Seasonal phytodetritus deposition and responses of bathyal benthic foraminiferal populations in Sagami Bay, Japan: preliminary results from “Project Sagami 1996–1999”

The seasonal carbon cycle was studied in the bathyal environment of Sagami Bay, Japan, to determine whether “benthic–pelagic coupling” takes place in this eutrophic marginal oceanic setting. Both Japanese sea color observation satellite (ADEOS) photography and sediment trap moorings have been used since 1996 for monitoring sea surface primary production. Video records at a real time deep-sea floor observatory off Hatsushima Island in Sagami Bay were also used to monitor the deposition of phytodetritus on the sea floor. At this location, a spring bloom starts in mid-February and ends in mid-April. About 2 weeks after the start of the spring bloom, phytodetrital material is deposited on the sea floor. Video records clearly show that phytodetritus deposition has taken place in the spring of every year since 1994, even though the exact timing is different from year to year. The population size of benthic foraminifera is highly correlated to this phytodetritus deposition. The phytodetritus triggers rapid, opportunistic reproduction of the shallow infaunal taxa, Bolivina pacifica, Stainforthia apertura and Textularia kattegatensis. Shallow infaunal species mainly occur in the phytodetrital layer or just below this layer during the spring. This indicates that such opportunistic species are key indicators of phytodetrital deposition. The deep infaunal taxa Globobulimina affinis and Chilostomella ovoidea show less pronounced seasonal fluctuations in population size, but nevertheless exhibit some response to phytodetrital deposition. Thus the seasonal flux of organic matter is the most important determinant of population size, microhabitats and reproduction of benthic foraminifera in Sagami Bay.