Benthic foraminiferal assemblage at a North Pacific deep-sea site

Benthic foraminifera play a very important role in the food web of the deep sea. Earlier investigations have shown that they build a very diverse and abundant group within the meiofaunal fraction. Samples were taken in September 2007 at Station M (34° 50‘ N, 123° 00‘ W), a 4000m deep observation station off the coast of California, U.S.A. The sampling was part of an in-situ feeding experiment with 13C-labeled food, investigating the reaction of the benthic consumers to a simulated food impulse composed of diatoms. Four days after food exploitation to the deep-sea floor, two untreated and two food-exposed cores were taken, sliced (1cm thickness) and frozen. Later in the laboratory, sediment samples were sieved (63µm) and wet-picked with a separation of the living and dead fauna.

First results show that the benthic foraminiferal assemblage is dominated by allogromids which were showing different morphologies, test shape and colour. High abundant agglutinated species were Rhabdammina, Hormosinelloides/Reophax, Lagenammina and Trochammina. While surface samples showed also a high percentage of bar-shaped agglutinated species, deeper samples showed
less bar-shaped but more spherical species such as Psammosphaera. Calcareous foraminfera showed a high percentage of rotalids while the miliolid fraction was dominated by Spiroglutina and Pyrgo. Species diversity and abundance of the benthic foraminifera decreased with depth.