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 we re to study foraminiferal recolonization patterns after a prolonged pe riod (> 5 years) of nearly permanent anoxic bottom water conditions in the lower parts of the transect, the seasonal stability of the assemb lages after recolonization, and interspecific tolerances to various en vironmental parameters (i.e., temperature, salinity, dissolved oxygen concentration, water depth). When the redox-boundary was at its shallo west position in the water column (30-35 m water depth; salinity 29-30 parts per thousand), Ammodiscus? gullmarensis was dominant adjacent t o the anoxic areas. This represents the first record of agglutinated d ominated assemblages bordering anoxic environments. It took more than one year after reaeration before the areas, where anoxic conditions ha d prevailed for more than five years, became suitable for colonization . By 1988, the foraminiferal standing crop had more than doubled in ar eas influenced by the transitional water masses and living (stained) i ndividuals were present down to the redox-boundary. Additionally, four species, which were not found along the transect in 1984, had been in troduced. These immigrants had probably been transported into the area in suspension from the south. Stainforthia fusiformis was the first a nd most successful species to recolonize the formerly anoxic areas and it showed exceptionally high densities in samples collected a few met ers above the redox-boundary. After recolonization, all species showed a distinct depth succession which, for most of them, prevailed throug hout the year. Possible lack of seasonal population fluctuations in se veral species is thought to be due to a permanently plentiful food sup ply. The nine abundant species have been ranked in accordance with the ir interspecific tolerance to increasing euryhaline and eurythermal en vironmental conditions.