Population dynamics of benthic shallow-water foraminifera: effects of a simulated marine snow event

Abstract

In this microcosm experiment, we determined the population dynamics of benthic foraminifera from the northern Adriatic Sea (from 32 m water depth) and how they were affected by an artificial marine snow event (AMS). The study focussed on the performance of individuals in 2 dif- ferent size fractions: one commonly used in micropaleontological studies (with foraminifera >63 µm) and 1 with smaller individuals (38-63 µm). Microcosms were incubated for 2 or 4 wk, and under AMS or control conditions. For each, the numbers of live (Rose Bengal-stained) individuals of different spe- cies in both size fractions were counted. From this, demographic parameters (fecundity, survival, growth) of specimens in different size classes were assessed. With these data, population projection matrices were constructed for the 4 most abundant taxa (Bolivina spp., Stainforthia fusiformis, Non- ionella turgida and Hopkinsina pacifica) and all foraminifera together. From the limit properties of these matrices, asymptotic population growth rates (λ) were derived. The results indicate that num- bers of larger benthic foraminifera decline more strongly because of AMS than those of the smaller individuals, and that growth is seriously inhibited. AMS, however, enhanced fecundity. Under AMS conditions (i.e. temporary anoxia and, concomitantly, extra food), the foraminifera seem to shift to an alternative life-history pathway of rapid reproduction and limited growth. After the first 2 wk, values of demographic parameters increase under AMS conditions, resulting in quickly recovering popula- tions. In our view, these demographic characteristics are beneficial for the observed species in peri- odically stressful and disturbed ecosystems, and could explain the increasing success of these species in the progressively eutrophicated northern Adriatic Sea.