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

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.