Abstract

Previous biological studies in the Northeast Atlantic have suggested that phytodetrital aggregates, which originate in the euphotic zone and settle rapidly to the seafloor following the spring bloom, constitute an important microhabitat and food source for certain bathyal and abyssal benthic foraminifera, including the rotaliids Alabaminella weddellensis, Epistominella exigua, E. pusilla and the allogromiid Tinogullmia riemanni. New data from a site at 4850 m depth on the Porcupine Abyssal Plain (49°N, 16°30′W), where phytodetritus deposition is an important phenomenon, support these observations. Here, the live foraminiferal populations which inhabit phytodetrital aggregates are dominated by E. exigua; A. weddellensis and T. riemanni are also abundant. Such species are much less common at a more southerly site on the Madeira Abyssal Plain (31°N, 21°W; 4940 m) where only traces of phytodetritus have been observed. The most common calcareous members of these assemblages share several characteristics. They have small tests with trochospiral coiling (indicating an epifaunal microhabitat preference) and thin, smooth, transparent or translucent walls. Almost certainly, they are opportunists, adapted to a fluctuating nutrient supply and able to grow and reproduce rapidly when a suitable food source (phytodetritus) is present. The best known species, Epistominella exigua, is abundant in areas of the Northeast Atlantic where phytodetritus deposition is known or believed to occur. It is tentatively suggested that the distribution and abundance of E. exigua is controlled largely by the presence of this organic material on the seafloor.

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