Abyssal hills – hidden source of increased habitat heterogeneity, benthic megafaunal biomass and diversity in the deep sea

Jennifer M Durden,Brian J Bett,Daniel O.B Jones,Veerle A.I Huvenne,Henry A Ruhl

doi:10.1016/j.pocean.2015.06.006

Jennifer M Durden, Brian J Bett + Show 3 more

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https://doi.org/10.1016/j.pocean.2015.06.006

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Abstract

Abyssal hills are the most abundant landform on Earth, yet the ecological impact of the resulting habitat heterogeneity on the wider abyss is largely unexplored. Topographic features are known to influence food availability and the sedimentary environment in other deep-sea habitats, in turn affecting the species assemblage and biomass. To assess this spatial variation, benthic assemblages and environmental conditions were compared at four hill and four plain sites at the Porcupine Abyssal Plain. Here we show that differences in megabenthic communities on abyssal hills and the adjacent plain are related to environmental conditions, which may be caused by local topography and hydrodynamics. Although these hills may receive similar particulate organic carbon flux (food supply from the surface ocean) to the adjacent plain, they differ significantly in depth, slope, and sediment particle size distribution. We found that megafaunal biomass was significantly greater on the hills (mean 13.45gm−2, 95% confidence interval 9.25–19.36gm−2) than the plain (4.34gm−2, 95% CI 2.08–8.27gm−2; ANOVA F(1,6)=23.8, p<0.01). Assemblage and trophic compositions by both density and biomass measures were significantly different between the hill and plain, and correlated with sediment particle size distributions. Hydrodynamic conditions responsible for the local sedimentary environment may be the mechanism driving these assemblage differences. Since the ecological heterogeneity provided by hills in the abyss has been underappreciated, regional assessments of abyssal biological heterogeneity and diversity may be considerably higher than previously thought.

Highlights

Habitat heterogeneity resulting from broad-scale topographic features is responsible for variations in the diversity and biomass of seafloor fauna in channels (Jones et al, 2007), canyons (De Leo et al, 2010; McClain and Barry, 2010; Ramirez-Llodra et al, 2010b; Sarda et al, 1994), trenches (Paterson et al, 2009), and on seamounts (McClain, 2007)
We found significant differences in the megafaunal assemblages related to the variations in environmental conditions on abyssal hills and the adjacent plain
We have quantified the differences in megafaunal assemblages between abyssal hills and the adjacent plain, and have linked them to changes in environmental conditions, testing a fundamental ecological heterogeneity paradigm in the most common habitat on the planet

Summary

Introduction

Habitat heterogeneity resulting from broad-scale topographic features is responsible for variations in the diversity and biomass of seafloor fauna in channels (Jones et al, 2007), canyons (De Leo et al, 2010; McClain and Barry, 2010; Ramirez-Llodra et al, 2010b; Sarda et al, 1994), trenches (Paterson et al, 2009), and on seamounts (McClain, 2007). Sediment-covered abyssal hills rise up to 1000 m above the seabed (seamounts by definition reach >1000 m above the seafloor), and exist in similar densities in the Atlantic, Indian and Pacific Oceans (Yesson et al, 2011). Despite their abundance, and potential importance to spatial variation in benthic communities, abyssal hills have been rarely studied. Billett et al (2010) found detectable differences in megafaunal community composition between trawls collected $100 km apart on the PAP, but little synoptic data exists for quantitative study of abyssal heterogeneity at the scale of hills Previous studies of the PAP, including a long-term time series, have focussed on the level bottom (Bett et al, 2001; Bett and Rice, 1993; Billett, 1991; Billett et al, 2001; Billett and Rice, 2001; Brunnegard et al, 2004; Danovaro et al, 2001; Turnewitsch and Springer, 2001), but few have examined the potential spatial variation. Billett et al (2010) found detectable differences in megafaunal community composition between trawls collected $100 km apart on the PAP, but little synoptic data exists for quantitative study of abyssal heterogeneity at the scale of hills

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