7th International Conference on Mercury as a Global Pollutant 1129 Distribution of mercury and methyl mercury in tissues of transplanted mussels Zorana Kljakovič-Gašpič1, Nikša Odž ak1, Tomislav Zvonarič1, Milena Horvat2 & Ante Barič1 'Institute of Oceanography and Fisheries, Split, Croatia; E-mail: kljakovic@izor.hr institute Jožef Stefan, Ljubljana, Slovenia; E-mail: milena.horvat@ijs.si Abstract: Total mercury and methyl mercury concentrations were determined in whole soft tissue, gills and digestive gland of mussels from the Kastela Bay, which is polluted with inorganic mercury. Concentrations of total mercury in the digestive gland were I.S-S.5 times higher than in the whole soft tissue and gills, while concentrations of MeHg were similar in all analyzed tissues. The percentage of methyl mercury in the digestive gland (4-11 %) was lower in comparison to the gills (12-25 %) and the whole soft tissue (13-27 %). A negative correlation was observed between the total mercury concentration in the tissues and the percentage of methyl mercury. Key words: mercury, mussels, transplatation, biomonitoring, Kastela Bay Introduction The Kastela Bay is the largest bay in the central part of the Croatian coast (Fig. I). In the period from 1949 to 1990, the Bay has been exposed to pollution by inorganic mercury derived from a chlor-alkali plant (Fig. 1). The total amount of elemental mercury discharged into the bay is estimated at 39 tons'1". Even though the chlor-alkali plant was closed 14 years ago, elevated levels of total mercury are still evident in surface sediments'2, 31, and surface waters of the bay'3,41. Previous studies showed that mercury concentrated in sediment close to the plant is being dispersed throughout the bay by remobili-zation processes'2, S 61. toring experiment in the bay in the period from September 2000 to March 2001. Mussels (M. galloprovincialis) were transplanted from the shellfish-breeding farm in the Mali Ston Bay to 4 stations in the Kastela Bay (Fig. 1). Samples were collected every second month. More detailed description of the experiment is given elsewhere'61. In order to assess the availability of remobi-lized mercury and methyl-mercury to marine organisms, we performed an active biomoni- Figure I. Kastela Bay with experimental stations. RMZ-M&G 2004, SI 14, I. ■h. i ■ 1396 Part 2 - Biogeochemistry Results and discussion Total Mercury: Temporal variations of total mercury in analyzed tissues are presented in Fig. 2. Concentrations of total mercury in the digestive gland (320-1349 ng g"1 d.wt.) were 1.5-5.5 times higher than in the whole soft tissue (138-325 ng g"1 d.wt.) and gills (121-380 ng g"1 d.wt.) (Fig. 2). These values are similar to the values usually found in the areas with the history of the contamination with mercury from different industrial sources. The process of mercury accumulation in the mussels transplanted from the referent station was very rapid at all stations. The highest differences were recorded at station 1. After 2 months, concentration of Hg in digestive gland at this station was 7 times higher in comparison to the initial period. In following months the decrease of Hg concentration in digestive gland was followed with the increase in the whole soft tissue. This is probably related to internal transport and redistribution. Spatial distribution of bioavailable species of total mercury was in the accordance with the prevailing circulation of water mass in the bay'1". The average concentrations of total Hg in mussel tissues decreased in an anticlockwise direction towards the exit of the bay (1>2, 3>4). Described spatial distribution of Hg in mussel tissues is similar to the spatial distribution of total Hg in the surface sediments'2", which indicates successful transport of Hg from sediments to the filterfeeding organisms. This could be expected since investigations in coastal areas showed that total mercury concentrations in mussels are strongly correlated with the mercury content of sediments and particulate matter'7". Figure 2. Variations of total Hg (ng g"1 d.wt.) in the whole soft tissue (WST), gills (G), and digestive gland (DG) of mussels. Methyl-mercury: Concentrations of MeHg were similar in all analyzed tissues, and covered a narrow range of 22.7-59.3 ng g"1 d.wt. (Fig. 3). These values are significantly lower in comparison to the period before the chloralkali plant was closed'81. However, they are still higher in comparison to the uncontami-nated areas. Enrichment factors of MeHg in tissues two months after transplantation were 1.6-2.5, 2.1-2.5, and 2-2.7, respectively, for digestive gland, whole soft tissue and gills. Increase of MeHg concentration in the whole soft tissue, and the decrease in digestive gland in period from November to January is probably related to internal transport. RMZ-M&G 2004, SI 7th International Conference on Mercury as a Global Pollutant 1131 Spatial distribution of MeHg was similar to the distribution of the total Hg. Thus, concentrations of MeHg decreased from Station I in an anticlockwise direction towards the exit of the bay. Ratio MeHg/total Hg: The percentage of methyl mercury in the digestive gland (4-11 %) was lower in comparison to the gills (411 %) and the whole soft tissue (12-25 %) (Table 1). The percentage of mercury in the methylated form in mussel tissues in the marine environment can range from 14% to almost 100 %[8-9], depending on the stage of contamination with mercury. However, the higher values are usually recorded in mussels from the uncontaminated areas. Our results support this statement. We observed that a negative correlation exists between the total mercury concentration in the tissues and the percentage of methyl mercury. Thus, the highest percentage of MeHg was recorded at station 4, where the concentrations of inorganic mercury are the lowest. This negative correlation is probably the result of the low production rate of MeHg in the ecosystem, which cannot compete with the large flux of inorganic mercury from the sediment'81. Figure 3. Variations of total MeHg (ng g"1 d.wt.) in the whole soft tissue (WST), gills (G), and digestive gland (DG) of mussels. Table I. Percentage of total mercury in the methylated form (%) in the whole soft tissue, gills and digestive gland of mussels from the Kastela bay. WHOLE SOFT TISSUE GILLS DIGESTIVE GLAND "~-\STATION 1 2 3 4 1 2 3 4 1 2 3 4 1X2000 27 27 27 27 11 11 11 11 12 12 12 12 XI 2000 20 24 25 27 14 25 18 25 4 8 7 11 1 2001 17 16 18 21 12 17 19 21 6 4 6 10 III 2001 13 15 15 21 14 16 21 25 6 5 7 11 RMZ-M&G 2004, SI 1396 Part 2 - Biogeochemistry Conclusions • Mercury is still one of the main pollutants in the Kastela Bay. • The concentration pattern of total and methyl mercury in mussel tissues is similar to the spatial distribution of total Hg in the surface sediment. • Spatial distribution of bioavailable species of mercury is in the accordance with the prevailing circulation of water mass in the bay. The percentage of methyl-mercury in mussel tissues (4-25 %) is characteristic for the areas contaminated with inorganic mercury. The digestive gland is very sensitive monitoring tool for the assessment of the bioavailability of remobilized mercury in the marine environment. 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