Abstract
Foraminiferal assemblages from a shelf environment (32 m water depth) in the northern Adriatic Sea were incubated in 26 mesocosms, in which six different environmental conditions were created. A number of mesocosms were sealed to initiate anoxic conditions, and different doses of organic matter were added under both the oxygenated and anoxic conditions. The mesocosms were harvested three times during an experimental period of two months and foraminifera were studied in the upper 3 cm of the sediment.
Certain taxa (Stainforthia fusiformis, Nouria polymorphinoides, Hopkinsina pacifica, Nonionella turgida) responded immediately to the anoxic conditions: standing stocks declined and migration towards the sediment-water interface was observed. Other taxa (Caronia silvestrii, Epistominella vitrea, Acostata mariae) only reacted to a pulse of organic matter. These taxa are also found in deeper infaunal habitats, but were observed to migrate towards shallower habitats. The quantity of organic matter appeared to be an important factor: the highest dose resulted in higher densities of certain taxa. Some taxa (H. pacifica, S. fusiformis) increased in abundance under anoxia when labile organic matter was present. A number of other taxa were not affected by organic flux; perhaps one of these (N. turgida) depends on another food source, e.g., bacteria, since it migrated to shallower depth in the anoxic treatments. A last group (Bolivina spp., Eggerella spp., Bulimina marginata) appeared to be less affected by or showed no clear response to the induced environmental changes.
In total, five different groups of foraminiferal taxa were distinguished, based on their response to the treatments. Oxygen depletion induced strong changes in the vertical distribution and density of the foraminiferal taxa over a short-term period (< 2 weeks). The addition of organic flux was more important in maintaining composition and density of the assemblage over a long-term period (> 4 weeks).
