PCDD/F and dioxin-like PCB bioaccumulation by Manila clam from polluted areas of Venice lagoon (Italy)
Introduction
The Manila clam Tapes philippinarum (Adams & Reeve, 1850) is an Indo-Pacific species which has widespread along the coasts of North America and Europe. The species was accidentally introduced in the 1930s along the Pacific coasts of the United States, probably by seed oysters imported from Japan (Chew, 1989), and it spread naturally along the Pacific coasts from California to British Columbia (Magoon and Vining, 1981). Due to its economical value the Manila clam was introduced along the American coasts of the North Atlantic, such as the Virginian coasts in the late 1960s, where it was farmed together with oysters (Murray and Kirkley, 2005). In Europe it was introduced in the 1970s, where production tests were carried out in several countries, including France, Britain, Spain and Ireland (Flassch and Leborgne, 1992).
In Italy clams were seeded in the southern basin of the Venice lagoon in March 1983 (Cesari and Pellizzato, 1985) with the purpose to diversify and strengthen the small lagoon shellfish industry, which until then, was devoted to mussels. In the following years the species was introduced in many other Italian lagoons (Di Marco et al., 1990, Giorgiutti et al., 1999, Mattei et al., 1990, Milia, 1990, Paesanti and Pellizzato, 2000, Zentilin, 1990).
Because of its significant growth rate, the capacity to reproduce and to adapt to different environmental conditions, the Manila clam quickly spread over other suitable areas situated in the north-western Adriatic coasts: particularly the lagoons of Grado and Marano, ponds and lagoons of the Po Delta in Veneto and Emilia-Romagna Regions. The extraordinary success of the species in those areas led to significant changes in the fishing sector from both a social and an economic point of view (Turolla, 2008), so that Italy became the first European producer in the early 1990s (Orel et al., 2000). The production reached a peak between 1995 and 2000 with 55,000 tonnes yr−1, of which ca. 40,000 tonnes yr−1 came from the Venice lagoon. The high exploitation of natural banks in proximity of industries and urban waste outflows by mechanical and hydraulic dredges (Orel et al., 2000, Torricelli et al., 2009) generated a variety of environmental and biological issues. Those areas are in fact characterized by high nutrient, organic matter, phytoplankton and microphytobenthos concentrations (Facca et al., 2002a, Facca et al., 2002b, Sfriso et al., 2003, Sfriso et al., 2005a, Sfriso et al., 2005b) which favor the clam growth but, unfortunately, in these same areas there are also high loads of pollutants such as heavy metals and organic compounds (Bellucci et al., 2002, Bernardello et al., 2006, Critto and Marcomini, 2001, Frignani et al., 2005, Guerzoni et al., 2007, Micheletti et al., 2007, Raccanelli and Bonamin, 2000, Raccanelli et al., 2009) and sediments and particulate matter can become potential sources of contaminants. In addition, studies on rates of settled particulate matter (SPM) in the lagoon (Sfriso et al., 2005c) show that high amounts of surface sediments (600–1000 kg dwt m−2 y−1) are re-suspended and can re-deposit in situ or spread favoring the dissemination of nutrients and pollutants (Bernardello et al., 2006, Secco et al., 2005, Sfriso et al., 2003). Tapes philipphinarum is a sediment-feeder and filter-feeder that lives sunken in the sediment. It can assimilate pollutants directly from the sediment or by straining suspended matter from water. However, the relationship between the persistent organic pollutant (POP) concentration in the surface sediments and in the clam soft tissues is not always clear.
This paper reports an investigation on the potential risk of dioxin (PCDD/F) and dioxin-like PCB (DLPCB) bioaccumulation by juveniles of T. philippinarum. The seed was produced in a clam hatchery placed in an area with low PCDD/F and DLPCB concentrations at Marano lagoon. It was reared both in an area close to the clam hatchery (reference site) and in two contaminated areas of the Venice lagoon. The experiment was scheduled to verify the difference between bioaccumulation patterns in relation to sediment grain-size and pollutant drivers (sediments or suspended matter) by studying clams sunken in the sediments or suspended. To compare the different behaviors in relation to grain-size, two sites with similar contamination level were chosen: Fusina, characterized by sandy sediments, and San Giuliano, where muddy sediments prevailed. To differentiate the sediment and SPM effects on the product quality, in each station, some clams were reared in nets placed on the bottom (B-clams), others in net suspended in the water column (S-clams). Clams were monitored during their life cycle from juvenile to market size. Moreover the separate contribution of PCDDs, PCDD/Fs, DLPCBs in the total clam toxicity was analyzed to highlight the presence of PCDD/Fs, which are mainly connected to the chlorine industry (Fattore et al., 1997, Raccanelli et al., 2009, Weber et al., 2008), and of DLPCBs, the role of which is often underestimated in the total product toxicity. The POP concentrations at the market size were deeply discussed to understand the potential clam toxicity for human health in relation to the highest values permitted by law.
Section snippets
Area description
Sampling sites were located in the lagoons of Venice and Marano (Fig. 1). In Venice, two stations were selected in areas where pollution and clam harvesting are high (Guerzoni and Raccanelli, 2003, Guerzoni et al., 2007, Marcomini et al., 1997, Micheletti et al., 2007, Raccanelli et al., 2009, Secco et al., 2005, Sfriso et al., 2008), one situated in a sandy area (Fusina) and the other in a muddy area (San Giuliano). A third station (Marano) was selected in a sandy area of Marano lagoon close
Environmental parameters
Parameters measured in the water column, surface sediments and SPM are shown in Table 1. Data resulting from the analysis of the samples taken in Marano (reference site), Fusina (polluted sandy site) and San Giuliano (polluted muddy site) show amounts of FPM (45.1–78.8 mg l−1) and total Chl-a (3.2–12.2 μg l−1), respectively, on the increase. The sediments at San Giuliano were characterized by 96.1% of fine particles (diameter <63 μm) and can be classified as mud, whereas at Fusina and Marano
Discussion and conclusions
Once generated by thermal and industrial processes, POPs persist in soils and sediments for decades even to centuries, becoming the major source of today's pollution (Weber et al., 2008) and representing a relevant risk for human health. It is well known that pollutant concentrations depend on sediment grain-size and that the particles of the 0.1 mm top sediment layer suspended in the water column are more rich in pollutants and nutrients (Sfriso et al., 1991, Marcomini et al., 1990). For this
Acknowledgments
The authors thank the Italian Ministry of Agricultural and Forestry Policies (MIPAF) that funded the present project in the framework of the ‘‘VI Triennial Plan for Fish and Aquaculture in Marine and Brackish Waters’’. The authors are grateful to Dr. Aurelio Zentilin for the technical assistance and to Dr. Orietta Zucchetta for the English editing.
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