How does macrofaunal bioturbation influence the vertical distribution of living benthic foraminifera?

The vertical distribution of benthic foraminifera in coastal sediments seems tightly controlled by bioturbation processes. However, it still remains unclear whether living specimens found at depth around biogenic structures actively migrated from the sediment surface to reach oxygenated food-rich micro-environments or have been passively transported by macrobenthic organisms. The present study experimentally demonstrates that the vertical distribution of Ammonia tepida, one of the dominant species of foraminiferal assemblages in temperate intertidal mudflats, is tightly dependent on macrofaunal bioturbation modes and rates. The high degree of similarity between vertical profiles of foraminifera and inert particle tracers revealed that this species is not able to (1) efficiently resist biologically induced downward transport and (2) initiate rapid upward migrations to the oxic zone. The strong decrease of locomotion activity recorded in anoxic conditions suggests that encystment and inactivity are the first strategic responses to sudden oxygen depletion.