Palaeolimnological and limnogeochemical features in the sedimentary record of the polluted Lake Lippajärvi in Southern Finland

A sediment core from Lake Lippajärvi, sampled by freezing it in situ, was investigated. The sedimentary sequence was dated using the ¹³⁷Cs method and the ²¹⁰Pb content of the core was also determined. It seems that, during the recent history of the lake, increasing eutrophication is the main factor controlling fluctuations in lake ecosystems. This increase is clearly reflected in the changes in the diatom stratigraphy of the core investigated. Sequential extraction analysis was performed for speciation of elements in the sediment. Ammonium acetate and hydroxylammonium chloride plus acetic acid were used as extractants; acid digestion and extraction by NaClO and NaDDTC/MIBK were also performed.

An increase in sedimentation rate is apparent for the upper part of the lithostratigraphy of Lake Lippajärvi. The increased nutrient load and heavy metal input have led to cultural eutrophication and pollution effects in the lake, and black bands of Fe monosulphides have emerged in sediments as a sign of oxygen deficit in the hypolimnion. The eutrophication of Lake Lippajärvi is also accompanied by biogeochemical cycles and limnogeochemical processes apparently responsible for the leaching of chemical elements from sediments and their subsequent redeposition in more loosely bound forms. Fe has been redeposited mainly as Fe oxyhydroxides and monosulphides, and Cu and Zn also seem to be associated with these precipitates. Ca, Mn and Zn are present to relatively large degrees as very loosely bound and sorbed forms. Fe, Mn, Cu and Zn are also associated with sulphides, and Fe and Cu with biogenic matter. There is a clear difference between man-made airborne forms of Pb and natural mineral-bound forms in the sediment column of the lake. In the polluted part of the investigated lithostratigraphy, Pb is mainly present in loosely bound reducible forms. Al and Si also seem to show leaching and redeposition in the sediments of Lake Lippajärvi.