A comparison of sediment and monitoring data: implications for paleomonitoring a small lake.

Long-term limnological monitoring data (from 1971 to 2001) were compared with sediment core record in Lake Viitna Linajärv (hereafter L. Viitna), a small lake in northern Estonia. The monitoring data show that L. Viitna changed from mesotrophic in the 1970s to eutrophic in the 1990s. The trends of paleodata that integrate the changes in the biogeochemical matter cycling in the lake over 2-3 years have clear signals about changes in the state of the ecosystem in L. Viitna during the last decades. A gradual increase in organic productivity should reflect a greater oxygen demand in the hypolimnion. As a result the hypolimnion became seasonally anoxic earlier and its pH level remained low for a longer time. These fundamental changes near the sediment-water interface were recorded in the sediment core. The greatest changes occurred at the beginning of the 1980s (layers at a depth of ca. 20 cm), when the meso(eu?)trophic conditions in L. Viitna started to become increasingly more eutrophic. The variations of paleorecords in the upper part of the sediment core coincide temporally with changes in the water level of the lake.