Changes in standing stock and vertical distribution of benthic foraminifera along a depth gradient (58–2750 m) in the southeastern Arabian Sea

Abstract

Benthic foraminifera are a reliable proxy to reconstruct past oceanographic conditions. The ecological preferences of benthic foraminifera are usually studied from the core-top samples. The habitat of living benthic foraminifera, however, can be much deeper. Here, we document the vertical distribution of living benthic foraminifera from the Gulf of Mannar, Southeastern Arabian Sea, to find out the sediment depth that contains representative living assemblage and to understand the relationship between ecological parameters and foraminiferal abundance. Additionally, as the region has a well-developed oxygen minimum zone, we also assess the effect of dissolved oxygen, organic carbon (%Corg) and other associated parameters on living benthic foraminifera. The multi-core samples were collected along a longitudinal transect covering the shelf, and slope (58–2750 m), during the post southwest monsoon season. We report that at stations where dissolved oxygen is low (≤1.5 mL/L), >85% of the total living benthic foraminiferal population is confined to the top 0–1 cm section of the sediment and the average living depth decreases up to 0.6 cm. However, in the well-oxygenated waters, especially in the lower slope region, a substantial fraction of the total living benthic foraminiferal population inhabits deeper sections of the sediments. The calcareous benthic foraminifera dominates the foraminiferal population both within and outside the oxygen minimum zone. The absolute abundance of agglutinated benthic foraminifera is more on the continental slope, as compared to the shelf. The infaunal and epifaunal calcareous and agglutinated foraminifera show a different trend. We report that while the bottom water dissolved oxygen and depth controls the absolute abundance of benthic foraminifera, both the %Corg and ambient temperature, affect the relative abundance of living calcareous and agglutinated benthic foraminifera in the Gulf of Mannar.