| Zusammenfassung: | This work deals with the biodiversity and distribution of benthic macrophytes in the Ghar El Melh lagoon, a Mediterranean coastal lagoon located in the North of Tunisia. An inventory was made of the benthic flora and submerged macrophyte communities were mapped during two successive campaigns (the summer of 1999 and the winter of 2000). The following 24 macrophyte species were identified: seven red algae, two brown algae, 11 green algae, and four marine angiosperms. The results were compared with available data from the literature. Ruppia cirrhosa is the most dominant species. It is found in all lagoon parts, except in the west sector. Ruppia beds are usually associated with Cladophora forming heterogeneous communities. During summer Ruppia cirrhosa shows a large distribution, covering an area of ca. 21.4 ${\rm km}^{2}$ , with dense, extensive beds covering 80-100%. In winter, several Cladophora species have a very large distribution as well, covering nearly an area of 28.5 ${\rm km}^{2}$ with an average cover of 46%. The green algae Caulerpa prolifera is confined to the eastern part of the lagoon which is mainly affected by seawater. In comparison with previous situations, many transformations were observed in biodiversity and spatial distribution of the dominant communities. Thus, Cymodocea nodosa and Zostera beds, which dominated in the 1970s, were replaced by Zostera and Caulerpa prolifera in the 1980s and are currently succeeded by Ruppia cirrhosa and Cladophora. Restoration of the Ghar El Melh lagoon will enable an increase in the exchange with the open sea and the circulation of water, in particular in the confined zones. This should considerably improve the water quality and would positively influence the phytobenthic communities.
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