BENTHIC FORAMINIFERAL DISTRIBUTION AND RECOLONIZATION OF FORMERLY ANOXIC ENVIRONMENTS IN DRAMMENSFJORD, SOUTHERN NORWAY

Investigations of living (stained) benthic foraminifera in the surface (0-1 cm) sediments along a depth transect in Drammensfjord, southern Norway, have been carried out on samples collected in 1984 and during all four seasons in 1988. The transect runs through strongly variable environments from a well oxygenated, brackish surface layer to anoxic waters of slightly less than normal marine salinity. The objectives were to study foraminiferal recolonization patterns after a prolonged period (> 5 years) of nearly permanent anoxic bottom water conditions in the lower parts of the transect, the seasonal stability of the assemblages after recolonization, and interspecific tolerances to various environmental parameters (i.e., temperature, salinity, dissolved oxygen concentration, water depth). When the redox-boundary was at its shallowest position in the water column (30-35 m water depth; salinity 29-30 parts per thousand), Ammodiscus? gullmarensis was dominant adjacent to the anoxic areas. This represents the first record of agglutinated dominated assemblages bordering anoxic environments. It took more than one year after reaeration before the areas, where anoxic conditions had prevailed for more than five years, became suitable for colonization. By 1988, the foraminiferal standing crop had more than doubled in areas influenced by the transitional water masses and living (stained) individuals were present down to the redox-boundary. Additionally, four species, which were not found along the transect in 1984, had been introduced. These immigrants had probably been transported into the area in suspension from the south. Stainforthia fusiformis was the first and most successful species to recolonize the formerly anoxic areas and it showed exceptionally high densities in samples collected a few meters above the redox-boundary. After recolonization, all species showed a distinct depth succession which, for most of them, prevailed throughout the year. Possible lack of seasonal population fluctuations in several species is thought to be due to a permanently plentiful food supply. The nine abundant species have been ranked in accordance with their interspecific tolerance to increasing euryhaline and eurythermal environmental conditions.