2949775681

78

A. Kaniecki

same time, the surface areas of the remaining lakes decreased considerably. This is especially true of the lakes with artificially lowered water levels, for example: Goplo Lakę whose water level decreased by 270 cm, Sadlogoszcz Lakę — by 130 cm, Ptureckie Lakę — by 50 cm, Mielno Lakę — by 116 cm, Pakoskie Lakę — by 76 cm and Bronislawskie Lakę — by 66 cm (Schulemann 1865).

In order to determine the changes in the surface area of lakes within the Wielkopolska Lowland a comparison of the surface areas of 326 lakes was madę using: a) Prussian maps from the years 1890-1894 (150 sheets) and b) the corresponding contemporary Polish maps from the 1980s, on the scalę of 1:25,000. The average ratę of lakę disappearance for this period is 13% (Jutrzenka 1989). While, according to A Choiński (1992), in this century, up to 1975 in the Wielkopolska-Kujawy Lakę District the total surface area of lakes decreased by 15.21%. It is true to an even greater extent of smali water reservoirs with surface areas below 1 ha. To evaluate the ratę of their disappearance during the last 100 years their numbers were calculated on 56 topographic sheets on the scalę of 1:25,000 from the years 1890-1894 and on the corresponding photographic maps from 1941 and on the maps from the 1960s on the same scalę (Stasiak 1991). On the maps from 1890-1894 11,068 of such smali bodies of water were recorded, on the photo maps from the year 1941 — 4,483 (44%), and on the maps from the 1960s — 2490 (22%). One should remember, however, that the 1890s witnessed the finał stages of the implementation of big dewatering projects in the Wielkopolska Lowland.

The same comparative materials proved that the most significant changes in the numbers of smali bodies of water took place in the regions of Jarocin and Dobrzyca — around 80% of such reservoirs disappeared (ibidem).

On the other hand, in the two decades between 1942 and 1963, significantly greater than average decreases in the number of smali lakes took place in the area of Dąbrowa Biskupia, Duszniki, Konojad and Jutrosin where it exceeded 40%. During the entire studied period (1890-1963) a particularly high number of formerly-existing lakes was recorded in the Southern part of the Wielkopolska Lowland, in the region of Dobrzyca, Borzacin, Rozdrażewo, Krotoszyn, Ostrów, and in watershed zones.

The decreasing surface area of lakes and their disappearance can be associated with land melioration and drainage, regulation of rivers, or eutrophication of lakes as a result of a considerable inflow of biogenic substances. As a result of dewatering Iow areas, Iow grasslands dried out and they were turned into arabie land, as a conseąuence of which the share of meadows and pastures gradually decreased. While at the close of the 18th century the areas of meadows and pastures still eąualled the surface area of arabie land (a 1:1 ratę), by 1815 the ratę had changed to 1:1.2 (Stryjewski 1980), and by the middle of the previous century it had changed to 1:3.7 (Falkowski, Skolimowski 1969), and currently it is around 1:5.