Polychaete community structure and biodiversity change in space and time at the abyssal seafloor

Abstract

The deep sea is a dynamic environment over various spatio-temporal scales. But, the characteristics of deep-sea natural variations and underlying processes remain poorly understood, which prevents contextualising any anthropogenic impact on this environment. Long-term observations, from which inter-annual variations can be detected, as well as detailed broad-scale spatial observations, are scarce in the deep sea. In this thesis, I examined changes in both spatial (~ 0.1-10s km scale) and temporal (inter-annual scale) domains in abyssal benthic fauna at two abyssal stations in the North East Pacific (Station M, 1991 to 2011) and in the North East Atlantic (Porcupine Abyssal Plain, PAP, 1991 to 2012) using Polychaeta as study organisms. Four main results were found.
1) At intermediate scales (0.1 to 10s km), the macrofauna and polychaete family composition were related to the sediment grain size distribution (Chapter 2). Differences in sediment grain size distribution between and among abyssal hills created habitat heterogeneity. Differences in near-bed current flows, and thus in particle size and food settling conditions, may be the primary factors influencing the sediment particle size distribution and macrofauna. Overall polychaetes and macrofauna may be more spatially variable than previously thought in abyssal areas when considering that abyssal hills might be one of the most common topographic features on Earth.
2) At inter-annual scale, polychaete families and functional groups varied in density, diversity and community structure at Sta. M and PAP (chapter 3). Sub-surface deposit feeders, such as Paraonidae, dominated the North East Pacific, whereas surface deposit feeders, such as Cirratulidae, dominated the North East Atlantic. Differences may be related to the quantity and quality of the particulate organic carbon reaching the seafloor. Overall, changes in the dominance of functional groups imply changes in abyssal ecosystem functioning.
3) At inter-annual scale, the dynamics of polychaete abundances and body sizes suggested that migration/enhanced survivorship and recruitment events occurred (chapter 4). The densities and median body sizes of the five density-dominant species at Sta. M were analysed over time to help elucidate population dynamics as population parameters such as reproduction or growth rate could not be directly measured. The results indicated that the density dynamics of Paradoneis cf. lyra may have be related to enhanced survivorship or immigration of adult individuals, while the density dynamics of Cossura cf. rostrata may have be linked to individuals shifting between growth and reproduction.
4) At inter-annual scale, Sta. M polychaete changes were detected in standing stocks, rank distributions, species and functional group composition (chapter 5). Although a change in sampling location occurred after 2005, preventing ascribing the assemblage change unequivocally to particular environmental drivers, analyses prior to 2005 showed that the polychaete assemblage composition dynamics were primary driven by food supply variation. Overall, energetic zero-sum dynamics were not clearly observed at polychaete assemblage level, but were evident in the greater macrofauna community that includes the polychaetes. Polychaete displayed stability in body size structure in space and time, following the predictions of the metabolic theory of ecology.
In conclusion, this work highlighted that abyssal polychaete communities can change over spatial scales of tens of square kilometres and time scales of months or more; this work found a large but as yet underappreciated variability.

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