many anthropogenic pressures (urban, domestic, agricultural and/or industrial effluents, port use and management, aquaculture, fishing, etc.), that are responsible for internal perturbations (pollution, sediment dredging, removal of indigenous species, changes in food web structure, etc.). In particular the introduction of exotic species, associated to aquaculture industry, endangers the ecological equilibrium of these fragile ecosystems (Micheletti et al., 2004).
The Manila clam (Tapes philippinarum) was introduced in the Italian lagoons in the 1980s and replaced almost completely the indigenous species Tapes decussatus, because of its higher growth and reproduction rates and greater resistance to pollution. Tapes philippinarum is a macrobenthonic filter-feeding species living in the uppermost oxidized layer of the sediment which is extensively farmed in Italy. In Italy, about 30000 tons of edible clams are produced per year (Binelli and Provini, 2003), in particular the main producers of shellfish in Italy are Venice and Sacca di Goro lagoons.
The Sacca di Goro lagoon is located in the northern part of the Po river delta and receives the high loads of nutrients and contaminants coming from agricultural areas of Burana-Volano watershed and Pianura Padana. Actually clam cultivation areas cover more than one third of the Sacca di Goro lagoon surface at densities attaining about 2000-2500 adult individuals m-2; such densities and sediment dredging related to harvesting activities, have a strong impact on the benthic system (Bartoli et al., 2001) and in the biogeochemical cycles in the lagoon.
Herbicides occur in the Sacca di Goro lagoon water and sediments as a result of the impacts of upstream agricultural activities (Baldi et al., 1991; Brown et al., 1996; Della Vedova et al., 1996; Riparbelli et al., 1996; Coppi et al., 1999; Galassi et al., 2000; Carafa et al., 2006). These contaminants have shown bioaccumulation tendency through the aquatic food web and toxic effects for aquatic biota.
Ecotoxicological effects of s-triazine on algae (e.g. Jianyi Ma et al., 2006; Podola and Melkonian, 2005; Dorigo U. and Leboulanger C., 2001; Nitschke et al., 1999; M. I., Abdel-Hamid, 1996) acquatic plants (e.g. Cedergreen and Streibig, 2005) and aquatic microorganisms in general (De Lorenzo M., 2001) have been investigated. A complete review of most important studies on toxic effects of these contaminants can be found in Eisler (2000).
A combined additive s-triazine effects on algae has been described (Faust et al., 2001; Strachan et al., 2001) and a low bioaccumulation potential of these contaminants has been noticed (Pérez-Ruzafa et al., 2000; Gluth et al., 1985; Carafa et al., 2006b). However, relatively few analytical field data are available for evaluating the toxic effects in marine fish (Hall et a., 1994; Ward and Ballantine, 1985) and in molluscs: Lawton et al. (2005) found in juvenile clam Mercenaria mercenaria, for atrazine, LOEC values of 1250 and 1000 μg L-1 in 96 h acute assay and 10 days chronic assay respectively (NOEC: 500 μg L-1), in this case sublethal endpoints were dry mass, shell size and condition index (dry mass/shell volume). Cheney et al. (1997) investigated the variation in metabolic activity in gill tissue of the mollusc Elliptio complanata in short time (20-50 min) exposure to atrazine for concentrations between 10-6 and 10-3 M.
Finally, Losso et al. (2004) measured embryotoxicity (malformed larvae and prelarval stages) of atrazine in early life stage of the bivalve Mitilus Galloprovincialis (48 h incubation test), and found EC50 value between 2.9-3.3 mg L-1. Data of organic contaminants accumulated in the sediment and in the tissues of target species may provide an assessment of pollutant occurrence and distribution in aquatic ecosystems, acting as a time integrated measure (Pereira et al., 1996). Specifically, in coastal lagoon ecosystems molluscs have been used as bioindicators of pollution because of their feeding behaviour and their scarce mobility which make them particularly exposed to contamination both through water column and sediment, directly or after resuspension. Among molluscs, clams show a tolerance to pesticide, but less is known of their metabolic responses to contamination. Moreover, clams are farmed for human consumption and, if contaminated, may represent a potential risk for human health. Although seafood represents a significant means of contamination of human diet, few legal thresholds have been established in order to protect human health from a number of toxic compounds and complex mixtures of chemicals. In particular the European Community introduced several laws in order to regulate the water quality parameters of bivalves farming zones (91/492/CEE), and restrictions for farming, transport and purchasing (78/923/CEE, 81/3796/CEE, 89/2886/CEE).
However these rules refer only to microbiological characteristics. In the directive 2000/60/CEE, with reference to decision n. 2455/2001/CEE, it is requested of European Countries to establish quality standard limits for priority hazardous substances in water; following this directive, threshold have been fixed by the Italian law (Decreto 6 novembre 2003, n. 367, Appendix 1).
Along with field studies and monitoring activities, model tools are necessary to understand the fate and transport of contaminants and to assess their impacts on communities and ecosystems (Carafa et al., 2006c). Modelling can also support the testing of different management strategies to improve ecosystem state (Marinov et al., 2006).
In the management of hazardous chemicals the prediction of bioconcentration and bioaccumulation factors from water in aquatic organism has become a very important tool.
The quantitative knowledge of uptake, metabolism, excretion and depuration processes of chemicals in the organisms is needed to predict the fate and bioaccumulation of contaminants along the food web (Moriarty and Walker, 1987).
However, little information exists regarding the uptake dynamics of herbicides in clams (e.g. Uno et al., 1997; Nordone et al., 1998), the retention time, the metabolism pathways, the excretion rates, etc. All these processes are strictly related to specific physiological characteristics, feeding behaviour and metabolism of the aquatic organism and to the particular chemical-physical features of the compound (Pereira et al., 1996), for this reason it is difficult to make comparisons between different studies and to determinate uptake and depuration constants.
In this work we used several general equations developed by Thomann (1989) and Thomann et al. (1992) for predicting chemical residues in aquatic food web and tested in fish species. A similar approach was developed also by Gobas (1993). Both models are based manly on the octanol-water partition coefficient of contaminants, Kow. Other
examples of predictive models for the bioaccumulation of pollulants in aquatic biota are listed below:
-Thomann and Conolly (1984) developed and validated a model for PCB in the Lake Michigan trophic network, taking into account: biomagnification, growth, respiration, metabolisation costants.