Living Foraminifera Assemblage of the West African Upwelling System

Living benthic foraminifera were studied in the highly productive West African upwelling system off Walvis Bay, Namibia. The Benguela Current System, influenced by the prevailing southeastern trade winds, governs the region's oceanography and is significantly impacted by climate variability. This results in seasonal upwelling, leading to high primary productivion and the formation of a Diatomaceous Mud Belt (DMB).  Inside this DMB anoxic and/or hypoxic conditions prevail between 150 and 450 m depths, impacting benthic foraminiferal communities (Inthorn et al., 2006). Our aims were to establish the vertical distribution patterns of foraminiferal species in the sediment (microhabitats) at each sampling station and to examine the ecological responses of foraminifera to varying oxygen conditions.

Sediment samples from NIOZ cruises 64PE449 and 64PE450 obtained by multicorer were analysed for living foraminifera identified by Rose Bengal staining. Subsamples were collected at 0.5 cm intervals from the top 2 cm of each sediment core and at 1 cm intervals from 2 to 10 cm depth. Isotopic analysis (δ¹³C) was conducted using IR-MS on cleaned specimens (20–45 µg) after ultrasonic washing to remove clays, using NFHS1 and NBS19 standards for calibration.

At the two deeper stations, at 750 and 324 m depth, bottom water oxygen concentrations were 105 and 44 µMol/L, respectively. The benthic foraminiferal assemblages were diverse and were largely limited to the top 1.5 cm of the sediment. Interspecific differences in microhabitat were limited.

At the six stations positioned around the 100 m isobath, oxygen concentrations varied between 3 and 20 µMol/L. The faunal diversity was much lower, with only five species being recorded: Bulimina elongata, Bolivina pacifica, Fursenkoina complanata, Nonionella stella, and Virgulinella fragilis. The assemblages were always strongly dominated by one or two species. At these stations, faunal penetration into the sediment was much larger, at some stations until 10 cm, again with rather limited interspecific differences in microhabitat. Remarkably, at three stations one or more conspicuous density maxima were found at depth in the sediment. This suggests at present, repetitive deposition of cm thiick sediment deposits takes place, burying living foraminiferal assemblages, which remain preserved, and stained by Rose Bengal, for some time in the deeper sediment layers.

A key finding is the significantly lower δ¹³C values observed in Virgulinella fragilis compared to co-occurring species at similar depths, in accordance to Bernhard, 2003., attributed to bicarbonate release during sulphate reduction, indicating environments with high sulphate reduction rates. Further investigation will explore these metabolic and biomineralization variations and their relationship to foraminiferal assemblages, environmental parameters, and overall ecosystem functioning within the West African upwelling system. As δ¹³C is measurable in fossil tests, it gives hope in future to interpret outlying overly negative δ¹³C values as probable anoxic metabolic pathway in case of fossil foraminifera.