The composition and distribution of meiofauna and nanobiota in a central North Pacific deep-sea area

A series of in situ subsamples from four box cores taken in the central North Pacific at 5800 m were analyzed for abundance and biomass of fauna and sediment porosity. Three classes of biota were distinguished (nanobiota, meiofauna and macrofauna) primarily on the basis of taxonomic affinity rather than size. Nanobiota and meiofauna occured in the deepest layer of sediment sampled (5.5 cm); however 90% of their numbers and biomass were found in the upper 3 cm of the sediment. In addition, nanobiota were significantly richer at the sediment-water interface than at depth. Metazoan meiofauna were most abundant in more porous sediments. Their density under manganese nodules did not differ significantly from that in open sediment. A characteristic scale of horizontal dispersion for metazoan meiofauna could not be discerned because their density showed both homogeneity and heterogeneity within and between box cores. Meiofauna and nanobiota dominated the fauna in numbers (0.3 and 99.7%, respectively) and biomass (63.8 and 34.9%, respectively). Foraminifera comprised 86.6% of the meiofaunal biomass, making them the singly most important component of the entire infaunal assemblage. The relative numerical density of meiofauna and macrofauna at the site did not differ from that in a shallow-water site at Martha's Vineyard. At both sites meiofauna were two orders of magnitude more abundant than macrofauna. The relative biomass of the two faunal groups did differ, however, with dominance of macrofauna in shallow water and dominance of meiofauna in the deep sea.