Influence of seasonally deposited phytodetritus on benthic foraminiferal populations in the bathyal northeast Atlantic. the species response

Abstract

Cores were obtained with a multiple corer at a bathyal site (1320 to 1360m depth) in the Porcupine Seabight during April and July 1982. In July (but not April) the sediment surface was overlain by a layer of phytodetritus, material rapidly sedlmented from the euphotik zone following the spring bloom. The phytodetrital fraction of samples (0 to 1 cm layer of subcores; 3.46 cm2 surface area) removed from the July cores harboured dense, low-diversity populations of benthic foraminifers which resembled the phytodetritus-dwelling assemblages already described from the much deeper (4550 m) BIOTRANS site in the northeast Atlantic. Our new observations consolidate the view that phytodetritus is a microhabitat for some deep-sea benthic foraminiferal species. The bathyal populations were dominated by Alabaminella weddellensis (75 '70 of total) and also included Episton~inella exigua and Tinogullmia sp. nov. These 3 species occurred also in the BIOTRANS phytodetrital assemblages. The April san~ples and the total July samples (phytodetritus plus sediment fractions) yielded diverse foraminiferal populations of similar density and species richness. However, there were some important taxonomic dffer-ences. In particular, the 8 species consistently present in the phytodetritus were significantly more abundant in the July san~ples, while the most common species in the April sanlples (01.dmrnina sp. nov. A) was entirely absent during July. We argue that the infhtence of phytodetritus, rather than spatial variability (patchiness), was responsible for some of the differences in species abundances. Other species, however, maintain more stable populat~on densities. Our results suggest that deep-sea benthic foraminifers, like those living in shallow water, probably display a variety of life-history strategies and populdtion dynamics.