Community structure and dynamics of the molluscan fauna in a Mediterranean lagoon (Gialova lagoon, SW Greece)

The molluscan community of a shallow Mediterranean lagoon (Gialova lagoon, SW Greece, Ionian Sea) was studied on a seasonal basis during 1995-1996. A total of 23 species were recorded from the lagoon. Two of them namely: Placida viridis and Polycerella emertoni are reported for the first time from the Greek seas and the Eastern Mediterranean respectively. The dominant species were the gastropods Bittium reticulation, Cerithium vulgatum, Pirenella conica, Hydrobia acuta, Cyclope neritea and the bivalves Cerastoderma glaucum, Abra ovata. Uni- and multivariate methods were employed to study the community structure and dynamics. On the basis of the multivariate pattern of the molluscan community a coenocline is observed which is strongly related with the degree of isolation. Two main zones with different faunal composition can be distinguished: one narrow zone close to the channel of communication with the sea and another in the innermost part of the lagoon. Following the confinement scale suggested for lagoonal ecosystems by GUELORGET & PERTHUISOT (1992) the first zone can be assigned to the ``zone III'' and the second to ``zones IV-V''. The distribution pattern of the molluscan community is governed by a different set of environmental variables in each season, discussed in detail. Although no disturbance due to anthropogenic impact was revealed, this narrow lagoonal habitat was proved to suffer severe ``dystrophic episodes'', during late summer and the beginning of autumn, but successfully recovered, demonstrating a seasonal community pattern.