Environmental history of an urban lake: a palaeolimnological study of Lake Jyväsjärvi, Finland

Sedimentary diatom frustules and chironomid remains, in addition to the chemical stratigraphy of 32 elements and organic pollutants such as resin acids, PCB, DDT and its metabolites, were studied from core samples taken from Lake Jyvasjarvi in Central Finland (64° 14′ N, 25° 47′ E). The sediment profile covered over 200 years, with the oldest samples representing a period of very low human interference. The town of Jyvaskyla was established on the lake shore in 1837, and the lake received untreated municipal wastewater from the town up until, 1977. A paper mill started operations in 1872 and began discharging effluent into Lake Jyvasjarvi. In recent years this effluent loading has been reduced. Based on the biological and chemical properties of the sediment strata, five developmental phases were distinguished and named as follows: (1) a pre-industrial phase (approximately up until the 1860s); (2) a phase of early changes in the lake ecosystem (from the ∼1870s to ca. 1940s); (3) a phase of increasing eutrophication (from ca. 1950 to ca. 1965); (4) a phase of severe pollution (from ca. 1966 to the early-1990s); and (5) a phase of recovery, which proceeded more quickly during the late-1990s following long-term hypolimnetic aeration. It was estimated that the ecological status of the lake changed from good to moderate during the second phase. Due to the poor chemical status of the lake (including increased concentrations of harmful substances) and the pronounced changes in diatom and chironomid communities, the ecological status from 1950 to the early-1990s was classified as bad. The present ecological status, after a slow recovery of about 20 years, can be classified as moderate/poor. Chemical analysis of the sediment revealed that some elements (C, N, P, S, Ba, Cd, Cu, Fe, Pb, Ni and Zn) followed the known history of municipal waste water discharge into the lake, but Hg, Cr and persistent organic pollutants had different stratigraphies, and therefore mainly originated from other sources. Pronounced changes in profundal benthic communities (chironomids) started about 80 years later than those in diatoms, but later changes in chironomid assemblages were greater than those in diatoms.