Species richness and distribution of benthic tidal flat fauna of the Banc d'Arguin, Mauritania

The Banc d'Arguin in Mauritania, West Africa, is an area of tidal flats and shallow inshore waters bordering the sandy desert of the Sahara. The project Banc d'Arguin 1985-1986 investigated predominantly benthic biomass and production, the same data allow investigation of the species diversity and the distribution of the tidal flat fauna of the Banc d'Arguin. We sieved and analysed 410 core samples (diameter 10 cm) from 82 locations with 0.6 mm mesh size. A total of 111 taxa were identified. Polychaeta were most diverse with 42 species belonging to 20 families. Other large species groups were Gastropoda (20 species, 14 families), Amphipoda (14 taxa, 8 families) and Bivalvia (15 species, 10 families). Densities ranged from 152 to 5635 ind. m-2, with an average of 1404 ind. m-2. Six taxa accounted for more than half of the total density. H' values were generally low with values ranging from 0 to 2.78 nats. Low evenness values indicated a high degree of dominance. The total density was independent of any sediment characteristic, probably because of the large range of species. Densities of dominant species did show a correlation with sediment size. A fraction of the individuals could have been lost with our use of a 0.6 mm sieve. Many species occurredonly at a few stations. An explanation for this could be that stations are dominated by different species, while the diversity within a station is relatively low. The occurrence of opportunistic species supports this explanation. The Banc d'Arguin is a reversed estuary, and the stress caused by high temperatures and salinity could result in opportunistic assemblages. The (theoretical) high turnover rate of these opportunistic assemblages could be the key to the importance of the Banc d'Arguin for wintering shorebirds. VA:IBN