Foraminiferal fauna from a deep basin in Gullmar Fjord: The influence of seasonal hypoxia and North Atlantic Oscillation

Historical benthic foraminiferal assemblages were analysed in two sediment cores recovered from the deepest basin of Gullmar Fjord by a Gemini and a gravity corer. A compilation of faunal data corresponding to the period 1935–2011 is presented together with the bottom water temperature, salinity and dissolved oxygen records for the same period. This study adds new information for the last decade (2001–2011) and investigates foraminiferal fauna after a prolonged period of positive North Atlantic Oscillation (NAO) and seasonal hypoxia (1979/80–1999). The earliest part of the record (ca. 1935–1998) reveals faunal changes similar to those reported in previous studies. The typical Skagerrak–Kattegat (S–K) faunas, which were present until ca. 1980–83, were replaced by Stainforthia fusiformis, often reported from environments with coastal hypoxia. A close relationship was noted between bottom water renewals, dominating weather regime (the NAO index) and geostrophic winds. After 1998, the NAO changed towards less extreme values, which facilitated regular bottom-water exchange events and the re-appearance of some S–K species: Cassidulina laevigata, Textularia earlandi and Bulimina marginata. However, the most recent severe hypoxic event (around 2008) again triggered the increase of low-oxygen tolerant species and interrupted the short re-establishment process of the S–K fauna.

► Foraminiferal faunas co-vary with the NAO and hydrographical parameters.
► Some Skagerrak–Kattegat species re-appear at better bottom-water ventilation.
► Stainforthia fusiformis quickly utilises the variations in dissolved oxygen content.
► Reestablishment of S–K fauna was terminated by a hypoxic event in 2008.