INTRA-ANNUAL CHANGES IN LIVING DEEP-SEA BENTHIC FORAMINIFERA ON THE SOUTHERN CALIFORNIA MARGIN: SEASONAL MIGRATION?

There are many well-known migrations of vertebrates on land and in the air and sea. Here we present evidence that there may also be seasonal migrations of single-celled organisms in the deep-sea. This study investigated the distribution of single-celled organisms, benthic foraminifera, within deep-sea sediments. Living (Rose Bengal stained) benthic foraminifera were examined in the >150 µm fraction of samples collected on the Southern California Margin (1330 m) every three months, from October 1995 to October 1996. Results reveal intra-annual variability in the foraminiferal assemblages between times with high surface-water productivity (May 1996 and October 1996) and those with lower productivity (October 1995 and August 1996). Abundances of benthic foraminifera were higher in samples collected during periods of higher surface productivity (May 1996 and October 1996) than those collected during periods of lower surface productivity (August 1996). In addition, foraminiferal species showed different temporal patterns of vertical distribution. During times of increased food supply (May 1996 and October 1996) deep infaunal species Globobulimina pacifica, Globobulimina hoeglundi and Chilostomella oolina showed peak abundances at 1-4 cm depth, whereas at times of low food supply (August 1996), these species were concentrated deeper down into the sediment (4-10 cm). In contrast, other species, such as Uvigerina proboscidea and Cibicidoides mckannai moved downward during May 1996 and October 1996. The distributional patterns of some species, such as Hoeglundina elegans did not change appreciably throughout the sampling period. Although longer records are required to confirm these patterns, the intra-annual variations in the distribution of benthic foraminiferal species suggest that some species migrate vertically in response to seasonal inputs of organic matter and associated changes in oxygen concentration.