Subtle variation in abyssal terrain induces significant change in benthic megafaunal abundance, diversity, and community structure

Brian J Bett,Jennifer M Durden,Henry A Ruhl

doi:10.1016/j.pocean.2020.102395

Brian J Bett, Jennifer M Durden + Show 1 more

Open Access

https://doi.org/10.1016/j.pocean.2020.102395

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Abstract

Bathymetric gradients in the deep sea are known to affect key benthic community characteristics such as diversity. However, most studies investigate large-scale bathymetric variation, while habitat heterogeneity related to modest bathymetric variation has generally been overlooked because of limitations to sampling technology. We investigate the role of modest bathymetric variation (~10 m water depth intervals) on an abyssal hill, and horizontal variation at the 0.1–10 km scale, in the structuring of abyssal megafaunal assemblages. We assess numerical density, biomass density, diversity, and assemblage composition using seabed photographs captured with an autonomous underwater vehicle and sediment characteristics determined from cores. We detect significant differences in sediment particle size and organic carbon content, in relation to modest topographic elevation, with a greater fraction of fine particles and organic carbon on the abyssal plain than the hill. Total megafaunal numerical and biomass density, diversity, and the numerical densities of feeding groups were significantly different with modest topographic elevation; similarly, megafaunal composition varied significantly between ~10 m depth intervals. In relation to mesoscale horizontal variation, we also record significant differences between megabenthic communities in two abyssal plain areas with no significant differences in measured sedimentary characteristics and only a 2 m difference in water depth. Differences in these communities were detected in terms of dominance, assemblage composition by density and biomass, and numerical densities of feeding groups. These observations strongly indicate that previous general concepts of the abyssal environment greatly underestimate this mesoscale heterogeneity, such that beta- and gamma-diversity in the abyss may be higher than estimated. Importantly, these results also have clear implications for the design and interpretation of environmental survey and monitoring programmes in the abyss.

Highlights

Bathymetric gradients in the deep sea are known to affect key benthic community characteristics such as diversity (Flach et al, 1998), zonation (Carney, 2005; Carney et al, 1983; Olabarria, 2005), body size (Rex & Etter, 1998; Rex et al, 2006), and standing stocks (Lampitt et al, 1986)
We investigate the role of modest bathymetric variation (~10 m water depth intervals), and horizontal variation at the 0.1–10 km scale, in the structuring of abyssal megafaunal assemblages in the well-studied seafloor environment of the Porcupine Abyssal Plain Sustained Observatory site (PAP-SO; Hartman et al, 2012)
The near-seabed suspended particle load, measured as turbidity, differed significantly with depth group (ANOVA F[6,85659] = 1168, p < 0.001). It was significantly increased over elevated terrain relative to the abyssal plain, with the magnitude of the increase being in the range 5–10%

Summary

Introduction

Bathymetric gradients in the deep sea are known to affect key benthic community characteristics such as diversity (Flach et al, 1998), zonation (Carney, 2005; Carney et al, 1983; Olabarria, 2005), body size (Rex & Etter, 1998; Rex et al, 2006), and standing stocks (Lampitt et al, 1986). Major bathymetric features in the deep ocean are known to affect the structure and function of benthic communities at large scales, such as noted in seamount studies Determining at what scale that heterogeneity becomes important (i.e. ecologically significant) is key to designing and executing an effective environmental survey that has sufficient power to detect any spatial and/or temporal change

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