Accumulation versus remobilization of mercury in sediments of a contaminated lagoon E. Ramalhosa a , P. Pato b, * , P. Monterroso b , E. Pereira b , C. Vale c , A.C. Duarte b a Escola Superior Agra ´ ria de Braganc ¸a, CIMO, Campus de Santa Apolo ´ nia, 5301-855 Braganc ¸a, Portugal b Department of Chemistry, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal c National Institute of Agronomy and Fishery Research, Av. de Brası ´lia, 1400-196 Lisboa, Portugal Once discharged in aquatic systems, mercury has a high affinity with suspended particles which contributes to its removal from water and to sink in nearby sediments (Wall- mann et al., 1996; Lee et al., 1998). Thus, mercury concen- trations in sediments are generally higher and therefore this compartment is considered a sink of mercury and simulta- neously a potential source to interstitial waters and to biota, mostly to the organisms that live in contact with the sediments (Arakel, 1995; May et al., 1997). The accu- mulation of mercury in aquatic organisms has been subject of many studies (Becher and Bigham, 1995; Chapman, 1997; Lawson and Mason, 1998) and it has been observed that the incorporation of Hg in fish depends on metabolic factors and metal bioavailability (Driscoll et al., 1995). Ria de Aveiro, a coastal lagoon in the Portuguese western coast, has for the last four decades, been subject to the discharge of several industrial and domestic effluents (Hall, 1982). A chemical complex located in Estarreja produced fertilisers, mineral acids, chlorine and soda, plastics, aroma- tics and other products. Its effluents have been, at least in part, rejected in a branch of the lagoon and represent the major contribution for the high accumulation of pollutants in sediments, namely metals (Pereira et al., 1997). One of the metals known to be present at higher concentrations than background levels is mercury (Lucas et al., 1986; Hall et al., 1987; Pereira, 1997; Pereira et al., 1998). The main anthro- pogenic source of mercury to the lagoon is the effluent from the chlor-alkali plant that discharged into a channel which drains to a basin where fisheries and recreation activities takes place. During the last decade, the industry changed the production process leading to a considerable decrease of mercury discharges (Pereira, 1997). Nevertheless, mercury discharged in past years has been deposited pro- gressively in sediments (Pereira et al., 1998). This work dis- cusses the extent of sediment mercury retention in the most contaminated sites in the lagoon, and its exportation to the water column as well as transfer to biogenic suspended particles and to sea bass (Dicentrarchus labrax). Therefore, the main question, with important environmental manage- ment implications, was to assess if mercury is effectively retained in the sediments or is progressively released to the water column in considerable amounts. In April 1997, water samples were collected from sam- pling stations in the most contaminated area of the lagoon (Laranjo Basin, Fig. 1), both in high tide and low tide con- ditions. Sediments cores (60 cm) were also taken from two stations (A and B) (Fig. 1). In June 1998 plankton samples (63 lm net) were collected in high tide and low tide conditions in Laranjo Basin and Muranzel (Fig. 1). During 1997 and 1998, in several periods of the year, specimens of sea bass (D. labrax) were captured at four stations located in different areas of the lagoon: Laranjo Basin, Muranzel, Costa Nova and I ´ lhavo (Fig. 1). Water samples were filtered with 0.45 lm Millipore filters. The filtrate was then acidified with HNO 3 (Merck, ‘‘mercury-free’’) to pH < 2 and stored at 4 °C until * Corresponding author. Tel.: +351 234 370 737; fax: +351 234 370 084. E-mail address: pmartins@dq.ua.pt (P. Pato).