Bert van der Zwaan

EXPERIMENTAL EFFECTS OF AN ORGANIC MATTER PULSE AND OXYGEN DEPLETION ON A BENTHIC FORAMINIFERAL SHELF COMMUNITY

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 ( 4 weeks).

Temporal variability of foraminiferal densities in the northern Adriatic Sea

Abstract Over a period of two years (August 1996 to July 1998) foraminiferal abundances have been monitored at two stations in the northern Adriatic Sea. Environmental variables such as oxygen concentrations, salinity and temperature were measured and the trophic state of the seawater at both localities was calculated. Foraminiferal numbers fluctuated considerably during the two years, but not in a straightforward yearly cycle. Most taxa, however, showed abundance patterns very similar to many of the others. Despite the lack of clear annual cyclicity in foraminiferal numbers, there appears to be a strong correlation between densities of several important taxa and high bottom-water oxygenation or a less extreme trophic state of water column. There is some time-lag, however, before the biotic response to environmental changes is reflected in the faunal patterns of foraminifera >63 μm.

The dynamics of the benthic foraminiferal microhabitat: recovery after experimental disturbance

We report the results of a microcosm experiment in which we studied the effect of disturbance on the occupation of microhabitats by benthic foraminifera. After destruction of the original microhabitat patterns through homogenization of the sediment, two experimental situations were established characterized by different population density regimes. The aim was to observe the rapidity and mode of recovery of microhabitat patterns within an experimental period of 22 days. Relative abundance patterns in the field, twice monitored as control, showed that the experiment reasonably reflected the natural situation. No significant difference was found between the two experimental situations; this suggests that density-dependent processes did not affect the vertical microhabitat distribution. Based on the way the assemblages recovered from the disturbances, two groups of taxa could be identified. The first group consisted of epifaunal to shallow-infaunal, opportunistic taxa: Hopkinsina pacifica, Nonionella turgida and Stainforthia fusiformis. After the onset of the experiment especially H. pacifica and N. turgida migrated rapidly to the uppermost sediment layer, but displayed a high net mortality. The second group consisted of deep infaunal taxa: Acostata mariae, Eggerella scabra/advena and Caronia silvestrii. During the experimental period, these taxa did not display significant net upward or downward change of the distribution patterns. Apparently, they survived successfully or even increased their standing stock, in deep sediment layers. Our data suggest that especially the so-called epifaunal taxa are able to recover rapidly after physical disturbance, but that complete restoration of microhabitat patterns including the deep taxa might take a considerable time.