Retarded response by macrofauna-size foraminifera to phytodetritus in a deep Norwegian fjord

Due to the scarcity of information concerning the role that large foraminifera play in deep-sea carbon cycling, the response of a foraminiferal community (>250 μm) to a simulated phytodetritus sedimentation event was assessed over two weeks using sediment cores collected from a deep-fjord environment. Sediment cores were collected from ~700 m water depth in the Korsfjorden, western Norway, and incubated ex situ with 1 g Corg m−2 of labile 13C-labeled Skeletonema costatum for 2, 7, and 14 days. We selectively picked (without prior staining) cytoplasm-containing foraminifera and found the foraminiferal community to be largely dominated (91%) by the deep-dwelling species Globobulimina turgida and Melonis barleeanum, as well as the shallow infaunal species Hyalinea balthica. None of the >250 μm, cytoplasm-containing fraction was involved in carbon uptake during the first 7 days. After 14 days, 3% of the foraminiferal samples possessed δ13C signatures indicative of carbon uptake, but the uptake was confined to the surface-living (0–2 cm) G. turgida. Foraminifera contributed 2.4% to faunal carbon uptake (foraminifera plus macrofauna) after 14 days, despite making up 24% of the combined biomass. Both the dominance of deep-infaunal species, such as G. turgida and M. barleeanum (68%), which are known to prefer degraded over more labile material, together with their large size, which often makes foraminifera respond slower to phytodetritus deposition than the more abundant, smaller-size foraminifera, are possible reasons for the retarded response observed. Overall, results from this investigation highlight that the response of large-size foraminifera to phytodetritus deposition is very slow. In addition, the results presented provide evidence that the foraminiferal response is most likely driven by differences in foraminiferal community composition and structure, with large, deep-infaunal species showing slower reactions to phytodetritus deposition compared to smaller foraminifera, metazoan meiofauna, and macrofauna.