Palaeoecology of a medieval fishpond system (Vajgar, Czech Republic)

Abstract

A palaeoecological investigation of a sediment core obtained from an artificial lake — the fishpond Vajgar (40 ha) near the town of Jindřichův Hradec, southern Bohemia, has been carried out in connection with extensive multidisciplinary research preceding the removal of its sediment. Samples from selected standard profile were subjected to chemical, pollen, and palaeoalgological analyses in order to study the chronology of vegetation development, anthropogenic impact on the landscape, and the development of an artificial fishpond ecosystem. The palaeoecological record under study comprises the period of extensive change from pre-cultural, silver fir-dominated forest vegetation into managed, agricultural landscape dominated by cereal fields and pastures. After Medieval colonization, the proportion between wooded and open landscape changed, and the composition of original forests shifted to pine. Pollen spectra from the High Middle Ages are characterized by high percentages of anthropogenic indicators and closely resemble the results from urban deposits (cesspits, wells, communication horizons, etc.). This period can be subdivided into a first phase correlated with the High and Late Middle Ages and a second phase correlated with the post Medieval period and partly maybe also with the agricultural collectivization of the 20th century. The earlier phase has a rather uniform character with a significant presence of grazing indicators and cereal weeds. After the collectivization of the 1950s and later, a substantial change in agricultural practices can be detected: the large-scale use of herbicides affecting the composition of agricultural ruderal species (e.g. decline of cornflower (Centaurea cyanus), rye (Secale), etc.). The development of the aquatic system of the fishpond Vajgar was studied by means of chemical and palaeoalgological analyses. On the basis of observed changes in the composition of subfossil algal communities, the fishpond history can be split into four distinct phases. It is assumed that the progression over individual phases was caused by changes in water chemistry that can be interpreted as shifts in water quality caused by the urbanization of the surrounding areas as well as by changes in agricultural practices. The results of the investigation of some chemical elements (especially heavy metals) show stratigraphic patterns that can be related to historical events. These results show that the mixing and bioturbation of the sediment was not so significant as might be expected.