Measurement of environmental trace-metal levels with transplanted mussels and diffusive gradients in thin films (DGT): a comparison of techniques
Introduction
Mussel-watch programs have been used worldwide throughout the last 25 years to monitor levels of various pollutants (Martin and Richardson, 1991; Boening, 1999). The use of mussels and other bivalves as bioaccumulating devices has several advantages over the testing of water samples for pollutants. Analysis of tissues of resident or transplanted bivalves gives a time-integrated measure of bioavailable toxicant levels (Phillips and Segar, 1986; Rainbow and Phillips, 1993). This avoids problems associated with small-scale spatial and temporal patchiness in water-borne toxicant levels (sensu Wu and Lau, 1996), and the question of which fraction, or chemical `species', of a toxicant should be measured to estimate bioavailable toxicant levels. Moreover, the concentration of toxicants found in bivalves may be several orders of magnitude greater than that found in surrounding waters (Phillips, 1976b; Rainbow and Phillips, 1993), reducing both the costs of analysis and problems associated with post-sampling contamination (sensu Florence and Batley, 1980).
For reasons such as these, mussels were identified as potential bioindicators of pollution in the mid 1970s (Phillips, 1976a, Phillips, 1976b). Since then, mussels and other bivalves have been used to assess patterns of distribution of a number of toxicants worldwide (e.g., Marcus and Stokes, 1985; Roper et al., 1996; Guillon-Cottard et al., 1998; Stein et al., 1998). The technique, however, has its shortcomings. Problems identified with the use of bivalves as bioindicators have included the effects of biotic factors such as age, size, sex, feeding activity and reproductive state, and abiotic effects such as organic carbon levels, temperature, pH, dissolved oxygen levels and hydrology, on the uptake of toxicants (Edler and Collins, 1991; Boening, 1999). Additionally, some toxicants do not accumulate predictably in tissues (e.g., copper and/or zinc; Phillips, 1976b; Klumpp and Burdon-Jones, 1982; Phillips and Rainbow, 1988, Phillips and Rainbow, 1989).
Diffusive gradients in thin films (DGT; Davison and Zhang, 1994) have been used to measure environmental concentrations of divalent metals over short periods of time (e.g. Zhang and Davison, 1995), and have been postulated as possible alternatives to bivalves as a means of assessing metal pollution (Davison and Zhang, 1994). The technique utilises an ion-permeable hydrogel as a diffusive layer of defined thickness over a second gel impregnated with a binding agent. The average environmental metal concentration (Cb) can be determined from the mass of metal (M) accumulated using:where Δg is the diffusive layer thickness, D is the diffusion coefficient of the metal, t is the exposure time, and A is the diffusive area. The technique has the same advantages as bivalves, in that a time-integrated measure is obtained and samples are pre-concentrated, but may have a potential additional advantage in that no biotic factors will affect metal uptake and effects of abiotic factors should be predictable. Reliable results have been found for most divalent metals tested (Zhang and Davison, 1995), and the technique can be adapted to measure other elements and compounds (e.g., Chang et al., 1998; Zhang et al., 1998). The technique also gives answers in terms of an actual water concentration of metal, which should simplify the interpretation of results. To generate accurate results, however, the technique does rely on the virtual absence of a diffusive boundary layer over the hydrogel diffusive layer (Davison and Zhang, 1994). Similarly, the effects of biofouling are likely to influence estimates of Cb (Davison and Zhang, 1994). Neither of these effects has been investigated in prolonged field studies.
This paper compares measured concentrations of metals in mussel tissues with environmental concentrations as measured by DGT. Measurements were made at four sites near Melbourne, Australia. Two sites were inside enclosed marinas and there was an expectation of higher metal levels at these sites. Four sets of mussels were transplanted to the sites over a one-year period, and accumulation of cadmium, copper, lead and zinc was monitored. DGT units were deployed at the same sites for two one-month periods the following year, and were tested for the same metals. The use of the two techniques at the same sites, although at different times, allowed a superficial comparison of the results gained.
Section snippets
Study sites
The surveys were conducted at four sites: Westernport Marina, Hastings Jetty, St. Kilda Marina and St. Kilda Pier. These sites have been described in detail elsewhere (Webb and Keough, 2000), but it is important to note that the sites at Westernport Marina and St. Kilda Marina are inside fully enclosed marinas, and that the sites at Hastings Jetty and St. Kilda Pier are in more open waters close to the two marinas (Fig. 1). The sites are referred to as `inside' and `outside' throughout this
Mussels
Over the one year of exposure, there were temporal trends in the levels of metals found in mussels transplanted to the experimental sites, but patterns differed for different metals. The levels of metals found in the reference mussels were uniform and low for the four samples, with the possible exception of cadmium, which showed some temporal differences (Fig. 2). Cadmium levels were similar for the four experimental sites, and were similar to the levels seen prior to deployment (level for
Comparison of results
The results for the DGT units and the transplanted mussels can be compared, but it is important to remember several confounding factors. Most important is that the two deployments took place in different years, and it is probable that spatial and temporal trends in metal levels differ from year-to-year. Secondly, the DGT units were deployed for two one-month periods designed to coincide with periods of warmest and coolest water. These periods do not correlate well with the three-month
Conclusions
Strengths and weaknesses exist for both techniques. For mussels, there is continuing uncertainty as to their ability to accumulate some metals (highlighted by the result for zinc at St. Kilda inside), and the results do not indicate over what period of time toxicants were accumulated (although this could be determined by different exposure times). However, the active filter feeding of mussels makes the uptake of metals from water an active process not affected by the vagaries of diffusive
Acknowledgements
We would like to thank Richard Lowe of Keefer's Mussel Farm for the regular donation of mussels used in this study. Hao Zhang supplied DGT components at short notice and was of great help with practical techniques for their use. Westernport Marina, Hayman-Pacific, St. Kilda Marina and Royal Melbourne Yacht Squadron provided unimpeded access to the field sites. Dustin Marshall provided field assistance. Comments on an earlier version of this manuscript by Alan Butler, Jan Carey, Emma Johnston
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2020, ChemosphereCitation Excerpt :There is evidence that trace metals represent natural constituents in tissues of marine organisms and their basic levels fluctuate seasonally (Regoli and Orlando., 1994), which is partly attributed towards seasonal penetration of gonadal tissues into the digestive gland during gametogenesis (Regoli and Orlando., 1994; Ivanković et al., 2005; Fattorini et al., 2008). Also, human activities (Webb and Keough, 2002) and the seawater convection in different seasons may influence the concentration of trace metals in seawater, and the bioaccumulation in mussels. The usage of bivalves as bioindicators for contaminant monitoring has inevitable defects.