Abstract
Foraminiferal propagule banks occur in fine sediment fractions that contain small individuals of benthic foraminifera. These sediments include locally sourced juveniles and propagules, as well as allochthonous propagules that have dispersed from surrounding areas. Such propagules can remain viable even under unfavorable local conditions. When exposed to more favorable conditions, they may grow to adult stages. Accordingly, during environmental changes, propagule banks have the potential to function as species pools and allow quick assemblage reactions. The propagule method was designed to study responses of foraminiferal assemblages by exposing propagule banks to controlled conditions in the laboratory, an approach that is applicable to a variety of ecological questions. Therefore it is important to understand the nature and dynamics of propagule banks, including local and seasonal influences. To obtain insights into the composition of local propagule banks, we studied experimentally grown assemblages from two shallow-water lagoons on Corfu Island in western Greece, and compared the results with in situ assemblages. We sampled in spring and autumn of 2017 and experimental treatments included the use of different substrates in our experiments to account for potential effects on assemblage compositions. Results revealed that sediments from each lagoon contained a distinct propagule bank. We found abundant allochthonous taxa among specimens grown in all experimental treatments, indicating dispersal of propagules, and possibly also juveniles, from adjacent regions into both lagoons. The time of sampling had a significant effect on experimental assemblages, indicating that the composition of propagule banks can vary throughout the year. However, no significant differences were found in assemblages grown in different substrata, suggesting a stronger influence of water variables (e.g., temperature or salinity) on assemblage compositions. Moreover, the experimental set-ups favored small, fast-growing, sediment-dwelling species tolerant of relatively high organic content. Our findings highlight the potential of propagule banks as species pools and will help to refine and improve future applications of the method.
Highlights
Assemblages of benthic foraminifera are widespread and nearly ubiquitous in the modern oceans
If propagules are transported into habitats outside their normal environmental conditions, they can become dormant [21], which has been observed in adult foraminifera [22], and can remain viable within the local propagule bank for at least two years [19]
We found 31 species belonging to Group 1, 17 species belonging to Group 2, 9 species belonging to Group 3 and only 1 species (Ammonia tepida) belonging to Group 4
Summary
Assemblages of benthic foraminifera are widespread and nearly ubiquitous in the modern oceans. In shallow-water, coastal environments they are found in almost all habitats and form habitat-specific assemblages, depending on environmental conditions and available microhabitats (e.g., [1, 2]). As such, they are widely applicable in ecological and paleoecological research and are increasingly used as indicators for bio-monitoring [2,3,4,5,6,7,8,9]. The fine size fraction of the sediment, contains numerous living small juvenile forms and cryptobiotic propagules. Such specimens, which can either derive from reproduction of local populations or be transported to the respective sites from adjacent areas, form the so-called propagule bank. Recent observations in the Bottsand lagoon (Baltic Sea) have provided field evidence of the presence of allochthonous propagules, as the previously absent Elphidium incertum suddenly appeared among local assemblages after a short period of increased salinity and even prevailed after conditions returned to their original state [23]
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