Abyssal hills: Influence of topography on benthic foraminiferal assemblages

Paris V. Stefanoudis,Brian J. Bett,Andrew J. Gooday

doi:10.1016/j.pocean.2016.09.005

Paris V. Stefanoudis, Brian J. Bett + Show 1 more

Open Access

https://doi.org/10.1016/j.pocean.2016.09.005

Copy DOI

Abstract

Abyssal plains, often thought of as vast flat areas, encompass a variety of terrains including abyssal hills, features that constitute the single largest landscape type on Earth. The potential influence on deep-sea benthic faunas of mesoscale habitat complexity arising from the presence of abyssal hills is still poorly understood. To address this issue we focus on benthic foraminifera (testate protists) in the >150-μm fraction of Megacorer samples (0–1cmlayer) collected at five different sites in the area of the Porcupine Abyssal Plain Sustained Observatory (NE Atlantic, 4850mwater depth). Three sites are located on the tops of small abyssal hills (200–500m elevation) and two on the adjacent abyssal plain. We examined benthic foraminiferal assemblage characteristics (standing stock, diversity, composition) in relation to seafloor topography (hills vs. plain). Density and rarefied diversity were not significantly different between the hills and the plain. Nevertheless, hills do support a higher species density (i.e. species per unit area), a distinct fauna, and act to increase the regional species pool. Topographically enhanced bottom-water flows that influence food availability and sediment type are suggested as the most likely mechanisms responsible for these differences. Our findings highlight the potential importance of mesoscale heterogeneity introduced by relatively modest topography in regulating abyssal foraminiferal diversity. Given the predominance of abyssal hill terrain in the global ocean, we suggest the need to include faunal data from abyssal hills in assessments of abyssal ecology.

Highlights

The abyssal zone ($3500–6500 m water depth) occupies 27% of the entire ocean depth range as well as almost 65% and 85% of Earth’s surface and ocean floor, respectively (Watling et al, 2013; Harris et al, 2014)
The aim of this study was to examine the potential effects of seafloor topography on benthic foraminiferal assemblages from the Porcupine Abyssal Plain Sustained Observatory area (PAP-SO, Hartman et al, 2012) in the Northeast Atlantic (4850 m water depth), a largely flat area populated by a number of abyssal hills
The questions we sought to answer in this study were: do abyssal hills modify the (i) density, (ii) diversity, (iii) and species composition of foraminiferal assemblages, and if so, (iv) is mesoscale diversity enhanced? we recorded enhanced density and diversity on hills, these differences were not statistically significant

Summary

Introduction

The abyssal zone ($3500–6500 m water depth) occupies 27% of the entire ocean depth range as well as almost 65% and 85% of Earth’s surface and ocean floor, respectively (Watling et al, 2013; Harris et al, 2014). Only an estimated 1.4 Â 10–9% of this large biome has been sampled to date (Stuart et al, 2008). Most of that sampling effort has been focused on abyssal plains, topographically flat (i.e. homogeneous) soft-bottom areas of the ocean (Heezen and Laughton, 1963). Marine geologists and geophysicists have reported the presence of numerous abyssal hills, small topographic rises 40% of the abyssal

Objectives

Methods

Results

Discussion

Conclusion