Seasonal changes in bathyal foraminiferal populations in response to the flux of organic matter (Sagami Bay, Japan)

Benthic foraminiferal populations through 1991–94 were investigated by taking cores and in situ observations from the submersible Shinkai 2000 in Sagami Bay, Japan (1450 m water depth). At this location, a strong spring bloom causes seasonal deposition of phytodetrital material to the sea floor. The population size of benthic foraminifera is mainly controlled by this seasonal flux of organic matter, which triggers rapid, opportunistic reproduction of the shallow infaunal taxa Bolivia pacifica and Textularia kattegatensis . We propose that these species have a one-year life cycle. The deep infaunal taxa Globobulimina affinis and Chilostomella ovoidea show less pronounced seasonal fluctuations in population size, and seem to have a life cycle longer than two years. The foraminifera migrate vertically through the sediment, down to the maximum depth to which the sediment is oxygenated. The seasonal flux of organic matter thus is the most important determinant of population size, microhabitats, and reproduction in Sagami Bay. Such foraminifera are extremely relevant in the functioning of the global carbon cycle, especially at the interface of the hydrosphere and lithosphere.