Sander Ernst

Benthic foraminifers: proxies or problems?: A review of paleocological concepts

In this paper, we review benthic foraminiferal distribution patterns in the context of their use as proxy to reconstruct paleoenvironments, in particular against the background of relevant biological data. These data suggest that benthic foraminifera, as most microbiota, are not stenotopic to most environmental variables. A more generalist strategy seems beneficial since the low degree of specialisation prevents rapid extinction. This immediately suggests that parameters like temperature and salinity are not very important in benthic foraminiferal distribution and thus not in proxy studies. Oxygen and organic flux, on the other hand, are of great ecological importance and it is not surprising that all viable proxies today are based on relationships with these parameters. Organic flux (food) is important in deciding on abundance, but is subordinate as soon as oxygen starts to be limiting. This is the case in almost all shallow water systems with muddy substrates, sometimes down to considerable waterdepth. Microhabitat patterns are shaped through the arrangement of species along a redox gradient, whereby species distribution seems to be correlated with specific redox levels. It cannot be excluded that a relationship with specific bacterial suites also plays a role here. However, the coupling to the successive redox stages provides a means for very detailed reconstructions of oxygenation. On the other hand, organic flux reconstructions can be distorted due to the fact that the same flux levels, though with different oxygenation, are characterised by different suites of species. Therefore, assemblage characteristics are discussed as additional means to constrain organic flux reconstructions. It is concluded that species distribution with depth is mainly a function of organic flux and oxygenation. In this sense, paleobathymetry should not be based on individual marker species, but preferably on quantitative assemblage characteristics such as P/B ratios. However, more generalised schemes of assemblage successions with depth are clearly helpful. It is further concluded that benthic foraminiferal distribution, and microhabitat occupation, are regulated by the interplay of organic flux, oxygen and competition. Each of these factors is acting in a different way and leads to the complex pattern as found in living associations.

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).

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.