The near-bottom plankton community at the Porcupine Abyssal Plain, NE-Atlantic: Structure and vertical distribution

Abstract

The study presents results on the composition and vertical distribution of the near-bottom plankton community at an abyssal site in the NE Atlantic. Plankton samples were collected at 1, 15, 50 and 100 m above bottom (mab). Whereas the composition within the upper three layers was very similar, a major shift occurred in the immediate vicinity of the seafloor. Between 100 and 15 mab, the plankton was dominated by Copepoda, making up more than 75% of the total abundance and biomass (without gelatinous organisms). At 1 mab, Copepoda were still abundant, but their share decreased to ca. 50%, while Polychaeta, Malacostraca and Chaetognatha became important groups. Within the Copepoda, the predominance of the genus Metridia (Calanoida) in the upper layers was replaced by the genus Benthomisophria (Misophrioida) at 1 mab. Despite enrichment in organic particles towards the bottom, the total abundance and biomass of plankton did not show marked differences between the four layers investigated. Several hypotheses are discussed which may explain why the presumably higher food concentrations near the deep-sea floor do not lead to increased standing stocks of the plankton community.

Near seafloor bioluminescence, macrozooplankton and macroparticles at the Mid-Atlantic Ridge

“…However, at these sites the authors suggested low biomass values may have made differences in biomass more difficult to detect. Strong taxonomic shifts with decreasing height above the seafloor have also been documented (Childress et al, 1989;Angel, 1990;Mees and Jones, 1997;Dauvin and Vallet, 2006), even in areas with low values and weak biomass depth gradients (Christiansen et al, 1999(Christiansen et al, , 2010. In the present study, although bioluminescent events could only be classified into broad faunal groups, different altitude patterns were discerned.…”

“…Other studies with good sample sizes confidently report faunal increases towards the seafloor (Hargreaves et al, 1984;Roe et al, 1990;Robison et al, 2010), while others report increases only in specific faunal groups (Domanski, 1986;Childress et al, 1989;Vinogradov, 2005). There are also reports of no significant increase in biomass or abundance (Roe et al, 1990;Christiansen et al, 1999Christiansen et al, , 2010, although weak trends may be noted. The non-universality of increased near seafloor biomass and abundance is not surprising.…”

“…However, weak altitude effects in abundance and biomass in the BBL have been previously reported. In the NE Atlantic weak vertical gradients were found in the biomass of near bottom zooplankton (Christiansen et al, 1999(Christiansen et al, , 2010. However, at these sites the authors suggested low biomass values may have made differences in biomass more difficult to detect.…”

“…However, reports of enhanced plankton biomass and abundance in the BBL are not universal (Wiebe et al, 1988;Roe et al, 1990;Christiansen et al, 1999;. Several hypotheses for the absence of an enhanced near seafloor layer have been proposed including top-down control of the zooplankton population by higher-order predators, low nutritional value of organic particles at abyssal depths and in oligotrophic regions, and sampling gear bias (Christiansen et al, 2010).…”

A Multidisciplinary Approach for Generating Globally Consistent Data on Mesophotic, Deep-Pelagic, and Bathyal Biological Communities

“…The organisms that inhabit the BBL can spend all or multiple periods of their lives in this zone. The hyperbenthic community composition differs significantly from that of the water column above it (Christiansen et al, 2010). These animals represent potential prey for benthic, pelagic, and demersal species, coupling pelagic and benthic food webs.…”