Living foraminiferal assemblages from the Southern California margin: A comparison of the > 150, 63–150, and > 63 μm fractions

Analyses of living (Rose Bengal stained) deep-sea benthic foraminifera from three size fractions (> 150 μm, 63–150 μm, and both combined into the > 63 μm) collected on the Southern California margin reveal changes in foraminiferal assemblages that provide clues about the ecology of some species. The top 5 cm of cores were examined from multicorer samples collected in May, August and October 1996 at a 990 m water depth site within the Southern California Bight. Replicate cores at each site were not examined, and results may be affected by spatial patchiness as well as seasonality. Calcareous foraminiferal abundances of the 63–150 μm size fraction were highest during surface bloom months (May and October) and lowest in August, while those of the > 150 μm size fraction remained relatively constant during the study. Small specimens (probably representing young individuals in the 63–150 μm fraction) and larger specimens (older individuals in the > 150 μm fraction) of most large species live in overlapping microhabitats. Older individuals were found slightly deeper in the sediment than younger individuals of some species such as Uvigerina peregrina and Bolivina spissa. Most species of foraminifera at the study site increase in abundance in one or both of the bloom months of May and October. Nonionella fragilis appears to be an opportunistic species that takes rapid advantage of fresh phytodetritus, increasing in abundance only during May and October. As a result, N. fragilis may be a useful paleoseasonality indicator. Other taxa that appear to prefer fresh phytodetritus include U. peregrina, B. spissa and Bolivina pacifica. The deep infaunal species, Globobulimina pacifica, instead increases in abundance only during August (no surface bloom). These results provide important clues about the ecology of benthic foraminifera in dysoxic, organic-rich, continental margins.