Bogumił Nowak, Barbara Brodzińska and Izabela Gezella-Nowak
Natural and economic factors of shrinkage of lakes of the Wielkopolska Lakeland
The paper discusses the issue of plant overgrowth in the littoral zone of lakes. It presents the plant encroachment process and its environmental and economic consequences based on the example of selected lakes of the Wielkopolska Lakeland. In order to estimate changes of quantity in the riparian vegetation structure, historical records of changes of reed bed areas were compared with their present areas. Historical data included bathymetric plans from the 1960's created by the Institute of Inland Fisheries and information obtained from interviews with the inhabitants of settlements located in the vicinity of the studied lakes. Present-day boundaries of reed bed zones were identified using orthophotos, aerial photographs and in situ inspections. In the course of the field research, identification of plant species and estimates of plant condition in the studied stands were carried out. Obtained data combined with data on local hydrological-meteorological conditions, water quality and land use in the direct vicinity of lakes allowed the authors to determine the conditioning factors of riparian vegetation encroachment. The collected evidence allowed the authors to establish that in the last five decades, the area of reed beds around the studied lakes increased by an average of 15-20%. This process was usually accompanied by the encroachment of hydrophytic plants consequently leading to the shrinkage of supralittoral zones. In the last 20 years, these two phenomena intensified considerably, particularly during two low water periods noted during this period. However, the encroachment of riparian vegetation proceeded differently in the case of each lake and it was most protrusive in highly eutrophicated reservoirs and those with a history of permanent or long-term lowering of the water table. The qualitative structure of the studied plant communities was conditioned mostly by nutrient inflow and the type of growing medium. In large clean lakes and in zones of sandy lake bottom, the reed beds were dominated by the common reed (Phragmitetum australis). In smaller, polluted lakes and shielded bays of larger lakes with stagnant water, beside multi-species reed beds, floating-leaf hydrophytic macrophyte plants of the genus Nymphaea, tall stands of sedges and helophytes were observed.