Mercury in the Bivalves Crassostrea tulipa and Perna perna from Ghana
The coastline of Ghana consists mainly of plains with numerous lagoons and estuaries; about 50 lagoons occur in the area, most of them very small (less than 1 km2). They are of two main types: `open’ and `closed’ lagoon systems. The open lagoons are in contact with the sea throughout the year and are therefore under tidal influence. The salinity is similar to that of the sea except during the wet season, when it is lowered as a result of dilution from surface run-off and streams; water temperature ranges from 24°C to 32°C, salinity from 10 to 40 PSU. The closed lagoons are cut-off from the adjacent sea by a sand bar (about 40 m wide) for the greater part of the year, while contact with the sea is re-established during the wet season; water temperature ranges from 27°C to 34°C, salinity from 27 to 70 PSU. Dilution is mainly from direct rainfall and small creeks, whereas hypersaline conditions result from evaporation during the dry season (Biney, 1986; Yankson, 1982). There are two wet seasons per year, the major one from March to July and a minor one from September to October, rated by a short cold dry season in August and a relatively long warm dry season from mid-October to March. The annual rainfall averages 2 m but varies greatly.
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