Heterocysts of Rivularia Type for Interpreting a Palaeoenvironmental Context of the Late Quaternary in Northern Italy

Abstract

This paper presents new results on a quali-quantitative analysis of the heterocysts of the Rivularia type as a key bioindicator informative on local eutrophic conditions. The Rivularia type is usually reported in palynological analyses due to the thick, multilayered envelope that ensures the preservation of heterocysts in sediments. Samples come from two continuous terrestrial cores (N-S3: 77 samples, C-S1: 20 samples) drilled in the area surrounding the Bronze Age site of the Terramara S. Rosa di Poviglio (Po Plain; N Italy) and spanning at least over the last 15 ka years. Concentrations and percentages of the Rivularia type, combined with pollen curves of wet environments, describe local variability and rapid changes in ecological conditions over the millennia of deposition. Given the abundance and diversity of heterocysts of the Rivularia type in the studied samples, this paper attempts to group these cells based on morphology (ellipsoidal or elongated) and the state of preservation of the sheaths (presence or absence). Actually, it is difficult to confirm a relationship between heterocyst morphologies and the presence of different cyanobacteria species. Increasingly accurate identification of heterocysts from biostratigraphical archives may improve the data available on these bioindicators for achieving more detailed decoding of wetland (and terrestrial) transformations. Since the Bronze Age and at the most recent levels, the Rivularia type may be a good indicator of the local presence of agriculture and livestock, which lead to trophic and water changes in the soil.