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INTRODUCTION
Coastal lagoons are among the most productive ecosystems supporting habitats for wildlife and commercially explored species (Costanza et al. 1997). These services are often at risk due to the increasing stress caused by contaminants from activities related to urbanization, industrialization, intensive agriculture in the watershed, massive tourism, and intensive aquaculture (e.g. Amiard et al. 2006, Cooper et al. 2013). Among numerous contaminants released to the environment, metals and metalloids are a worldwide concern due to their persistence and toxicity (ATSDR 2015). Organisms exposed to trace elements may accumulate them in tissues leading to various inductive responses, such as production of metallothioneins to sequestrate toxic metals (Bebianno and Langston 1993), and triggering antioxidant defence systems to prevent the formation of reactive oxygen species and their deleterious effects (Ivanina et al. 2013, Lushchak 2011). Trace elements may interfere with the variation of physiological parameters related to the organism’s wellbeing (Yatoo et al. 2013). In the case of bivalves, several works have reported high consumption of glycogen, lipids and proteins in response to environmental contamination (Hamza-Chaffai 2014, Luna-Acosta et al. 2015), leading to changes in the condition index (Cravo et al. 2012) and eventually in the gametogenesis cycle (Breitwieser et al. 2016, Gauthier-Clerc et al. 2002). According to Mantel and Farmer (1983), metal exposure may alter the fluid ion level in clams, causing the decrease of Na+, Cl- and K+ contents as a compensatory process in the internal osmolarity regulation.
In biomonitoring studies, an important step is to evaluate whether trace element distribution in key organisms is determined by inherent processes, such as those associated with the sexual cycle, as expected at low impacted environments, or is highly influenced by human activities. The objective of this work was to examine whether human activities in Ria Formosa and the surrounding region have a major influence on the trace element partitioning in the clam Ruditapes decussatus produced on inter-tidal flats, superimposed on changes related to metabolic processes. This hypothesis was tested by transplanting clams from a natural bank to three sites: one under the influence of urban activities, one of fish aquaculture earthen ponds, and one near the lagoon inlet, which was used as a reference site. Concentrations of Zn, As, Cu, Mn, Ni,...