Community structure and microhabitats of deep-sea benthic foraminifera of the North Pacific

Benthic foraminifera are a diverse group of free-living protists found in almost all marine habitats. They are an especially important element of deep-sea benthic ecosystems. As inhabitants of the abyssal sea floor, they are exposed to variable food regimes, controlled by the seasonality of primary production in the surface waters. Population density and composition can therefore differ through time following the variation in availability of food and oxygen concentration. We investigated the standing stock of benthic foraminifera in 4100 m depth at Station M (34°50’ N, 123°00’ W) from sediment cores taken in May and September 2007. The foraminiferal assemblage (>63 µm) contained more than 100 species. In both seasons (spring/fall), it was numerically dominated by soft-walled saccamminids, although the greatest diversity was found among agglutinated foraminifera. The total population density was highest at the sediment surface and decreasing with sediment depths. Distinct variation in species composition was observed between seasons. Living foraminifera were present down to 5 cm depth, with individual species often linked to preferred microhabitats. The prevalence of epifaunal and shallow-infaunal species underlines the abyssal character and indicates an environment with little organic matter flux at the sea floor. Statistical analyses (e.g. rarefaction, component analysis) show a varying distribution of diversity on a spatial and temporal scale.