Carbonate contents of bottom sediments of Todos os Santos Bay, Bahia, Brazil: their importance for biodiversity

Todos os Santos Bay, located on the Brazilian coast between 12 degrees 35'to 13 degrees 07'S and 38 degrees 29' to 38 degrees 48'W, has an area of 1086 Km(2) and contains a variety of ecosystems. The bay is surrounded by the densest urban development in the state of Bahia. The present investigation aimed to evaluate the carbonate contents of bottom sediments in this bay and their importance for the biodiversity of the benthic fauna. Sediment samples were obtained using a van Veen grab, from 32 stations during 1997, Total carbonate contents ranged from 3.3% in the northern part of the bay, to 99.9% west of Mare Island, and were higher than 50% at 25% of the stations. The proportion of carbonate decreased with decreasing grain size. Polychaeta, Mollusca Bivalvia and Crustacea Decapoda all had frequencies of occurrence above 85%. Biodiversity, considered as the richness of major taxonomic groups, ranged from 3 to 23. The highest biodiversity values were found in the area between Itaparica Island and the western shore of the bay. In this area, sediment texture and composition varied widely. The lowest biodiversity values occurred in the northern part of the bay, where the bottom is muddy with very low carbonate content. Stations with higher biodiversity (richness values over 50%) had sediments with the highest carbonate contents, a statistically significant relationship. In Todos os Santos Bay, then, the biodiversity of the macrobenthos increases with increasing structural heterogeneity of the environment, and carbonate sediments seem to play an important role.