Temporal changes (1989-1999) in deep-sea metazoan meiofaunal assemblages on the Porcupine Abyssal Plain, NE Atlantic

Trends among major metazoan meiofaunal taxa were investigated based on 56 deployments of a multicorer at 10 time points over a period of eleven years (1989-99) at a site on the Porcupine Abyssal Plain Sustained Observatory (PAP-SO: 48°50 ́N 16°30 ́W, 4850 m depth). This area is characterised by a strong seasonality in the deposition of organic matter to the seafloor and by the massive increase in the density of holothurian species since 1996, the so-called ‘Amperima event’. Total meiofaunal densities ranged from 346 to 1074 ind. 10 cm and showed a significant increase with time when time was represented by cruises, years and the ‘Amperima period’ (1996-99) vs. the pre-Amperima period (1989-94). This pattern was driven mainly by the nematodes, which were the dominant taxon (~90% of total abundance). The third most abundant group, the polychaetes, also increased significantly in abundance over the time series, while the ostracods showed a significant decrease. Most other taxa, including the second-ranked group, the copepods (harpacticoids and nauplii), did not exhibit significant temporal changes in abundance. Ordination of taxon composition showed a shift from the pre-Amperima to the Amperima periods, a trend supported by the significant correlation between the x-ordinate and time. The majority (52-75%) of meiofaunal animals inhabited the top 2 cm of the 5 cm sediment cores analysed. There were significant increases in the proportion of total meiofauna, nematodes and copepods (but not polychaetes) inhabiting the 0-1 cm layer over time (represented by cruises) and between the pre-Amperima and Amperima periods in the case of copepods and polychaetes. During the intensively sampled period (1996-97), there were indications of seasonal changes in the vertical distribution patterns of total meiofauna and nematodes within the sediment.