Preliminary investigations into the vertical structure of propagule banks of temporary freshwater rockpools in the Maltese Islands

Abstract

The vertical structure of the propagule bank of five angiosperms (Elatine gussonei, Damasonium bourgaei, Callitriche truncata, Ranunculus saniculaefolius, Zannichellia palustris) and one charophyte (Chara vulgaris) was studied in eight ephemeral autumnal rockpools from two localities in the Maltese Islands. Distribution of seeds and oospores was distinctly non-uniform, with 94% of propagules being located in the top 4cm {)f sediment. Propagules that were recovered from depths exceeding 4cm are unlikely to have been buried in situ but are probably due to percolation from the surface during the dry season when cracks and fissures in desiccated sediment provide such a route. Operation of this process is indicated by over-representation of the smallest propagules in the deepest strata. The presence of large numbers ofpropagules in the surface layers is adaptive, facilitating reception of germination cues and attainment of the soil surface by young shoots. However this strategy also exposes propagules to predation as well as to high temperatures during the dry season. The data obtained in the present study suggest that quantification of contributions to and losses from the propagule bank should be the focus of subsequent work aimed at constructing a predictive model of the population dynamics of these species.