Abstract
The Ciénaga Grande is a low salinity lagoon situated in the delta of the Magdalena River on the Caribbean Coast of Colombia. Water depth is generally less than 2,3 m. Temperature is stable near 30° C the year around. Tidal influence is confined to the proximity of the small inlet. The hydrography of the lagoon is thus controlled by the rates of river discharge. The mollusc fauna is characterized by a few species able to tolerate low salinities and temporary exposure to freshwater. The impact of the freshwater from a catastrophic flood of the Magdalena in December 1970 is documented. The lagoon is fringed by mangroves, which grade into freshwater swamps farther inland. The shallow, broad lagoon gradually came into existence by a relative rise in sea level, about 2 m, during the last 2 300 years. Before the transgression this area had been an Everglades-type swamp. Extensive peat deposits represent that early period. Recent lagoonal sediments are primarily gray or black soft muds. Sandy sediments occur in the vicinity of the barrier island to the north and of some scoured channels. In the central parts of the lagoon, a thin blanket of skeletal carbonates fornjs the only recent sediment.
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