A number of palaeoecological investigations have proved that the geochemical composition of lake sediments serves as a valuable archive of information for the reconstruction of natural as well as human-induced processes that shape the present character of a lake's ecosystem (Engstrom & Wright, 1984; Birks & Birks, 2006; Battarbee et al., 2007). The use of chemical proxies is particularly important for analysis of sediments that accumulated during the last centuries when intensified human activities began to alter natural biogeochemical pathways, causing disturbance of natural systems at an elemental level (Boyle, 2001; Tylmann, 2005; Punning et al., 2007).
Previous investigations show that during the last century the natural ecosystem of Lake Nommejarv in NE Estonia became affected by human activities that resulted in significant alterations in its sediment and water composition as well as in its ecological status (Vesiloo, 1987; Sagris, 1989; Varvas, 1994; Punning et al., 1997).
As a result of the implementation of environmental protection measures and the political transition at the beginning of the 1990s, the industrial impact in the area connected with oil shale mining and power plants has become less pronounced. A complex palaeoecological study, which was carried out at the beginning of the 1990s, showed that this factor seems to positively contribute to a decrease of pollution connected with atmospheric fallout as well as to the overall tendency of the lake ecosystem to return to pre-disturbance conditions (Punning et al., 1997).
Based on geochemical and stable isotope proxies, this study aims to describe the changes in the geochemical composition of the sediment as a response to the presumed reduction in the human impact on the lake during the last 20 years. It also attempts to detect what anthropogenic factors continue to control the present state of the lake. To document and understand the ways in which human impact on the ecosystems interacts with natural processes is one of the essential prerequisites for establishing sustainable and efficient land management (Oldfield & Dearing, 2003).
Lake Nommejarv is located in NE Estonia (58[degrees]03' N and 26[degrees]30' E) in the western part of the Kurtna lake district (Fig. 1). The lake is 15 ha in area and its maximum depth is 7 m (Varvas & Punning, 1993). Its catchment has a forested area in its western and eastern parts, peatland to the north, and arable land in the south (Fig. 1a). Human influence began to intensify at the end of the 19th century when drainage of the surrounding bogs and pastures started. During the 1920s, the natural flow of the Raudi Stream (Fig. 1a) was directed to the lake, changing it from a closed to an open system. Local human impact on the lake continued to increase with the construction of a military camp on the shore at the beginning of the 1930s, which was used then for military and later, including today, for tourist activities (Punning et al., 1997).
At the beginning of the 1950s, oil shale mining began in the vicinity of the Kurtna lake district. Expansion of oil shale mining and related industries such as oil-shale-based power engineering became a strong industrial factor in the region (Erg, 2003). Local kukersite oil shale forms horizontal sequences in the limestone from which it has been extracted in underground mines. To facilitate the discharge of groundwater from mines, several Kurtna lakes have been utilized and interconnected with channels. In 1970 the Raudi Stream was completely transformed into an artificial channel and mine water started to flow through Lake Nommejarv. Variation in the discharge has been largely governed by seasonal fluctuations and the volume of mining waters (from 25 000 to 54 000 [m.sup.3] [h.sup.-1]) (Sagris, 1989). Water is also carrying mine clastic mineral debris, which consists of carbonate matter, siliciclastic (silica-bearing sedimentary rock) particles, and oil shale remains and is high in sulphate ions and calcium (Bauert & Kattai, 1997; Erg, 2003). …