Living benthic foraminiferal assemblages of a transect in the Rockall Trough (NE Atlantic)

Abstract

Foraminifera are unicellular protists and play a major role in the deep-sea ecosystems. This study focusses on the faunal analysis of living benthic foraminiferal assemblages and their vertical distribution along a transect in the Rockall Trough in the NE Atlantic. Therefore, seven multicorer cores from the Rockall Trough (RT-1 – RT-7), NE Atlantic, were collected at water depths between 1008 and 2205 m on the RRS Discovery cruise DY051 in spring 2016.Surface sediment samples of the collected material were analysed up to 10 cm sediment depth for living (Rose Bengal stained) benthic foraminifera to gain insight into the recent distribution of the foraminiferal fauna. The cores were sliced from 0 to 3 cm into 0.5 cm intervals and from 3 to 10 cm in 1 cm slices. Sediments were divided into the fractions >125 μm and 125–63 μm by wet-sieving. The analysis showed that stations differed in live assemblages, diversity, abundances and vertical distribution. Standing stock values of 92–168 ind./10 cm2 for the total, 54–130 ind./10 cm2 for the calcareous and 24–59 ind./10 cm2 for the agglutinated assemblages were obtained.Living deep-sea foraminiferal assemblages differed between the three shallower (RT-1 – RT-3) and the four deeper sites (RT-4 – RT-7), indicating that the different water masses could be a main factor controlling the deep-sea benthic fauna in the Rockall Trough. This area is characterised by two main water masses: the upper more saline Eastern North Atlantic Water (ENAW) and the lower North Atlantic Deep Water (NADW).Basically, Nonionella iridea occurred at all sites, while Cibicidoides species (except C. wuellerstorfi) and Melonis barleeanus were limited to water depths between 1008 and 1610 m (RT-1 – RT-3). Soft-shelled foraminifera increased sharply with water depth. Saccamminid sp. 1 contributed significantly to this increasing proportion of soft-shelled individuals with its maximum at 1857 m water depth (RT-5). Further characteristics of this transect include a mass occurrence of Uvigerina mediterranea, which was only present at 1008 m (site RT-1) or Globobulimina sp. 1 with its highest densities at 1610 m (RT-4), where it accounted for 20% of the total living foraminiferal fauna. The general microhabitat preferences where consistent with those of other studies, although the Rockall Trough is a very dynamic region considering food supply or other physical parameters.