Reconstruction and interpretation of the local vegetational succession of a Lateglacial deposit from Usselo (The Netherlands), based on the analysis of micro- and macrofossils

Analysis of plant micro- and macrofossils from a sequence from Usselo (The Netherlands) allows a detailed reconstruction of local developments during a period of more than one and a half millennia, starting before the Bolling period and lasting till the end of the Allerod period. The sequence started in an oligotrophic shallow pool with a very low organic production in a barren sandy landscape. An early phase with Gloeotrichia type may be connected with the ability of N-fixation of these blue-green algae. This capacity may have initiated nutrient availability. Characeae played an important role on the initially sandy substrate. These pioneers opened up the site for other aquatics and helophytes, e.g. Potamogeton alpinus, Carex rostrata, Phragmites australis, Equisetum fluviatile, followed by Menyanthes trifoliata, Myriophyllum spicatum and various Carex species. This sequence indicates a gradual eutrophication of the site, accompanied by an increase in production of organic matter (interrupted during the Older Dryas), as substantiated by the organic/inorganic ratio of the substrate. The tendency towards eutrophication ended when the vegetation at the mire surface lost its contact with the ground water. The sequence ended with a vegetation type dominated by Sphagnum and the occurrence of oligotraphentous Thecamoeba. These changes in local vegetation are explicable without resort to temperature fluctuations. The nutrient status of the habitat, and the water level (the latter influenced by sand and organic sediment deposition) were apparently the most important factors in the vegetation succession.