Cyanobacteria and testate amoeba as potential proxies for Holocene hydrological changes and climate variability: Evidence from tropical coastal lowlands of SW India

Cyanobacteria and thecamoebians are the two major constituents of Non Pollen Palynomorphs (NPP) found in the palynological preparations. The former is well represented in the sediment archives while the latter has restricted occurrence in the Holocene sequence apart from the conventional organic matter of pollen and spores. Well preserved morphotypes of Gleotrichia and Rivularia are the best forms of cyanobacteria found in almost all the studied profiles and their abundance towards Early to Middle Holocene coincides with the heavy rainfall and wet climate conditions during the Holocene climate optimum (HCO). Contrary to cyanobacterial diversity, thecamoebians are restricted to Late Holocene in most of the studied profiles and their occurrence is correlated with the prevailing dry and stressful environmental conditions. The major elements of thecamoebians identified belong to Arcellaceans, Centropyxids and Cucurbitella types. Abundance of Centropyxids and Arcellaceans in Varapuzha, West Kadungallur, Koonamavu, Pachcha and Ernakulam indicates low humidity and low precipitation. However, the relative abundance of Centropyxids in Mid Holocene at Ernakulam and Koonamavu reflects the early colonization of this type competing with the cyanobacteria, whereas, Arcellaceans are essentially confined to Late Holocene. Dominance of cyanobacteria and its proliferation is directly related to freshwater influx of the rainfall as well as the local hydrological regimes associated with the active aquifers controlled by the geomorphological features of the region in which the wetland system sustained. Considering the signatures of cyanobacteria and thecamoebians observed in the sediment archives their occurrence in the wetlands during the Holocene corresponds to the hydrodynamic regimes brought in by the freshwater influx as a result of monsoon variability. The well preserved cyanobacterial signatures in sediment archives of Kerala coast and their scarcity or even absence elsewhere along the west coast of India indicates that they are exclusively adapted to the wetland habitats where the hydrodynamic regimes are regularly controlled by fresh water influx in the form of extended rainy days of the monsoon system.