Seasonal variation of recent benthic deep-sea foraminifera in the Northeast Pacific

Benthic foraminifera are important contributors to the faunal community and diversity in deep-sea sediments. Abyssal plains are often characterized by oligotrophic conditions with seasonal increased primary production in the surface waters and a resulting higher flux of organic matter to the sea floor. Population density and distribution of foraminifera are mainly influenced by the availability of food and the oxygen concentration in the bottom water. Therefore, foraminiferal assemblages could in theory mirror the unstable delivery of food as differences in the composition and density of the resident fauna. To study the influence of different seasons on the foraminiferal community, we investigated the standing stock of benthic foraminifera at 4,100 meter depth in the Northeast Pacific (Station M, 34°50’ N, 123°00’ W) in September (fall) 2007 and May (spring) 2008. Sediment cores of 7 cm inner diameter were taken within close vicinity of each other, sliced on board in 1 cm steps and frozen (-20°C). Living and dead foraminifera were identified, classified to species level and counted. The total foraminiferal assemblage (>63 µm) contained about 200 taxa and showed a higher number of species in fall (~190 taxa) than in spring (~150 taxa). In both seasons, living foraminifera were present down to 5 cm sediment depth with highest total numbers at the sediment surface and a clear decrease with sediment depth. Individual species showed microhabitat preferences with the epifaunal and shallow-infaunal habitats being most common. During spring mainly soft-walled and agglutinated species contributed to the living fauna while in fall soft-walled foraminifera were found to clearly dominate in individual numbers. Dead foraminifera showed no trend in their vertical distribution and predominantly agglutinated tests were found. Calcareous foraminifera were also present, with few main species and numerous species with single appearances. Variation in species composition and population density of foraminifera will be compared statistically on a spatial and temporal scale. In summary, we found the foraminiferal community at Station M to reflect a typical abyssal environment in terms of species composition, density and microhabitats. Spatial patchiness has been observed as well as seasonal variation in the benthic foraminiferal assemblage.