guillard, 1976; Anderson et al., 1978). aquatic systems with respect to metal concentrations. Yet, In contrast to these controlled laboratory studies, studies this information does not give an indication on the vari- on periphyton have rarely examined the relationships ous forms of metals occurring in waters or the fraction of among metal content in benthic algal communities, the water concentration of the metals (Ramelow et al., 1987; Whitton et al., 1989; Gupta, 1996) and metal speciation. * Corresponding author phone: +41 1 823 5119; fax: +41 1 823 5315; e-mail: [email protected] Most studies only report on the metal content and the de- Published on Web: October 17, 2002 rived accumulation data are used directly to infer envi- Aquat. Sci. Vol. 64, 2002 Research Article 301 ronmental changes in metal concentrations. Moreover, Thur River, has background levels of metals. The “An- metal content measurements often rely on a single sam- delfingen” site at km 125 is a contaminated site affected ple despite the fact that data are subject to temporal vari- by agricultural and sewage inputs. Both sites are similar ability. Factors that may be important include the metal with respect to the geochemical background, consisting speciation in the water column, the periphyton composi- mostly of carbonate rocks. tion in terms of algal species and the quantity of metal ox- ides associated with the periphyton mat. These factors are all subject to temporal dynamics in the environment. The Sampling and analytical methods appropriateness of periphyton as an indicator of bioavail- Sampling of periphyton, sediments and water was coor- ability of metals thus depends on an understanding of the dinated. The Birs sites were sampled 3–5 times for deter- variability of the metal content in communities from un- mination of intracellular metal (see below) over nine polluted sites. Before using metal content data for man- months starting in June 1996. The Thur sites were sam- agement purposes, relationships between content and pled 4 times over six months starting in November 1998. metal concentrations should be made clear and variabil- The river bed of the Birs River was covered with stones. ity should be quantified. This is particularly important at Fine sediments were only present as a thin layer on the metal concentrations lying only slightly above corre- stones and were collected as a slurry. The slurry of sur- sponding water quality criteria limits. Previous studies on face sediments was collected from the Birs River bed by the long-term effects of metals on periphyton did show sucking with a hand pump producing a vacuum in a flask the sensitivity of some algal species towards very low connected to teflon tubing. This sediment suspension was metal concentrations (Gustavson and Wängberg, 1995; collected in 2-L polyethylene bottles. In the laboratory, Soldo and Behra, 2000). the sediments were allowed to settle, and then sieved se- In this study, we have measured the temporal and spa- quentially through 122 μm- and 63 μm-mesh nets. The tial distribution of copper and zinc in two rivers moder- fraction <63 μm was freeze-dried and stored for analysis. ately polluted by metals. Our aim was to quantitatively Aliquots (40–50 mg) were digested with concentrated evaluate the relationship between ambient dissolved HNO3 (4 mL) and H2O2 (1 mL) in a microwave digestion metal concentrations and the metal content of periphyton system (Microwave Laboratory Systems, mls 1200 by collecting water and periphyton simultaneously. mega). The digested samples were diluted to 50 mL un- Moreover, we examined and compared intra-site variabil- der a clean bench. The concentrations of Zn and Cu were ity of metal content in periphyton taken from stone sur- measured by ICP-OES (Perkin-Elmer Elan 5000). The faces and from synthetic glass substrates. The results are accuracy of the metal measurements in sediment samples discussed in respect to the utility of periphyton for show- was checked regularly using a reference sediment (NBS ing patterns of metal contamination in rivers. Buffalo River sediment 2704). Water samples for metal analysis were collected in clean, acid-washed polypropylene bottles and placed in Materials and methods plastic bags in the field. Plastic gloves were worn for sample handling. Samples were collected by hand from Study sites the Birs River and using a peristaltic pump from the Thur. The Birs River (Switzerland) flows over a length of 73 km Before measurement, samples from the Birs were filtered from the Jura region to the Rhine River through a catch- under a clean bench using an acid-cleaned polysulfone ment dominated by carbonate rocks. Its alkalinity ranges filtration unit and acid-cleaned cellulose nitrate filters from 2 to 4 mM and its pH from 8.0 to 8.5. Based on pre- (0.45-μm, Sartorius), and acidified to 0.01 M HNO3. De- vious metal analyses, the Birs is considered as moder- tails of the analytical procedure to determine dissolved ately contaminated (Jakob et al., 1994). Three sampling metal concentrations in the Thur samples have been re- sites were selected along the first 35 km of the river as ex- ported previously (Sigg et al., 2000). amples for various pollution levels. Site 1, which is un- Cu in whole and in filtered water samples was mea- contaminated by metals, is located 2.9 km from the river sured directly by graphite furnace AAS (Varian GTA-95) source. Site 2, at km 6.1, is located downstream from a for the Birs samples and by ICP-MS for the Thur samples. water treatment plant that receives the input of a metal- In the Birs samples, Zn was determined by flame AAS lurgical plant. Site 3 is located 9 km downstream from a after preconcentration as described in Sigg et al. (1996). metallurgical plant at km 34.1. Zn in the Thur samples was determined by ICP-MS. The The Thur River (Switzerland) flows over a length of detection limits were 0.2μg L–1 (3 nM) for Cu and Zn in 134 km from the Toggenburg region to the Rhine River. the Birs samples, and 0.01 μg L–1 (0.2 nM) for Cu and Zn in Its alkalinity ranges from 3.5 to 4.5 mM and its pH from the Thur samples, as determined from calibration curves. 8.0 to 8.5. Two sampling sites were selected. The upper- Field blanks (bottles treated in the same way as sam- most site (“Necker”), at the mouth of a tributary of the ples) were taken on a regular basis. Blank values were 302 R. Behra et al. Copper and zinc availability in two rivers <0.03 μg L–1 (0.4 nM) for Cu and < 0.3 μg L–1 (5 nM) for Statistical analysis Zn. The accuracy of the metal determinations by ICP-MS Metal content data were evaluated statistically in two was checked on a regular basis using SLRS-3 reference ways. First, Cu and Zn content in periphyton were re- water (National Research Council Canada).