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Distribution of epiphytic diatoms on five macrophytes from a Pampean shallow lake: host-specificity and implications for paleoenvironmental reconstructions

Epiphytic diatoms are an important component of death and fossil assemblages in macrophyte-dominated shallow lakes. In the Argentinean Pampas, the lack of studies on the composition, diversity and distribution of epiphytic diatoms limits the extent of the paleoenvironmental inferences that can be made from their fossil record. We studied the composition of epiphytic diatom biofilms living on five macrophytes in a Pampean shallow lake (Nahuel Rucá, Buenos Aires Province). Samples of emergent, free-floating, rooted-floating and submersed macrophytes were collected in triplicate over one year and analyzed for diatoms. Epiphytic assemblages on the free-floating species Azolla filiculoides and Ricciocarpus natans were dominated by Lemnicola hungarica, whereas the remaining macrophytes (the emergent Schoenoplectus californicus, the submersed Ceratophyllum demersum and the rooted-floating Hydrocotyle ranunculoides) were dominated by Cocconeis placentula. Although all assemblages were dominated by epiphytic taxa, low proportions of planktonic and benthic taxa were also recorded. Composition, diversity, richness and evenness of diatom assemblages found on free-floating macrophytes differed significantly from the rest of the samples analyzed. The strong association of the epiphytic diatom, L. hungarica, with free-floating macrophytes, together with the high dominance of C. placentula on emergent and submersed plants provides a potential method for inferring past fluctuations in aquatic vegetation coverage in the palaeolimnological record of Pampean shallow lakes.