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

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Abstract
Cores were obtained with a multiple corer at a bathyal site (1320 to 1360 m 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 sedimented from the euphotic 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% of total) and also included Epistominella exigua and Tinogullmia sp. nov. These 3 species occurred also in the BIOTRANS phytodetrital assemblages. The April samples 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 differences. In particular the 8 species consistently present in the phytodetritus were significantly more abundant in the July samples, while the most common species in the April samples (Ovammina sp. nov. A) was entirely absent during July. We argue that the influence of phytodetritus, rather than spatial variability (patchiness), was responsible for some of the differences in species abundances. Other species, however, maintain more stable population densities. Our results suggest that deep-sea benthic foraminifers, like those living in shallow water, probably display a variety of life-history strategies and population dynamics.

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