Abundance and distribution of recent benthic foraminifera from the southwestern part of the Bay of Bengal

Foraminifers are ubiquitous and abundant organisms in the marine environment, sensitive to different environmental variables such as pH, dissolved oxygen, salinity, temperature, organic matter and substrate. In the present study, environmental variables and benthic foraminiferal assemblages from 32 surface sediment samples belonging to different water depths were studied to understand the shelf and slope environment of the southwestern Bay of Bengal. Statistical methods such as cluster and principal component analysis (PCA) were used to infer the relationship existing among various foraminiferal assemblages, bathymetry, sediment and bottom water characteristics. Statistical analysis yielded four assemblages, of which assemblage-I is dominated by Nonion faba, Rotalidium annectens, Asterorotalia pulchella, Elphidium crispum, and Amphistegina radiata, and is confined to the shallowest inner shelf region (5 to 35 m water depth) with sandy sediments indicating high-energy condition. Assemblage-II is marked by the abundance of Cibicidoides wuellerstorfi, Hanzawaia concentrica, Quinqueloculina agglutinans, Quinqueloculina seminula, Quinqueloculina lamarckiana, Ammonia beccarri, and Ammonia tepida which are associated with the middle shelf region (water depth 35 to 70 m) with sandy to slightly muddy sand sediments. Assemblage-III is dominated by Asterorotalia dentata, Quinqueloculina venusta, Cancris communis, Cassidulina laevigata, and N. costiferum which occur at a depth of 70 to 170 m in the outer shelf, and are highly associated with slightly muddy sand to muddy sand. Whereas, assemblage -IV includes Uvigerina mediterranea, Bolivina dilatata, Bolivina robusta, and Bulimina marginata foraminiferal species that are abundant in the upper slope (170–244 m water depth) and are confined to muddy sand and sandy mud with low dissolved oxygen in the bottom water. The above assemblages depict that the abundance and distribution of foraminifera are largely controlled by substrate, energy conditions and environmental variables such as temperature, salinity, organic matter and dissolved oxygen.