This thesis is a summary of six previously published or submitted articles on (1) the significance of the geographic distribution of Recent benthic foraminifera in the Baltic Sea and the central North Atlantic, and (2) the significance of the stratigraphic distribution of Pliocene benthic foraminifera from the Blake Plateau, western North Atlantic.

Recent benthic foraminifera in the Baltic Sea have a geographic distribution strongly correlated to the highly variable hydrographical conditions. In this area low salinity, low temperature and low CaC03 concentration are factors causing environmental stress, leading to the development of a specific brackish fauna which is characterized by low diversity and high relative abundance of arenaceous species.

Assemblages of Recent benthic foraminifera from the mid- Atlantic Ridge also show correlation to the hydrographical properties of the principal water masses, resulting in a bathymetric control of the distribution of the present species.

The Pliocene faunal assemblages observed in core V26-145 from the Blake Plateau show that prior to 3-3 Ma two different water masses influenced the coring site, North Atlantic Deep Water and a current originating in shallow water areas. At 3-3 Ma ago the latter water mass becomes replaced by the Antilles Current, which successively diminishes the influence of North Atlantic Deep Water.