Temporal patterns among meiofauna and macrofauna taxa related to changes in sediment geochemistry at an abyssal NE Atlantic site

Two major size classes of the sediment community, meiofauna and macrofauna, and four classes of lipid compounds, fatty acids, alkanes, alcohols and st erols, were investigated using multicorer and USNEL boxcorer samples, colle cted during six cruises over a two year period (September 1996 to September -October 1998), at the Porcupine Abyssal Plain (similar to 48 degrees 50'N 16 degrees 30'W, 4850 m depth) within the framework of the MAST 3 BENGAL pr oject. This site was known to be subject to seasonality in the input of org anic matter to the seafloor. Results are given for each faunal size class i n terms of taxonomic structure at the level of phylum, class or order, depe nding on the taxon, and for the dominant faunal components in terms of dens ity and vertical distribution. For each lipid compound class, results are g iven in concentration and vertical distribution. The taxonomic structure of each size class did not change within the study period. Total meiofaunal a nd macrofaunal densities were particularly high, probably reflecting the hi gh quantity and quality of organic matter inputs to the site. The dominant components of the two size classes presented different temporal patterns in their responses to changes in their environment. Populations of mciofaunal species, a foraminifieran and an opheliid polychaete, which inhabit the su rface or sub-surface of sediment and feed on phytodetritus, responded with a rapid increase in abundance to a pulse of organic input in summer 1996. T he macrofaunal polychaetes showed a lagged response to the same event by sl owly increasing in density. Other components of the sediment community, tha t can live deeper in the sediment, moved down the sediment, column, in resp onse to 1) the impoverishment and bioturbation of the surface layer, and 2) the downward mixing of organic matter in the sediment by larger organisms. In this study, different temporal patterns were demonstrated for the first time in different size classes of the sediment community, and in the biolo gical and environmental parameters that were studied simultaneously. (C) 20 01 Elsevier Science Ltd. All rights reserved.