Anthropogenic changes in the sediment composition of Lake Gosciaz (centralPoland), during the last 330 yrs

According to historical sources, the development of settlement around Lake Gosciaz during the last 330 yrs was intense at two time periods: the second half of the 1700's, and from ca. 1880 until 1944. The small farms were the n abandoned, following which the lake surroundings were planted with forest trees. The presented study of human influence on the lake ecosystem and su rrounding vegetation has been based on analyses of general sediment composi tion and its chemistry, pollen, Cyanobacteria, Chlorophyceae, Rotatoria, Cl adocera, and a preliminary diatom survey. The history of human impact has b een divided into four phases: 1. Phase of small local hamlets (before ca. 1770): The human impact was rat her moderate then, but the cultivation of Canabis sativa, Secale cereale an d later of Fagopyrum is evidenced from that time. 2. Phase of 'Hollandii' settlement (ca. 1770-1863). Its influence is indica ted first by the recession of deciduous wood (Corylus, Carpinus) stands, wh ich triggered drastic drop of calcium in sediments. The development of rura l economy in the area, including, an extension of agriculture (Secale and o ther cereals, crucifers, potatoes), and animal breeding based partly on gra zing in the forest, is evidenced only after 1820. 3. Phase of German colonization (1863-1944): In the early periods (before 1 910) the pollen spectra do not document any essential change in the type of farming, however, a serious disturbance of the lake ecosystem and sediment chemistry is evidenced by the blooms of Araphidinae diatoms and Tetraedron minimum, a maximum frequency of Bosmina longirostris, disturbances of the regular spring blooms of Chrysophyceae, appearance of vivianite, distinct m axima of organic matter, potassium, and iron concentration in sediments and an increase of the sedimentation rate. An intensification of agricultural activities commenced around 1910; woods, including also pinewoods, were the n heavily devastated, and farming extended on poor soils, what was symptoma tic for the general poverty of population. Coincidently in the lake, Centri ceae showed blooms, Araphidinae diatoms and Tetraedron minimum developed, a nd the content of potassium, iron and phosphorus increased substantially, i ndicating altogether rising eutrophication. 4. Phase of restoration of the natural landscape (after 1945): The farm deg radation from ca. 1944 is very weakly expressed in pollen data, which show a substantial fall of farming indicators from 1953/6 only, when the whole a rea was used for forest plantation. It was accompanied by a certainly spont aneous development of Betula and Alnus woods, this process progressing till recent time. The gradual extinction of farming activity near Lake Gosciaz was accompanied by abrupt changes in the lake ecosystem, expressed by the r estored blooms of Chrysophyceae expansion of planktonic Cladocera, rapid de cline of phosphorus and extinction of vivianite from sediments. As document ed by the drop of Cu/Zn ratio, lake hypolimnion has been weakly oxidized si nce 1949, what was probably responsible for the drop of Fe and Mn content i n sediments. Increasing strength of overturns affected preservation of lami nae in sediments, which almost completely disappeared after 1966.