Assessing the ecological preferences of agglutinated benthic foraminiferal morphogroups from the western Bay of Bengal

Abstract

Agglutinated benthic foraminiferal morphogroups have often been used as a proxy in paleoecological studies. The response of various agglutinated morphogroups may, however, vary regionally. Here we report the ecological preferences of agglutinated morphogroups from the fresh-water influx dominated western Bay of Bengal. The abundance of both living (rose-Bengal stained) as well as dead agglutinated benthic foraminifera and their morphogroups, has been studied in a total of 42 core-top samples from the depth range of 25-2540 m, covering the entire shelf and slope. Agglutinated foraminifera are divided into four morphogroups (A-D). Out of these, two morphogroups, namely B and C are sub-grouped into (B1-B4) and (C1-C2), respectively. We report exceptionally high agglutinated benthic foraminiferal abundance on the slope of the western Bay of Bengal. The depth and temperature affect the total abundance of agglutinated benthic foraminifera. Morphogroups A (tubular), B1 (globular), and B3 (planispiral/trochospiral, only dead) are positively correlated with water depth and negatively correlated with the ambient temperature, suggesting its preference for deeper waters. Both the dead and living assemblages of morphotype B2 as well as C1, are negatively correlated with dissolved oxygen (DO) and positively correlated with organic carbon (%Corg) suggesting its preference for depleted oxygen environment. The morphotype B4 prefers relatively deeper depths. The morphotype C2 (quinqueloculine/milioline forms) is positively correlated with DO, temperature, coarse fraction and organic carbon/nitrogen, and negatively correlated with %Corg and water depth, thus indicating that species belonging to this morphotype prefer shallow water, oxic, coarse-grained environment. The relative abundance of morphogroup D (dead) is negatively correlated with ambient salinity, thus suggesting its preference for hyposaline conditions in the shallow marine environment. The study suggests a clear ecological preference of a majority of the agglutinated morphotypes and morphogroups and thus their potential application in paleoecological interpretations.