Benthic foraminifera as trophic links between phytodetritus and benthic metazoans: carbon and nitrogen isotopic evidence

Abstract

We examined the trophic levels of deep-sea benthic foraminifera and metazoans based on stable carbon and nitrogen isotopic compositions of soft tissue to evaluate the role of foraminifera in deep-sea benthic ecosystems. Living benthic foraminifera and metazoans were collected from 2 bathyal sites in Sagami Bay, Japan (water depths 750 and 1430 m) on 3 occasions (April 2004, November 2004 and August 2005). Both carbon and nitrogen isotopic compositions significantly differed among the analysed foraminiferal species. At the deeper site 15 N of the benthic foraminifera ranged from 6.7 to 11.0 (typically 7 to 10 ) with considerable interspecies variations. This implies that most benthic foraminifera utilize primarily surface sediments (4.5 ) or particulate organic matter (6.4 ) as their food sources. Many metazoan meiobenthic organisms, in particular polychaetes of meiofaunal size, some harpacticoid copepods, and kinorhynchs (examined only at the shallower site), exhibited 15 N heavier than foraminifera, suggesting that they occupy higher trophic levels than benthic foraminifera. Macro-and megabenthos (spatangoids, ophiuroids and Dentalioida) exhibited 15 N of 10 to 14 , suggesting they belong in trophic levels 1 to 2 steps higher than metazoan meiobenthos and benthic foraminifera. Similar isotopic trends were observed at the shallower site. Combining the isotopic evidences and the observations on gut contents of some metazoan meiobenthos, together with previous experimental results, the benthic foraminifera in the bathyal Sagami Bay are considered a bridge in the energy flow from phytodetritus and sediments to metazoans.